I was born in a small town and I live in a small town. I’ll probably die in a small town, but only if you want to call a blue-collar city of 75,000 people small. My friends are small town and my parents still live here in this same small town. It was here in this small town that I started my career as a surveyor and circled back for an opportunity to be near family and friends.
There are many towns, villages and cities in the Midwest that fit this description — mostly because of the agricultural background, but also because of the labor-intensive industries that provide most of the local jobs. Like those who prefer the large city hustle-bustle lifestyle, the small town attitude is the same, but in the opposite direction.
My career as a surveyor has provided me with opportunities to work in both small and heavily populated areas. I have seen the definite distinction between the two environments, and have seen many of the technological advancements of the past few decades. Having worked in both the large urban multi-discipline engineering firm and the small town surveying firm, I see much disparity between how surveying is done today from varying firms. The biggest difference I see today is how surveyors are using GPS technology for their field operations.
For the modern surveyor, GPS has become an everyday tool for measuring and data collection, but it wasn’t always that way. I began my career in the early 1980s working for a seven-person engineering firm in my small town, so GPS was never in our budget. Like most surveyors, I read about GPS use in technical publications, and was amazed what these magical boxes could do. The first generations of static receivers produced were very complicated and expensive. With long occupation times and even longer processing times, only the few companies who could afford the high-priced equipment and software were buying these units. Our small-town market could not justify this purchase, so we made do with conventional equipment as the norm.
Today’s environment requires the working knowledge of handheld data collectors that are more powerful than the computer on the Apollo moon mission, laser scanners that collect millions of points per second, and GPS receivers that talk through cell phones to get sub-centimeter accuracy. Around the corner is wider use of lidar data collection as well as the unlimited use and application of UAVs with cameras and scanners, so there is no end in sight for technology and the surveyor.
Most of the larger urban firms have established equipment and training budgets stay current with technology and not lose ground with the competition. Because of these strategic and spending plans, fee structures have increased over the years, and thus the cost of surveying has increased accordingly. This has also afforded those who stay up-to-date with current technology to push the limits of the equipment, and to continue to find new and useful ways to perform our work and provide newer services.
By keeping their costs down, small-town and rural surveyors have kept their fees down as well, but to the detriment of the profession. In fact, the cost of a typical land survey in a small town has come nowhere close to inflation for the past 40 years in most places.
In comparison, however, many industry partners of the surveyor have spent a great deal of money and time staying current with technology and production methods. Excavators and earth movers now have GPS-based computers controlling their movement and placement of material, as well as utilizing robotic instruments and GPS for layout of improvements and utilities. Architects and structural engineers are utilizing scanners for building and piping as-builts for existing and future improvements. Almost all farmers have tractors equipped with GPS-based control systems to help them plant and harvest with pinpoint accuracy, as well as apply herbicides or pesticides based upon high-tech mapping performed by crop analysts. Many land-management companies are gearing up with UAV technology to assist future operations with the information gathered by these flying wonders. Everyone around us is tech savvy, but the small surveyor seems to lag behind.
I wanted to be exposed to new technology when I moved from a small town to a large urban city in 1998. The big improvement for the surveying community was the introduction of real-time kinematic (RTK) methods. The firm I worked for during the late 1990s and early 2000s had been using RTK systems since their introduction. Being able to collect points “on the fly” both manually and with ATVs greatly increased our productivity as well as accuracy.
By the mid-2000s, almost every big-city firm utilized this technology as standard equipment for their crews. The only drawback to the RTK system was the need to leave a base-station receiver, so the introduction of the real-time network via cellphones in large market areas was another step in solidifying GPS use for everyday work.
Another benefit of extensive GPS use in the large city area is that most of the firms keep their surveys on state plane coordinate systems, so exchange and verification of data is a much easier process. These larger urban firms also continue to upgrade to newer equipment as more satellite systems are introduced. The addition of GLONASS has increased our precision and coverage levels, with future systems including Galileo, IRNSS and BeiDou set to raise that bar even higher.
On the contrary, many firms in small-town and rural areas have not progressed into new technologies because of cost and lack of cellular coverage needed for RTN systems. I moved back to my small town several years ago and have experienced this slip in technology firsthand. The big cellular carriers say they have 3G and 4G service in most places, but I can tell you from my travels that there are many places I have not been able to use our RTN receiver because of lack of cell signal. Most of the surveyors in our area still utilize an RTK system and will establish a position through OPUS or will assume a local coordinate system. Not many have upgraded their equipment to take advantage of GLONASS, so there are several steps they will need to take in the future to catch up to the industry. We also must travel greater lengths to recover NGS monuments for our positional verification, which will become more important as static monuments become a thing of the past.
So with my apologies to Mr. Mellencamp, my job as a surveyor has not been just small town, and has provided me with many big-city opportunities. Our crews face challenges here every day that many take for granted while in the big city market, including RTN coverage, GLONASS constellation usage and many more NGS monuments for QA/QC. Having more of these items could greatly help our productivity, especially when stronger cellular coverage is expanded to more of our rural areas.
I look forward to these improvements but will continue to work with our existing systems until that time. So look for us, surveying on in our small towns and locating all those little pink houses.
This column introduces Tim Burch, GPS World’s new co-contributing editor for survey. Tim will alternate with Dave Zilkoski in contributing monthly columns to the Survey Scene e-newsletter. Tim is survey department manager for Chastain & Associates LLC in the Decatur, Ill., area. He has been working as a professional land surveyor since 1985, and is the secretary, Board of Directors, National Society of Professional Surveyors.
For his next column in January, Tim plans to write about farmers and their technology in his area, focusing of course on GPS use. His article will compare surveyors’ processes with those used by farmers. (For Dave Zilkoski’s last column in October, see Establishing Orthometric Heights Using GNSS — Part 3.)
Part 1 of this column appeared in the June Survey Scene newsletter, Part 2 appeared in the August newsletter. Upcoming Survey Scene newsletters will carry additional columns in this series.
Basic Understanding of Scientific and Hybrid Geoid Models
David B. Zilkoski
In my first newsletter column of this series, I discussed the basic concepts of GNSS-derived heights. I discussed the three types of heights involved in determining GNSS-derived orthometric heights: ellipsoid, geoid and orthometric.
In my second column (Part 2), I discussed guidelines for detecting, reducing, and/or eliminating error sources in ellipsoid heights. The column focused on guidelines for establishing accurate ellipsoid heights in a local geodetic network.
This column, Part 3, will describe the differences between a scientific gravimetric geoid model and a hybrid geoid model, and why it is important to use both geoid models in your analysis. The latest published United States National Geodetic Survey (NGS) hybrid geoid model, Geoid12B, is made consistent with the United States National vertical height reference frame, that is the North American Vertical Datum of 1988 (NAVD 88). This means a user will be consistent with NAVD 88 when using GEOID12B to estimate GNSS-derived orthometric heights. However, this doesn’t guarantee that your GNSS-derived orthometric heights are accurate.
NGS’ new Beta experimental geoid height models xGEOID14B and xGEOID15B are not distorted to fit the published NAVD 88 heights so they are useful for identifying valid NAVD 88 bench marks (that is, ensuring the monuments haven’t moved since their last survey and their published heights are still valid). Therefore, it is extremely important to validate all NAVD 88 height constraints used to estimate accurate GNSS-derived orthometric heights. Understanding NGS’ scientific and hybrid geoid models will help the user perform the appropriate analysis to determine which leveling-derived orthometric height constraints should be used as constraints. This newsletter will focus on differences between geoid models in a local project area.
Information on NGS’ experimental geoid models can be found here.
Thursday, August 20, 2015
Yearly Experimental Geoid Model Available for Public Review
In 2022, NGS will replace the current North American Vertical Datum of 1988 with one that is based on the geoid — a model of global mean sea level that is used to measure precise surface elevations. NGS created and released annual experimental models of the geoid starting in 2014. This year’s models, xGEOID15A and xGeoid15, are now available for public comment on the NGS beta website. The annual experimental models include new data from the Gravity for the Redefinition of the American Vertical Datum project, which has systematically collected airborne gravity data across the nation since 2008. For more information, contact: [email protected]
A depiction of the United States geoid. Areas in yellow and orange have a slightly stronger gravity field as a result of the Rocky Mountains.
While we often think of the earth as a sphere, our planet is actually very bumpy and irregular.
The radius at the equator is larger than at the poles due to the long-term effects of the earth’s rotation. And, at a smaller scale, there is topography—mountains have more mass than a valley and thus the pull of gravity is regionally stronger near mountains.
All of these large and small variations to the size, shape, and mass distribution of the earth cause slight variations in the acceleration of gravity (or the “strength” of gravity’s pull). These variations determine the shape of the planet’s liquid environment.
If one were to remove the tides and currents from the ocean, it would settle onto a smoothly undulating shape (rising where gravity is high, sinking where gravity is low).
This irregular shape is called “the geoid,” a surface which defines zero elevation. Using complex math and gravity readings on land, surveyors extend this imaginary line through the continents. This model is used to measure surface elevations with a high degree of accuracy.
How Does the U.S. National Geodetic Survey Generate a Geoid Model?
Generating geoid models is a fairly complex process and is performed by individuals with expertise in physical geodesy and geophysics. It is too complex of a topic for this newsletter but the following excerpt from an NGS publication by Dan Roman provides a good overview of NGS’ process.
Development of the North American Gravimetric Geoid: Adapting the Process to Determine a Unified Central American Geoid
D.R. Roman National Geodetic Survey, 1315 East-West Highway, Silver Spring, MD, USA, 20910
2 Data & Process Improvements
Techniques discussed here have already been addressed previously in Roman and Smith (2001) and Smith et al. (2001), hence only a summary of the approach discussed in those papers is given here. Essentially, the approach currently under investigations seeks to take advantage of recent and pending gains in various data sets related to the gravity field and significantly reduce approximations considered acceptable in the past.
The first thing to consider is the justification for using a geoid over a quasi-geoid, or more accurately, orthometric heights over normal heights. Convincing arguments have been made for orthometric heights (Holdahl 1984) and normal heights (Heiskanen and Moritz 1967). While orthometric heights require extensive knowledge of the gravity field, it is just that reason that warrants their use. Given the extensive knowledge and available data sets, it is incumbent on governmental agencies to generate such models. With a model of the gravity field from the surface to the geoid at hand, anyone subsequently desiring to transform from orthometric to normal heights need only apply it. However, if normal heights are developed and orthometric heights are later desired, the development of such a model will then be required. Clearly, this is a task best suited to national and international organizations that have access to such data and methods. It should not be left to those researchers desiring to use height models in their studies that may not have access to sufficient resources to accomplish this.
With that understanding then, the development of a gravimetric geoid model follows as a mechanism to readily convert between ellipsoidal and orthometric heights. The method summarized here seeks to break the gravity field into three components and solve them separately. In fact the long wavelength component will be derived from a global reference gravity model. The short wavelength will be determined from the terrain. Both of these components will be removed from available gravity observations, which will then reflect the intermediate wavelength signal. A flowchart depicting the determination of these three signals and the generation of a gravimetric geoid is given in Figure 1. Paths shown in red highlight the use of the reference model, paths in green show the determination of the terrain effects, while paths shown in purple highlight the main path to determining Helmert anomalies and then a gravimetric geoid model.
Fig. 1 Determination of a gravimetric geoid using Helmert anomalies.
The expected accuracy of global gravity models in the near future is expected to vastly improve with commission errors below 1-2 cm at wavelengths of 200-300 km (Tscherning et al. 2000). Use of a remove and restore technique (Bašiæ and Rapp 1992) will then result in significantly reduced errors in the residual signal that will be manipulated.
The approach discussed in Roman and Smith (2001) develops the North American gravimetric geoid by removing the terrain effects, downward continuing the residual values, and then restoring the effects of the condensed terrain to generate Helmert anomalies (Heiskanen and Moritz 1967).
To this end, the gravitational attraction of the terrain (TgP) will be calculated and removed from the gravity observations. It will be split into inner and outer zones to reduce computation times. Smith et al. (2001) showed that the effects of using FFT to determine gravitational attraction and potential for both condensed and 3D masses is negligible beyond about a 4 degree cap radius from the point of interest (P). Inside that zone, DEM’s are employed to capture the spherical relationships between the points and more accurately determine the attraction. With available or pending 1 and 3 arc-second DEM’s (Smith and Roman 2001a, NIMA 2001), the signal that may be determined is limited mainly by the computational facilities available to a researcher.
Additionally, the DEM’s will be used to construct grids for the attraction and potential of the condensed terrain (cgPo and cWPo), as well as the potential of the actual terrain (TWPo), all on the geoid. This will capture the short wavelength gravity signal represented by the terrain to the resolution of the grid generated and facilitate later incorporation of this signal into Helmert anomalies.
The resulting point values should be composed mainly of intermediate features in the gravity field with sources deriving from variations in the Moho depth and lateral density variations. This signal should be sufficiently smooth to reduce errors resulting from downward continuation. It should also sufficiently sample the intermediate field to permit the use of minimum curvature (Smith and Wessel 1990) to generate a grid at the same interval as that of the above terrain effects.
Once these terrain effects are restored, these extremely high resolution grids represent residual Helmert anomalies and may be processed using the Stokes integral to determine a best fitting residual gravimetric geoid. Adding the reference geoid derived from the selected global coefficient model will create an equally high-resolution regional gravimetric geoid model.
For a specific country, GPS-derived ellipsoid heights at leveled bench marks (GPSBM’s) provide control information for generating a hybrid geoid model that can be used to specifically, easily, and accurately transform heights between ellipsoidal and orthometric heights (Smith and Milbert 1999, Smith and Roman 2001b).
What are Hybrid Geoid Models and how are they Generated?
NGS’ hybrid geoid model GEOID12B is computed based on the gravimetric geoid USGG2012. As described above, the gravimetric geoid is computed using the satellite model (GOCO3S), terrestrial gravity data, and the altimetric gravity anomaly over oceans. The heights of USGG2012 represent an equipotential surface relative to the reference ellipsoid. The differences between USGG2012 and the zero height surface of NAVD88 are represented by NAD 83 (2011) GNSS-derived ellipsoid heights on NAVD 88 published benchmarks (GPSBM data). See article by Milbert, D.G., 1998: “Documentation for the GPS Benchmark Data Set of 23-July-98,” IGeS Bulletin N. 8, International Geoid Service, Milan, pp. 29-42.) for a excellent description of NGS’ GPSBM dataset.
Currently, the USGG2012 is fitted to the GPSBM data by using the method of least squares collocation. (See section labeled “Excerpts from NGS’ Geoid 12 Web Page” for specific details on how NGS generated hybrid geoid model GEOID12B.) Areas where there are no GNSS observations on published NAVD 88 benchmarks are filled in by USGG2012 geoid. This means a user will be consistent with NAVD 88 when using GEOID12B to estimate GNSS-derived orthometric heights. Being consistent with NAVD 88 is important but being consistent doesn’t guarantee that your GNSS-derived orthometric heights are accurate. The documentation of GEOID12B states that “The relative accuracy of GEOID12B to NAVD88 is characterized by a misfit of +/-1.7 centimeters nationwide.” However, if a published NAVD 88 height used in the development of the hybrid geoid model isn’t valid, then the model is precise but not accurate. That’s why it is important to ensure the monuments used in hybrid geoid models haven’t moved since their last survey and that their published heights are still valid. We will discuss this in more detail later in this newsletter.
Hybrid geoid model, GEOID12B is computed based on the gravimetric geoid USGG2012 . More specifically, they are computed using the satellite model GOCO3S, terrestrial gravity data, and the altimetric gravity anomaly over oceans. The heights of USGG2012 represent an equipotential surface relative to the reference ellipsoid. The differences between USGG2012 and the zero height surface of NAVD88 are represented by GPSBM data.
Currently, the USGG2012 is fitted to the GPSBM data by using the method of least squares collocation. That implies that the voids or empty areas where there are no GPSBM data are filled in by USGG2012 geoid.
There are over 500,000 leveled marks and 80,000 GPS marks over U.S. territory. Of those, there are only 26,000 GPSBM, with half of them concentrated in 5 states. The data density is uneven and sparse in some states. Lists of GPSBMs can be downloaded from the GEOID12B home page.
The GPSBM data provide the geoid height ‘N’ by differencing the ellipsoidal height ‘h’ from the orthometric height ‘H’:
N = h – H
The difference between the geoid height N and that of USGG2012 is computed at every GPSBM. Then, a mathematical model using Least Squares Collocation (LSC) fitting Gaussian functions to describe the behavior seen at the GPSBM is developed. Figure 1 shows empirical data versus the model.
Figure 1: Covariance functions of the geoid differences between USGG2012 and GPSBMs.
Once the relationship between the points is modeled, the model is used to generate a regular grid for interpolation purposes. Figure 2 shows the final conversion surface. This surface represents the difference between NAVD 88 as a datum and the geopotential (geoid) surface used in the gravimetric geoid and is representative of what the datum transformation surface will be when the new geopotential datum is released in 2022. (Similar to VERTCON, which transforms heights from NGVD29 to NAVD88.)
Figure 2: GEOID12B conversion surface.
Summary and Recommendations
Three hybrid geoid models GEOID12, GEOID12A, and GEOID12B are created. They are very similar, but have distinctive differences in few areas. GEOID12A differs from GEOID12 in that it does not use GPSBM data collected in the southern tier states along Gulf Coast, while GEOID12B differs from GEOID12A only in Puerto Rico.
Data in the database are constantly updated, hence older geoid models do not reflect the newer data. To guarantee data consistency, latest model should be used. At this time, GEOID12 and GEOID12A should be superseded by GEOID12B.
Use data conversion outside the GPSBM data areas with caution. Significant extrapolation errors are expected in areas where there are no GPSBM data.
The relative accuracy of GEOID12B to NAVD88 is characterized by a misfit of +/-1.7 centimeters nationwide.
As previously stated, NGS released its latest gravimetric geoid model, xGEOID15. This site will allow the user to compare geoid heights from GEOID12B, USGG2012, xGEOID14 and xGEOID15. (See an example of an input and an output file below.) There are some limited features to this tool. It only provides the results in IGS08 and you are limited to the number of coordinates you can submit at once (20 stations).
Saying that, this tool can be useful for identifying valid NAVD 88 published monuments to be used in the development of future hybrid models. More importantly, it can be used to identify monuments that should NOT be used in future hybrid geoid models or used as constraints in GNSS survey project adjustments.
First, let’s look at the hybrid geoid model GEOID12B values compared with computed geoid height values using the equation N (Computed Geoid Height) = [h (NAD 83 (2011) Ellipsoid Height) – H (NAVD 88 Orthometric Height)]. Table 1 lists the differences between the modeled GEOID12B values and the computed geoid height values for a few stations in an area in eastern North Carolina. Figure 1 depicts the stations locations and values. Many of the differences are less than 1.5 cm which is consistent with NGS’ documentation of GEOID12B that states “The relative accuracy of GEOID12B to NAVD88 is characterized by a misfit of +/-1.7 centimeters nationwide.” However, what is important to notice is that two stations have large differences; station LILIPUT’s difference is 7.4 cm and station BR 7’s difference is -4.6 cm (See highlighted rows in table 1 and boxed area on figure 1). This means that the relative difference between stations LILIPUT (EA0875) and BR 7 (EA0873), which are only 3.3 km apart, is 12.0 cm. This is a large difference and may be indicating a large error in the ellipsoid height and/or the orthometric height at station LILIPUT (EA0875) or station BR 7 (EA0873). In the second newsletter we highlighted that stations LILIPUT and BR 7 were only 3.3 km apart but were not simultaneously observed during the same session. Since the relative difference is 12 cm, the ellipsoid heights of these two should be investigated. It should also be noted that the difference between stations BR 7 (EA0873) and TOWN CREEK (EA0883) is only 3.2 cm. This implies that station B 7 (EA0873) is consistent with some of its neighbors. In the second newsletter we noted that stations B 7 (EA0873) and TOWN CREEK (EA0883) were simultaneously observed during the same session. This may be an indication that B 7 is stable relative to its neighbors and that the orthometric and/or the ellipsoid height of station LILIPUT needs to be investigated.
So what does this mean to the user? If the user establishes a GNSS-derived orthometric height near station LILIPUT using GEOID12B, their results will disagree with the published NAVD 88 heights to around 7 cm; if they establish a GNSS-derived orthometric height near station BR 7, they will disagree with published NAVD 88 heights to around –5 cm. This could also mean that the results in a project could really disagree by more than 7 cm if station LILIPUT moved since its last survey. At this moment, we don’t have enough information to determine if the ellipsoid height or the orthometric height is the problem, or which station may have moved since its last survey.
Table 1. Geoid Height Comparison using GEOID12B Hybrid Model Values.Figure 1. Geoid12B minus Computed Value on NAVD 88 Benchmarks.
Next, let’s look at the differences using the experimental geoid models which are not distorted to be consistent with the NAVD 88 published heights. There will be a bias and a tilt between the systems but in this small areal extent the tilt should not be significant to our analysis. The bias can be removed by looking at relative differences between stations. Table 2, titled “Geoid Height Values for Various NGS Models using xGeoid15 Web Tool,” provides the modeled geoid height minus the computed geoid height where N (Computed Geoid Height) = [h (IGS08 Ellipsoid Height) – H (NAVD 88 Orthometric Height)]. Figure 2, titled “Various Geoid Models minus Computed Geoid Height,” depicts the differences between the various experimental models and computed geoid heights.
Table 2. Geoid Height Values for Various NGS Models using xGeoid15 Web Tool.Figure 2. Various Geoid Models minus Computed Geoid Height.
What is important to note is that stations LILIPUT (EA0875) and WATERWAY (EA0665) seem to be outliers compared to the other stations in the area of study (red boxes on figure 2); and station B 7 (EA0873) seems to be consistent with its neighbors (yellow box on figure 2). For example, station LILIPUT (EA0875)’s residual using xGeoid15B is 25.5 cm and station BR 7 (EA0873)’s residual using xGeoid15B is 13.1 cm, a relative difference of 12.4 cm. Similarly, station TOWN CREEK (EA0883)’s residual using xGeoid15B is 16.3 cm and station BR 7’s residual is 13.1 cm, a relative difference of only 3.2 cm. In my opinion, station LILIPUT (EA0875) needs to be investigated to determine if it has moved since it was last surveyed. In addition, stations east of LILIPUT (EA0875) such as WATERWAY (EA0665) should also be investigated for an ellipsoid and/or orthometric height issue. As previously mentioned, it is also important to note that station BR7 (EA0873), the box in yellow, appears to be consistent to the 3 cm level with its westerly neighboring stations (the boxes in green). This is important to note because the hybrid geoid model could be significantly difference around stations LILIPUT and BR 7 if station LILIPUT was not used in the development of the hybrid geoid model. I am not suggesting that NGS did anything incorrect by including these stations. The goal of the hybrid geoid model is to be consistent with published NAVD 88 values. Unless there is enough information to determine that a station has moved since the last time it was surveyed, the station should be included in the hybrid model. This is where the user may be able to help NGS. If users would investigate outliers like LILIPUT and BR 7 and provide new GNSS survey data and/or leveling data, NGS may have the appropriate information to determine if the monument should be included in the hybrid model.
Part 2 in this Survey Scene series discussed procedures which need to be followed to detect, reduce, and/or eliminate error sources to estimate accurate GNSS-derived ellipsoid heights. This column, Part 3, discussed why a user should understand the differences between NGS’ scientific gravimetric geoid model and hybrid geoid models, and why it is important to use both types of geoid models in their analysis. It demonstrated how to use these geoid models and ellipsoid heights to identify potential issues with published NAVD 88 heights.
My next newsletter column will focus on analyzing the NAVD88 orthometric heights in this area. It will provide basic procedures for validating NAVD 88 height constraints used to estimate GNSS-derived orthometric heights.
Reliable, consistent positioning accuracy has always driven new product development in the survey and mapping sector of the GPS/GNSS market. It’s remarkable how quickly the provided accuracy in successive new survey products over the years has increased the required accuracy from users and customers in the field, and consequently the desired accuracy in a feedback loop to the product developers.
In other words, the degree of required accuracy has risen steadily over the three and a half decades since GPS was born. “Accuracy is addictive.” Somebody said that in the second decade of GPS development, that is, sometime in the 1990s. This statement continues to hold true, as true for this industry as Moore’s Law does for computer technology as a whole.
Moore’s Law states that overall processing power for computers will double every two years; as a corollary or an extension, the size of said computers gets cut in half every two years, and the cost (sometimes) also comes down by 50 percent. Moore’s Law in action in the GPS/GNSS industry has driven the product developments that we have consistently seen for many years.
We have seen the gradual tightening of accuracy requirements across all sectors of the positioning, navigation and timing (PNT) community with each passing year and with each new State of the Industry Report. This is the first time we have seen it cross the 1-centimeter line. Not in capability; sub-centimeter capability has been available for some time. But now that level of performance is the minimum acceptable “good enough” for more respondents in the survey and high-precision sector than any lesser degree of accuracy; in fact, greater than all other ranges combined.
To put this into measurable, statistical form, GPS World has just released its fourth annual “State of the GNSS Industry Report.” In the years that we have conducted the survey, the accuracy required for the majority of survey applications has steadily come down. No surprises here.
In 2013, those who said that the majority of this market sector needed accuracy of better than a centimeter amounted to only 8 percent of total respondents.
In 2014, this group rose dramatically to 35 percent, while close to a majority, or 47 percent, held that a range of 1 to 5 centimeters was “good enough.”
Now, in this year of 2015, the majority has shifted clearly to the side of 1 centimeter or better as the new standard of required precision; 51.25 percent held this view. From 8 percent to more than half in just two years — that’s some change!
How accurate is good enough for the majority of this sector?
Fewer people believe that a survey done completely on a computer and driven by remote-sensor data will occur in less than five years. Counter to last year’s expectations, most now think it will take longer than five years to come about.
How soon will a survey be performed entirely from a computer, using high-resolution satellite and/or UAV-collected data, without any instrumented field work?
Those who are addicted to 1-centimeter accuracy form the new majority. Their preferences and their behaviors will rule the positioning world, not just in survey, but across all sectors supplied by GNSS and increasingly by a broad range of PNT technologies: defense, transportation, UAVs, machine control, precision agriculture, and much more. These other sectors will presumably answer likewise — “1 centimeter accuracy, that’s what I need!” in coming years, following the trail blazed by the you high-precision surveying pioneers.
We have crossed the Rubicon. Unlike other obsessive behaviors, there is no going back in our case. This path is a one-way road to to the promised land of always-on, always-true, near-perfect provision of positioning.
How much effort are you devoting to mitigation of GNSS jamming or spoofing?
Part 1 of this column appeared in the June Survey Scene newsletter.
Basic Procedures for Establishing Accurate GNSS-Derived Ellipsoid Heights
David B. Zilkoski
In my first newsletter column of this series, Part 1, I discussed the basic concepts of GNSS-derived heights. My article discussed the three types of heights involved in determining GNSS-derived orthometric heights: ellipsoid, geoid, and orthometric. I also mentioned that each of these heights has its own error sources that need to be detected, reduced or eliminated by following specific procedures or applying special models.
GNSS-derived ellipsoid heights are the basis for GNSS-derived orthometric heights, so it makes sense to make these ellipsoid heights as close to error free as possible. This article will discuss guidelines for detecting, reducing and eliminating error sources in ellipsoid heights. It will focus on guidelines for establishing accurate ellipsoid heights in a local geodetic network.
Based on the Federal Geographic Data Committee publication “Geospatial Positioning Accuracy Standards, Part 2: Standards for Geodetic Networks,” guidelines were developed by the National Geodetic Survey (NGS) for performing GNSS surveys that are intended to achieve ellipsoid height network accuracies of 5 cm at the 95 percent confidence level, as well asellipsoid height local accuracies of 2 cm and 5 cm, also at the 95 percent confidence level. These guidelines were developed in partnership with federal, state and local government agencies, academia and private surveyors, and are the result of processing various test data sets and having extensive discussions with various GNSS users groups. These guidelines, known as NGS 58, have been documented in a publication titled “Guidelines for Establishing GPS-derived Ellipsoid Heights (Standards: 2 cm 9and 5 cm), Version 4.3″ and can be downloaded from the NGS website. NGS is reevaluating the guidelines and, based on its research results, will update the document appropriately (NGS, Personnel Communication).
Guidelines have also been written to establish GNSS-derived orthometric heights that approach these same accuracies, 2 cm and 5 cm. The slight differences between the accuracies of GNSS-derived ellipsoid heights and GNSS-derived orthometric heights will be generally due to the accuracy of the geoid model and published orthometric heights used to evaluate the differences between the three height systems: ellipsoid, geoid and orthometric heights. The topic “procedures for estimating accurate GNSS-derived orthometric heights” will be addressed in a future newsletter in this series.
If users follow the NGS guidelines, they will reduce or eliminate errors in ellipsoid height or, at a minimum, they will detect problems or errors in data. If these problems or errors are detected and corrected before the project is completed, then they will not be problems to the end users.
Basic Procedures for Detecting, Reducing, and Eliminating Errors in GNSS Ellipsoid Heights
The basic concepts listed below are very simple, but they all need to be followed as prescribed.
First and probably one of the most important procedure is to repeat baselines on different days and at different times of the day. This helps to detect and reduce the effects of: multipath, differences in height values due to different satellite geometry, and the amount of time a user must occupy a station for a short baseline, for instance, 30 minutes of good, valid data over baselines less than 10 km. (Although, it should be noted that to obtain 30 minutes of good, valid data, the user may have to obtain 45 to 60 minutes of data.)
The observing scheme for all stations requires that all adjacent stations (base lines) be observed at least twice on two different days and at two different times of the day. The purpose is to ensure different atmospheric conditions (different days) and significantly different satellite geometry (different times) for the two baseline measurements.
Keep baseline lengths under 10 km. The closer the two stations are, the better chance that common errors will cancel or nearly cancel, such as unmodeled atmospheric errors. It helps to reduce the amount of time the user must occupy a station in order to collect enough good, valid data to correctly fix all the integers.
Use fixed height poles. This helps eliminate errors due to incorrectly measuring the height of the antenna above the mark. Of course, when listening to GNSS users, nobody has ever measured the height of the tripod wrong. But, it’s strange how that turns out to be the most common error when fixed-height poles are not used.
Antenna set-up is critical. Plumbing bubbles on the antenna pole of the fixed-height tripod must be shaded when plumbing is performed. Plumbing bubbles must be shaded for at least 3 minutes before checking and/or re-plumbing. The perpendicularity of the poles must be checked at the beginning of the project and any other time there is suspicion of a problem. The user should also ensure the antenna is properly seated in the mount.
Use a geodetic antenna with ground plane and/or choke ring. This helps reduce effects of local multipath.
Final processing shall consist of fixing all integers for each vector for all sessions except to some control sites. Users should be able to fix the integers over baselines that are less than 10 kilometers. If the integers cannot be fixed, there is probably something wrong with the data, such as bad multipath effects, missing data due to blockage, or interference. Baseline solutions with fixed integers prove to be more reliable, consistent and accurate.
Simultaneously observe baselines between neighboring stations. This helps to ensure that closely spaced stations (neighboring stations) will have the desired local accuracy and are the stations that most users will want to use to validate their classical leveling results.
Establish a high-accuracy 3-D fiducial network that encompasses the entire project. This network helps to detect and reduce the effects of remaining systematic errors in the local network observations. This also ensures that when two local networks are eventually connected, they will be consistent with each other. This is a very important aspect of establishing accurate GNSS-derived ellipsoid heights using the guidelines documented in NGS 58. The survey should be referenced to at least three existing Continuous Operating Reference Stations (CORS) [NOAA CORS or equivalent] near the project area. The survey should also consist of at least three control stations that are referenced to the three CORS and interspersed throughout the project. For these control stations, receivers should collect data continuously and simultaneously for at least three, 5-hour sessions on three different days at different times of the day during the project. As previously stated, NGS is reevaluating the guidelines and will update them based on the results of their research. Until NGS updates the guidelines, the user should continue to collect long data sets at these control stations, because they are extremely important to detecting potential errors in the stations established using short data observing sessions.
Evaluating the Quality of Published NAD 83 (2011) Ellipsoid Heights
A description of the National Adjustment of 2011 Project (Alignment of passive control with the latest realization of the North American Datum of 1983: NAD 83(2011/PA11/MA11) epoch 2010.00) is available online.
I’ve listed a few paragraphs (and highlighted a few statements) from the write-up that I believe are important to anyone using published NAD 83 (2011) ellipsoid heights as control stations.
As part of continuing efforts to improve the NSRS, on June 30, 2012, NGS completed the National Adjustment of 2011 Project. This project was a nationwide adjustment of NGS “passive” control (physical marks that can be occupied with survey equipment, such as brass disk bench marks) positioned using GNSS technology. The adjustment was constrained to current North American Datum of 1983 (NAD 83) latitude, longitude and ellipsoid heights of NGS Continuously Operating Reference Stations (CORS). The CORS network is an “active” control system consisting of permanently mounted GNSS antennas, and it is the geometric foundation of the NSRS. Constraining the adjustment to the CORS optimally aligned the GNSS passive control with the active control, providing a unified reference frame to serve the nation’s geometric positioning needs.
For the final constrained adjustments, the median network accuracy for all stations was 0.9 cm horizontal and 1.5 cm vertical (i.e., ellipsoid height) at the 95% confidence level. The median change in coordinates from the previous published values was about 2 cm horizontally and vertically. However, some station coordinates changed by more than 1 meter horizontally and 60 cm vertically. Although some of the large coordinate changes resulted from new data and adjustment strategies, most horizontal changes greater than about 6 cm occurred in geologically active areas and were likely due to tectonic motion.
Results of the 2011 national adjustment for 79,677 passive control marks are available on NGS Datasheets, including their network and local accuracies.Of these passive marks, 79,161 are referenced to the North America tectonic plate as the 2011 realization (including CONUS, Alaska and the Caribbean); 345 are referenced to the Pacific plate as the PA11 realization (the central Pacific, including Hawaii, American Samoa and the Marshall Islands); and 171 are referenced to the Mariana plate as the MA11 realization (the western Pacific, including Guam, Palau and the Commonwealth of the Northern Mariana Islands). Although the passive marks are referenced to three different tectonic plates, all refer to a common 2010.0 epoch date. With the completion of the national adjustment, all passive marks on NGS Datasheets with NAD 83(2011/PA11/MA11) epoch 2010.00 coordinates will be consistent with results obtained using CORS and the NGS Online Positioning User Service (OPUS). Note that 183 stations were excluded from the final national adjustments due to lack of enabled vector connections; where possible, these stations will be reconnected to the network in subsequent individual adjustments.
Other technical issues addressed in the project include:
1. appropriate down-weighting of the up component of GNSS vectors to account for subsidence in the northern Gulf Coast region of CONUS;
2. use of variable weighted (stochastic) constraints for CORS based on formal accuracy estimates derived from the NGS MYCS1;
3. scaling of GNSS vector error estimates for all projects to ensure consistent weighting of observations;
4. use of down-weighting (rather than removal) for vector rejections;
5. splitting the conterminous U.S. into a Primary and Secondary network, as mentioned above, such that vectors observed prior to about 1994 were assigned to the Secondary network. This allowed the Primary network to be adjusted separately without the problems associated with older observations (e.g., single frequency receivers, no antenna phase center models, poor orbit accuracy, incomplete satellite constellation, lack of CORS, etc.).
Each of these technical challenges (and others) was satisfactorily resolved, and completion of the National Adjustment of 2011 Project represents a significant step toward a more integrated, consistent, and accurate NSRS.
First, I’d like to commend NGS for performing the NAD 83 (2011) national adjustment; it was a great accomplishment by NGS. It provides users with a consistent, accurate set of geodetic coordinates (latitude, longitude and ellipsoid height) that should serve the nation’s positioning requirements for many years. Saying that, there are some issues that the user needs to consider when using published NAD 83 (2011) ellipsoid heights as constraints in GNSS network adjustments:
Generally, the NAD 83 (2011) network design was sufficient for determining accurate horizontal coordinates (latitude and longitude) but may not have been sufficient for establishing the vertical component (ellipsoid height) accurate enough for use as control stations in NGS Height Modernization Projects (see this webpage for more information on NGS’ Height Modernization Program) . Many of the earlier GNSS projects, prior to the publication of NGS 58, did not repeat baselines; stations were, however, usually occupied at least twice and observing sessions lasted for two hours or more. They were generally evaluated using loop closures and adjustment statistics, but loop analysis and adjustments do not always detect, reduce and/or eliminate all problems.
In addition, prior to NGS 58, not all closely spaced stations (neighboring stations) were simultaneously observed during the same session. In my opinion, the published formal errors may be too optimistic for some of these stations. These stations may be very precise but based on the survey field procedures performed prior to the publication of NGS 58, it is my opinion that the relative ellipsoid height accuracy for closely-spaced stations that were not simultaneously observed during the same session may not be as accurate as their listed median accuracy value.
Stations that were observed following the NGS 58 document are labeled as Height Modernization stations on the NGS datasheet and their ellipsoid height values should be good to the 2-cm level if they were involved in the same project.
It is important to understand the quality of published NAD 83 (2011) ellipsoid heights because your project’s GNSS-derived ellipsoid height values will be evaluated by them. The project’s control stations help to detect and reduce the effects of remaining systematic errors in the local network so they need to be very accurately determined.
Identifying good, valid published NAD 83 (2011) ellipsoid heights accurate enough to evaluate the results of a GNSS project isn’t an exact science, but there are ways to identify good candidates. I’ve listed three ways of using NGS published datasheets to help the user evaluate the quality of NAD 83 (2011) ellipsoid heights.
Identify stations that were established in Height Modernization Projects (that is, the stations were established following NGS 58 guidelines).
Analyze the network and local accuracy values to identify stations with accuracy values less than 2 cm.
Use local accuracy tables of stations to determine if closely spaced monuments (neighboring stations) were occupied during the same session.
The user can retrieve NGS datasheets in text form or as a shape file using NGS’ Datasheet retrieval program. Identifying stations involved in a NGS Height Modernization Project is simple because the datasheet adds a note stating that a particular station is a Height Modernization Survey Station. The user can assume these stations were determined following NGS 58 guidelines. An example of a station involved in a height modernization project is station CARGO, DJ5933 (see the datasheet below). The NGS datasheet also lists the station’s network and local accuracies. On the datasheet, the network accuracy value is listed below the coordinates (for instance, 1.39 cm for station CARGO). Below the network accuracy value, the user can obtain the local accuracy values by clicking on the following link in the datasheet: “Click here for local accuracies and other accuracy information.” You can obtain the full NGS datasheet for CARGO.
The NGS Data Sheet for Height Modernization Station CARGO (DJ5933) PROGRAM = datasheet95, VERSION = 8.71 National Geodetic Survey, Retrieval Date = JULY 12, 2015 DJ5933*********************************************************************** DJ5933 HT_MOD – This is a Height Modernization Survey Station. DJ5933 DESIGNATION – CARGO DJ5933 PID – DJ5933DJ5933 STATE/COUNTY- NC/NEW HANOVERDJ5933 COUNTRY – US DJ5933 USGS QUAD – WILMINGTON (1979)DJ5933DJ5933 *CURRENT SURVEY CONTROL DJ5933 ______________________________________________________________________ DJ5933* NAD 83(2011) POSITION- 34 12 27.89075(N) 077 57 16.40009(W) ADJUSTED DJ5933* NAD 83(2011) ELLIP HT- -34.732 (meters) (06/27/12) ADJUSTED DJ5933* NAD 83(2011) EPOCH – 2010.00 DJ5933* NAVD 88 ORTHO HEIGHT – 2.05 (meters) 6.7 (feet) GPS OBS DJ5933 ______________________________________________________________________ DJ5933 NAVD 88 orthometric height was determined with geoid model GEOID03 DJ5933 GEOID HEIGHT – -36.78 (meters) GEOID03DJ5933 GEOID HEIGHT – -36.80 (meters) GEOID12BDJ5933 NAD 83(2011) X – 1,101,934.174 (meters) COMPDJ5933 NAD 83(2011) Y – -5,164,049.037 (meters) COMPDJ5933 NAD 83(2011) Z – 3,565,508.167 (meters) COMPDJ5933 LAPLACE CORR – -5.30 (seconds) DEFLEC12B
DJ5933
DJ5933 Network accuracy estimates per FGDC Geospatial Positioning Accuracy
DJ5933 Standards:
DJ5933 FGDC (95% conf, cm) Standard deviation (cm) CorrNE
DJ5933 Click here for local accuracies and other accuracy information.
Local accuracies provided on the NGS datasheet can be used to determine if closely spaced stations were simultaneously observed during the same session. If two stations were simultaneously observed during the same session, they will have a local accuracy value listed in their table. Station TOWN CREEK (EA0883) is an example of a station that was simultaneously observed by BR 7 (EA0873) in one GNSS project and by LILIPUT (EA0875) in a different project. (Figure 1 depicts these stations and their NAD 83 (2011) network accuracy values.) Looking at the highlighted section of the tables below, station EA0883 is listed in the local accuracy tables for EA0873 and EA0875, so it was simultaneously observed during sessions with EA0873 and EA0875.
Saying that, we can also use the tables to show that EA0873 and EA0875 were not simultaneously observed during the same session. That is, EA0873 is not listed on EA0875 local accuracy table and EA0875 is not listed on EA0873 local accuracy table so they were not processed simultaneous in a session. Figure 2 depicts the two GNSS projects that include observations involving stations EA0873 and EA0875. The user can perform the same procedure to determine that stations EB0217 and EA0873, 8.3 km apart, were not simultaneously observed during the same session, and similarly EA0873 and EA0665, 7.5 km apart, were not simultaneously observed during the same project. Please note I am not suggesting that anything is wrong with these surveys; there may be good reasons why these stations were not simultaneously observed during the same project. I am only using it as an example in this column. Network and local accuracy values are good indicators of potentially “how good” a station is relative to its neighbor, but they should always be evaluated and investigated. My intent is to provide the user with tools for evaluating the quality of published NAD 83 (2011) ellipsoid heights. This is important because published coordinates are used to evaluate the adjustment results of new projects.
Local and Network Accuracy Data for NGS Datasheet – EA0873 Program lna_ret Version 2.7 Date April 6, 2015 National Geodetic Survey, Retrieval Date = JUNE 30, 2015 EA0873 ************************************************************ EA0873 ACCURACIES – Complete network and local accuracy information. EA0873 DESIGNATION – BR 7 EA0873 PID – EA0873 EA0873 EA0873 Horiz and Ellip are the horizontal and ellipsoid height accuracies EA0873 at the 95% confidence level per Federal Geographic Data Committee EA0873 Geospatial Positioning Accuracy Standards. SD_N, SD_E and SD_h are EA0873 the standard deviations (one sigma) of the coordinates (NETWORK) or EA0873 of the difference in the coordinates (LOCAL) in latitude, longitude EA0873 and ellipsoid height. CorrNE is the (unitless) correlation EA0873 coefficient between the latitude and longitude components of either EA0873 the coordinate (NETWORK) or coordinate difference (LOCAL). Dist is EA0873 the three-dimensional straight-line slope distance, in km, between EA0873 station EA0873 and the corresponding local station. Local stations EA0873 are stations processed simultaneously in a session regardless of EA0873 distance. EA0873EA0873 Accuracy and standard deviation values are given in cm.EA0873EA0873 Type/PID Horiz Ellip Dist(km) SD_N SD_E SD_h CorrNEEA0873 ——————————————————————-
Local and Network Accuracy Data for NGS Datasheets – EA0875 Program lna_ret Version 2.7 Date April 6, 2015National Geodetic Survey, Retrieval Date = JUNE 30, 2015 EA0875 ********************************************************** EA0875 ACCURACIES – Complete network and local accuracy information. EA0875 DESIGNATION – LILIPUT EA0875 PID – EA0875 EA0875 EA0875 Horiz and Ellip are the horizontal and ellipsoid height accuracies EA0875 at the 95% confidence level per Federal Geographic Data Committee EA0875 Geospatial Positioning Accuracy Standards. SD_N, SD_E and SD_h are EA0875 the standard deviations (one sigma) of the coordinates (NETWORK) or EA0875 of the difference in the coordinates (LOCAL) in latitude, longitude EA0875 and ellipsoid height. CorrNE is the (unitless) correlation EA0875 coefficient between the latitude and longitude components of either EA0875 the coordinate (NETWORK) or coordinate difference (LOCAL). Dist is EA0875 the three-dimensional straight-line slope distance, in km, between EA0875 station EA0875 and the corresponding local station. Local stations EA0875 are stations processed simultaneously in a session regardless ofEA0875 distance.EA0875EA0875 Accuracy and standard deviation values are given in cm.EA0875EA0875 Type/PID Horiz Ellip Dist(km) SD_N SD_E SD_h CorrNE
I haven’t discussed all procedures documented in NGS 58 here. There are other minor, but very important, procedures that the user must follow, such as use of precise ephemerides, taking a rubbing of the mark; the reader is referred to NOAA Technical Memorandum NOS NGS-58, “Guidelines for Establishing GPS-derived Ellipsoid Heights (Standards: 2 cm and 5 cm), Version 4.3,” for more details.
This column discussed procedures that need to be followed to detect, reduce and eliminate error sources to estimate accurate GNSS-derived ellipsoid heights. Analysis of the quality of project data should be based on repeatability of measurements, adjustment residuals and analysis of loop closures. Please be aware that repeatability and loop closures do not always disclose all problems, and that is why it is important to adhere to the procedures outlined in NGS’ publications.
It is important to understand geoid models when estimating GNSS-derived orthometric heights. The user should understand the differences between NGS’ scientific gravimetric geoid model and hybrid geoid models, and why it is important to use both types of geoid models in an analysis. As I mentioned in Part 1, the latest NGS hybrid geoid model, Geoid12B, is made consistent with the published NAVD 88 heights. This means you will be consistent with NAVD 88 when using GEOID12B to estimate GNSS-derived orthometric heights. However, this doesn’t guarantee that your GNSS-derived orthometric heights are accurate. NGS’ new Beta experimental geoid height model xGEOID14B is not distorted to fit the published NAVD 88 heights so it is useful for identifying valid NAVD 88 benchmarks. In my next column, I’ll address how to use these geoid models and published NAD 83 (2011) ellipsoid heights to evaluate potential issues with published NAVD 88 heights.
Figure 1. NAD 83 (2011) Ellipsoid Network Accuracies – units cm (Network accuracies were obtained from NGS datasheets).Figure 2. NAD 83 (2011) Network Design for Stations EA0873 and EA0875. [Note: GNSS Vectors for GNSS projects GPS 1588 and GPS 2057 were provided by NGS].
Editor’s Note: This month, we introduce a column by David B. Zilkoski, one of our two new Survey Scene editors. Zilkoski has worked in the fields of geodesy and surveying for more than 40 years, including serving as director of the National Geodetic Survey. See his full bio at the end of this article. He is joined by coeditor David Doyle, who contributed the May column.
The Three Types of Heights Involved in Computing GNSS-Derived Orthometric Heights
By David B. Zilkoski
David B. Zilkoski
This column is the first in a series of newsletters discussing issues associated with establishing orthometric heights using GNSS. The purpose of my columns is not to promote a particular procedure or process, but to provide the reader with information and analysis tools to consider when using GNSS to estimate orthometric heights.
This information is not new. During the past two decades, I have written several articles and papers on estimating GNSS-derived orthometric heights and presented numerous seminars describing guidelines on how to estimate GNSS-derived heights. However, due to the automation of technology and “blackbox” processes, many users are accepting results without performing the proper analysis to ensure that their results are reasonable and correct. These processes and procedures are not difficult to perform, but they can be very beneficial to obtaining an understanding of the accuracy of your results and ensuring your results are correct.
To understand how to estimate GNSS-derived orthometric heights at centimeter-level accuracy, you must have a basic understanding of the types of heights involved, how these heights are defined and related and how accurately these heights can be determined. In other words, you need to obtain a basic understanding of ellipsoid, geoid and orthometric heights and how they are related and their estimated accuracies.
To adequately address these topics, a series of Survey Scene newsletters will be separated into several sections. Some of this material will be a review (and probably boring) for those of you that have been performing GNSS-derived orthometric height surveys but, hopefully, you will gain a little benefit from the review. For those of you just starting out, I hope this will whet your appetite to obtain a better understanding of heights.
The following is a brief outline of what the columns will address:
Description of the three types of heights involved in computing GNSS-derived orthometric heights. That is, the definition of ellipsoid, geoid and orthometric heights, and how they are related. The user should understand what potential issues can arise due to how each height was defined, modeled and published. For example, in the United States, what errors exist in the published NAVD88 heights due to the leveling network design and remaining systematic errors in the leveling data? Constraining a North American Vertical Datum of 1988 (NAVD 88) published height that’s less accurate than your GNSS-derived orthometric height may allow your results to be consistent with the surrounding published heights, but could be distorting the rest of your results. In the end, you may need to do that, but you should know how your decision has influenced the rest of your results. I was the NAVD 88 project manager, so I know where all the problems are hidden. I am just kidding about knowing where all the problems are hidden, but there are issues associated with performing a nationwide network adjustment. NGS’ latest scientific geoid models can be useful in identifying potential issues in NAVD88.
Basic procedures for detecting published NAD 83 (2011) ellipsoid height outliers and how repeatability does not mean accuracy. Why you can’t assume that the published ellipsoid heights between two closely spaced stations is accurate to the published formal errors.
A description of the differences between a scientific gravimetric geoid model and a hybrid geoid model, and why it is important to use both geoid models in your analysis. The latest NGS hybrid geoid model, Geoid12B, is made consistent with the published NAVD 88 heights. This means you will be consistent with NAVD 88 when using GEOID12B to estimate GNSS-derived orthometric heights. However, this doesn’t guarantee that your GNSS-derived orthometric heights are accurate. NGS’s new beta experimental geoid height model xGEOID14B is not distorted to fit the published NAVD 88 heights, so it is useful for identifying valid NAVD 88 benchmarks.
Basic procedures for validating NAVD 88 height constraints used to estimate GNSS-derived orthometric heights. How to ensure your monuments haven’t moved since their last survey, and how good are your leveling-derived orthometric height constraints? Based on all available information and data, basic procedures to determine how good the final set of GNSS-derived orthometric heights really are. NGS 59 guidelines outline basic rules and procedures that need to be adhered to for computing accurate NAVD 88 GNSS-derived orthometric heights.
A description of NGS’ proposed 2022 Vertical Reference Frame and why it will be a good replacement for NAVD 88.
Background
Since 1983, NOAA’s National Geodetic Survey (NGS) has performed control survey projects in the United States using GPS satellites. NGS used these early GPS surveys projects to develop guidelines and procedures to estimate GPS-derived orthometric heights. These publications are known as NGS 58 and NGS 59.
Over the past three decades, GNSS surveying techniques have proven to be so efficient and accurate that they are now routinely used in place of classical line-of-sight surveying methods for establishing vertical control networks at the 2-cm level. Understandably, interest has been growing in using GNSS techniques to replace all leveling requirements. During the next decade, scientists will continue to develop better models and tools to facilitate GNSS-derived orthometric heights replacing classical line-of-sight surveying for many applications. In the meantime, it is important to have a clear understanding of the basic concepts of establishing GNSS-derived orthometric heights, otherwise water (or something worse) may not flow “down hill.”
Let’s start with a review of the three types of heights used when estimating GNSS-derived orthometric heights and how they are related.
Types of Heights and Their Relationship
Orthometric heights (H) are referenced to an equipotential reference surface, e.g., the geoid. The orthometric height of a point on the Earth’s surface is the distance from the geoidal reference surface to the point, measured along the plumb line normal to the geoid. These are the heights most surveyors have worked with in the past and are often called mean sea-level heights.
Ellipsoid heights (h) are referenced to a reference ellipsoid. The ellipsoid height of a point is the distance from the reference ellipsoid to the point, measured along the line that is normal to the ellipsoid. Years ago, the term ellipsoid height may have been a new concept to many traditional surveyors, but prevalent today because ellipsoid heights are readily derived from GNSS measurements.
At the same point on the surface of the Earth, the difference between an ellipsoid height and an orthometric height is defined as the geoid height (N). It should be noted that h=H+N is an approximate equation because H is measured along the plumb line normal to the geoid, where h is measured along a line normal to the ellipsoid (see Figure 1). For all practical survey projects, this small difference can be ignored.
Figure 1. Relationship of ellipsoid, geoid and orthometric heights.(Figure from POB article by David Zilkoski, The GPS Observer column, Feb. 28, 2001)
Several error sources that affect the accuracy of orthometric, ellipsoid and geoid height values are generally common to nearby points. Because these error sources are in common, the uncertainty of height differences between nearby points is significantly smaller than the uncertainty of the absolute heights of each point. This is the key to establishing accurate orthometric heights using GNSS.
Orthometric height differences (dH) can then be obtained from ellipsoid height differences (dh) by subtracting the geoid height differences (dN):
dH = dh – dN
Each of these heights and height differences have systematic errors that are accounted for by following appropriate procedures during data acquisition, by applying corrections based on environmental conditions and models, and/or estimating parameters using adjustment techniques. There will always be remaining errors that are not accounted for, and you must perform the appropriate procedures to detect, reduce or eliminate these errors in the final set of GNSS-derived orthometric heights.
Relative Accuracy Estimates
Adhering to NGS guidelines (NGS 58), ellipsoid height differences (dh) over short baselines (less than 10 km) can now be determined with 2 sigma uncertainties that are typically better than +/ 2 cm. The requirement that each baseline must be repeated and agree to within 2 cm of each other, and they must be repeated on two separate days, during different times of the day, should provide a final GNSS-derived ellipsoid height better than 2 cm at the 2-sigma level. The requirement that spacing between local network stations cannot exceed 10 km helps to keep the relative error in geoid height small.
Adding in the small error for the uncertainty of the geoid height difference and controlling the remaining systematic differences between the three height systems will produce a GNSS-derived orthometric height with 2-sigma uncertainties that are typically +/- 2 cm. Therefore, it is possible to establish GNSS-derived orthometric heights to meet certain standards, not millimeter standards, but 2-cm (95%) standards are routinely met now using GNSS.
When high-accuracy field procedures are used, orthometric height differences can be computed from measurements of precise geodetic leveling with an uncertainty of less than 1 cm over a 50 kilometer distance. Less accurate results are achieved when third-order leveling methods are employed. Depending on the accuracy requirements, GNSS surveys and present high-resolution geoid models can be employed as an alternative to classical leveling methods.
In the past, the primary limiting factor was the accuracy of estimating geoid height differences. With the computation of the more accurate National high-resolution geoid models, e.g., GEOID12A, the limiting factor is ensuring that the NAVD 88 orthometric height values used to control the project are valid. Strategically occupying benchmarks with GNSS that have valid NAVD 88 height values is critical to detecting, reducing or eliminating blunders and systematic errors between the three height systems. (Note: Valid NAVD 88 height values include, but are not limited to, the following: benchmarks that have not moved since their heights were last determined, were not misidentified, and are consistent with NAVD 88.)
Conclusion
This newsletter addressed the basic concepts of GPS-derived heights. To reiterate, it is important that you understand there are three types of heights involved with estimating GNSS-derived heights: ellipsoid, geoid and orthometric. Each of these heights has its own error sources that need to be detected, reduced or eliminated by following specific procedures or applying special models. This series of newsletter columns will address these potential errors sources and provide procedures to assist you in identifying these errors.
My next column in this series, coming in the August Survey Scene, will review guidelines for detecting, reducing or eliminating error sources in ellipsoid heights, and provide a brief discussion on using published NAD 83 (2011) ellipsoid heights in your analysis.
References
NOAA Technical Memorandum NOS NGS-58, Guidelines for Establishing GPS-derived Ellipsoidal Heights (Standards: 2 cm and 5 cm), Version 4.3.
NOAA Technical Memorandum NOS NGS-59, Guidelines for Establishing GPS-derived Orthometric Heights (Standards: 2 cm and 5 cm), are available. These guidelines address the establishment and densification of vertical control networks through the use of GPS surveys and valid NAVD 88 orthometric control.
David B. Zilkoski has worked in the fields of geodesy and surveying for more than 40 years. He was employed by National Geodetic Survey (NGS) from 1974 to 2009. He served as NGS director from October 2005 to January 2009. During his career with NGS, he conducted applied GPS research to evaluate and develop guidelines for using new technology to generate geospatial products. Based on instrument testing, he developed and verified new specifications and procedures to estimate classically derived, as well as GPS-derived, orthometric heights.
Now retired from government service, as a consultant he provides technical guidance on GNSS surveys; computes crustal movement rates using GPS and leveling data; and leads training sessions on guidelines for estimating GPS-derived heights, procedures for performing leveling network adjustments, the use of ArcGIS for analyses of adjustment data and results, and the proper procedures to follow when estimating crustal movement rates using geodetic leveling data.
Editor’s Note: This month, we introduce a column by David Doyle, one of our two new survey editors. Doyle brings to GPS World more than 40 years of experience as a geodesist and surveyor with the National Geodetic Survey — see his full bio at the end of this article. He will be joined by coeditor Dave Zilkoski, who will contribute the June column.
David Doyle
Since the mid-1980s, thousands of articles have appeared in peer reviewed journals, trade magazines and professional organization publications that describe the phenomenal capabilities of contemporary space-based positioning systems. The majority have been about various uses of the United States Global Positioning System (GPS) and increasingly include the potential for the inclusion of the Russian GLONASS, European Union Galileo and China’s BeiDou collectively referenced as Global Navigation Satellite Systems (GNSS).
Without meaning to understate the process, the ability for almost anyone, anywhere, at any time to determine a three-dimension position accurate to within a few centimeters is well established. I often comment that the systems are generally so easy to use that if you have the IQ of a squirrel you can obtain pretty good quality data. A feat that until recently was achievable only by the small community of geodesists and geodetic surveyors is now a near-trivial process for anybody who can make a modest investment in some form of positioning system device — and it’s getting better, faster, cheaper and more accurate all the time.
Our ability to collect, manage and display monumental amounts of positional data is also enhanced by the advances in Geographic Information Systems (GIS).
I have been privileged to be a part of this revolution since my initiation into the world of geodetic positioning in 1967, courtesy of the Selective Service System and the U.S. Army, and their use of geodetic triangulation combined with emerging artificial satellite systems such as SECOR (Sequential Collation of Range). These introductions to geodesy eventually led me to a position with the National Geodetic Survey and a career that spanned 40+ years.
During that time, we watched as the centuries-old method of triangulation was replaced by GPS, and as the prices of equipment plummeted with the integration of this technology into a multitude of public, private and academic disciplines — everything from geophysical sciences to weather prediction, precision agriculture, improved marine and aeronautical navigation. The list goes on and on and is well known to those who read this magazine.
So where is this going? What does the title of this article mean?
Doyle working on the Washington Monument.
If we accept, which we do, that all these things are true, then why is it that the world of sharing positional information is filled with scenarios that go something like this? “I got a cm and you got a cm, but our centimeters differ by a meter.” What this means is if these systems are so capable, then virtually all positional data integration should be a snap — everything should fit like a bespoke shirt. Unfortunately, that is often just not so.
Take the case of a decree issued by the U.S. Supreme Court in December 2014 delineating the offshore boundary between the United States and the state of California. The boundary is defined as a set of Universal Transverse Mercator (UTM) grid coordinates published to the nearest mm and referenced simultaneously to the North American Datum of 1983 (NAD 83) and the World Geodetic System 1984 (WGS 84), which the decree states are interchangeable. At the mm level, this is not true. In this area, they differ by approximately 1 m.
The decree provided no information on how these positions were derived, how accurate they really are, and who performed the computations — it certainly was not the Supreme Court. Without pointing fingers at the responsible agency, these all-too-common occurrences seems to be rampant among the many users of high-accuracy positional data, both horizontal and vertical. The crime is often the sin of omission.
The failure in many cases is a lack of knowledge on the part of many GNSS users of some of the basic principles of geodesy and geodetic surveying guidelines and providing complete metadata such as:
what geodetic datums and potentially which realization of those datums were referenced?
what are the units of measure?
how accurate are the positions/heights really?
It’s important to note that the number of digits to the right of a decimal point have nothing to do with accuracy. Land surveyors are taught from their first day on the job that they are following in the footsteps of the surveyor that went before them. It is not unusual for surveyors to struggle with incomplete information from previous surveys to be able to make accurate interpretations of what the original or other previous surveyors intended — a lack of complete metadata.
Today, the massive amount of coordinate and height information being generated by thousands of surveyors, engineers and other disciplines are those footsteps — albeit digital. The multitudes of high-quality data being collected around the world is only as good as the associated information about those values.
As we rapidly approach a time when there will be vastly improved GNSS constellations and very likely cm-level positioning available to millions if not billions of people in cheap handheld devices, the issues of professional education and attention to detail are more important than they have ever been. While it would be really nice if everyone who picked up a GNSS receiver had an advanced degree in geodesy, obviously that is not only unrealistic, it’s senseless. What does need to happen is a comment that I’ve made in hundreds of seminars on these topics — those in professions and disciplines where high-accuracy coordinates are important should know enough to qualify for the Junior Geodesist Secret Decoder Ring!
There are efforts in the works at this time that may bring us a step closer to making this a reality. The American Association for Geodetic Surveying (AAGS) is working on a geodetic surveying certification initiative in collaboration with the National Society of Professional Surveyors (NSPS). This effort will be aimed at anyone who is inclined to collect, manage, distribute and/or utilize the increasing amounts of high-quality positional information.
Watch this space for more details next time.
David Doyle joined the National Geodetic Survey in 1972, and held the position of chief geodetic surveyor for 12 years before his retirement in January 2013. He was responsible for the development, technical design and management of plans and programs that enhanced the United States National Spatial Reference System. During his career with NGS, his experiences included all phases of geodetic triangulation, astronomic positioning, leveling, GPS data collection, data analysis, datum transformations, network adjustments, data publication and outreach in the form of seminars, workshops and webinars. His efforts also included extensive activities to direct and coordinate the modernization of national geodetic reference frames in countries in Africa, Central, Caribbean and South America, Eastern Europe and the Pacific.
Doyle is a past president of the American Association for Geodetic Surveying and a Fellow member of the American Congress on Surveying and Mapping. He has served on the U.S. delegation to the International Federation of Surveyors and is an active member of the District of Columbia, Maryland and Virginia professional surveyors associations. Doyle now operates Base 9 Geodetic Consulting Services.
Image showing projected Florida flooding, from flood.firetree.net, using Google Earth with NASA data. Image from flood.firetree.net, using Google Earth.
Surveyors, prepare to get your feet wet. Global warming is about to hit you in the job list. By 2050, a majority of U.S. coastal areas are likely to be threatened by 30 or more days of flooding each year. This according to a December report in Earth’s Future, a journal of the American Geophysical Union.
[Parenthetically, the next issue of Survey Scene, in May, will be written by an actual geodesist. Until then, you have to put up with GPS World’s editor in chief — by no means a surveyor. Patience.]
The study used data from National Oceanic and Atmospheric Administration (NOAA) tide gauges to show the annual rate of coastal floods has accelerated in recent years. These are now five to 10 times more likely today than 50 years ago — and getting worse.
Mitigation decisions could range from retreating further inland to coastal fortification or to a combination of “green” infrastructure using both natural resources such as dunes and wetland, along with “gray” man-made infrastructure such as sea walls and redesigned storm water systems. And that’s not even mentioning such basics as redrawing property lines. Any way you look at it, surveyors are going to be involved.
“As communities across the country become increasingly vulnerable to water inundation and flooding, effective risk management is going to become more heavily reliant on environmental data and analysis,” said Holly Bamford, NOAA acting assistant secretary for conservation and management.
The recent U.S. Hydro 2015 conference in National Harbor, Maryland — an area particularly called out for vulnerability to the oncoming floods — naturally found a lot to talk about in this and related areas of interest for surveyors, with session tracks including: Effects of Climate Change on our Oceans and Waterways; Coastal and Ocean Mapping Initiatives; Advances in Unmanned System Technology, and several more.
Some of the papers presented that GPS World found of interest, and hopes to present or encapsulate in some form in the near future, include:
Resolving Systematic GPS Interference from Aeronautical Distance Measuring Equipment during Mission-Critical Shallow Water Multibeam Surveys
GPS Water-Level Buoy for Hydropgraphic Survey Operations
Examining the Uncertainty Associated with the Establishmenbt of an Ellipsoid to Chart Datum Separation Surface Using GNSS Buoys
Comparison of Horizontal and Vertical Resolvable Resolution between Repetitive Multibeam Surveys Using Different Kinematic GNSS Methods.
And those just came from the poster sessions. In the technical sessions, Jack Riley from the NOAA Coast Survey’s Hydrographic Systems and Technology Program presented a GPS Buoy Water Level Uncertainty Case Study.
Data from on High
Since you can’t get at a coastline from all angles — with any degree of stability, that is — data from overhead, sometimes far overhead, proves invaluable. Such as that provided by aerial digital imagery, LiDAR, and increasingly, satellites.
Because digital aerial images are already in electronic form, they can quickly be processed and made available to users. Most of the special cameras in use nowadays provide direct georeferencing capability, which allows camera position and orientation to be determined automatically using GPS and inertial measurement equipment. An entire mini-industry has grown up around integrating aerial data with that taken from ground surveys.
Light detection and ranging (LiDAR), a remote sensing system, became available for commercial topographic mapping in 1993. An airborne laser scanning system paired with a kinematic GPS receiver and an inertial navigation system can calculate and produce a highly accurate spot elevation. It is possible to obtain point densities that would likely take months to collect using traditional ground survey methods. The National Geodetic Survey (NGS) is currently implementing LiDAR into their shoreline mapping production process.
Our Record So Far
Coverage of these salty issues has been sparse in GPS World and associated newsletters, but not entirely absent. In 2006, the May issue featured “GPS Buoys Nautical Measurement.”
In 2008, Richard Langley edited an Innovation column on “Tsunami Detection by GPS,” featuring work for which co-author Attila Komjathy eventually won a GPS World Leadership Award in 2013. And in 2010, Langley brought forth an Innovation column on “Monitoring Water Level with GNSS.”
And way, way back in 2005, we published “Abreast of the Waves: Open-Sea Sensor to Measure Height and Direction.” This was prior to our digital era, so until we can scan a paper copy into here, we’ll simply give the abstract: “Accurate and timely information on open-sea wave conditions can help in preventing large-scale maritime disasters. This article describes a new, low-cost Global Positioning System (GPS)-based sensor that measures wave height with an accuracy of several centimeters and direction with an accuracy of 5 degrees. The receiver is mounted on a buoy, and a high-pass filter is used to extract the movement of the buoy and thus minimize GPS positioning errors. The data provided by the sensor is intended to improve wave prediction models. In addition, since this GPS-based sensor transmits only analyzed ocean wave data, it reduces the volume of data and leads to lower operating and acquisition costs. The article describes the concept of the GPS-based wave sensor, algorithms that are used for filtering and extracting wave data, as well as the results of open-sea trials.”
So there’s more to come. Watch this space. In the meantime, we leave you with Bob Dylan’s prophetic words, circa 1967.
Well, it’s sugar for sugar And salt for salt If you go down in the flood It’s gonna be your own fault.
You may have heard about “NMEA data” with respect to GPS.
NMEA is an acronym for the National Marine Electronics Association. NMEA existed well before GPS was invented. According to the NMEA website, the association was formed in 1957 by a group of electronic dealers to create better communications with manufacturers. Today in the world of GPS, NMEA is a standard data format supported by all GPS manufacturers, much like ASCII is the standard for digital computer characters in the computer world.
The purpose of NMEA is to give equipment users the ability to mix and match hardware and software. NMEA-formatted GPS data also makes life easier for software developers to write software for a wide variety of GPS receivers instead of having to write a custom interface for each GPS receiver. For example, VisualGPS software (free), accepts NMEA-formatted data from any GPS receiver and graphically displays it. Without a standard such as NMEA, it would be time-consuming and expensive to write and maintain such software.
What makes NMEA a bit confusing is that there are quite a few “NMEA” messages, not just one. So, just like there are all kinds of GPS receivers with different capabilities, there are many different types of NMEA messages with different capabilities. Furthermore, NMEA data can be transmitted via different types of communications interfaces such as RS-232, USB, Bluetooth, Wi-Fi, UHF and many others.
NMEA Message Structure
To understand the NMEA message structure, let’s examine the popular $GPGGA message. This particular message was output from an RTK GPS receiver:
13 denotes number of satellites used in the coordinate
1.0 denotes the HDOP (horizontal dilution of precision)
495.144 denotes altitude of the antenna
M denotes units of altitude (eg. meters or feet)
29.200 denotes the geoidal separation (subtract this from the altitude of the antenna to arrive at the Height Above Ellipsoid (HAE).
M denotes the units used by the geoidal separation
1.0 denotes the age of the correction (if any)
0000 denotes the correction station ID (if any)
*40 denotes the checksum
The $GPGGA is a basic GPS NMEA message. There are alternative and companion NMEA messages that provide similar or additional information.
Here are a couple of popular NMEA messages similar to the $GPGGA message with GPS coordinates in them (these can possibly be used as an alternative to the $GPGGA message):
In addition to NMEA messages that contain a GPS coordinate, several companion NMEA messages offer additional information besides the GPS coordinate. Following are some of the common ones:
$GPGSA – Detailed GPS DOP and detailed satellite tracking information (eg. individual satellite numbers). $GNGSA for GNSS receivers.
$GPGSV – Detailed GPS satellite information such as azimuth and elevation of each satellite being tracked. $GNGSV for GNSS receivers.
$GPGST – Estimated horizontal and vertical precision. $GNGST for GNSS receivers.
Rarely does the $GPGGA message have enough information by itself. For example, the following screen requires: $GPGGA, $GPGSA, $GPGSV.
VisualGPSView screenshot. (Photo: VisualGPC LLC)
The following screen, focused on the time capabilities of GPS, requires a slightly different set of NMEA messages: $GPGGA or $GPRMC or $GPZDA, $GPGSA, $GPGSV.
NMEATime. (Photo: VisualGPC LLC)
The above screenshot examples are useful for the general GPS user. The $GPGST message is particularly useful for high-precision GPS mapping and surveying. In fact, I would say it’s a requirement for high-precision users. The reason is that GPS metadata is very important for the high-precision user as a method of assisting in determining the quality of a particular GPS coordinate. Typical GPS real-time metadata used in understanding the quality of the GPS coordinate include: PDOP, number of satellites tracked, correction method and horizontal/vertical standard deviation values. If a GPS receiver user has the ability to see this information in the field during data collection, they have a level of confidence in the precision of the GPS data they are collecting. If you’ve used RTK before, you probably recall the familiar horizontal RMS (HRMS) and vertical RMS (VRMS) values displayed on your data collection device. The $GPGST message generates those values.
DD.MMMMMMM, DDMM.MMMMM, or DDMMSS.SSSSS
One of the challenges in dealing with raw NMEA data (data not using a software like VisualGPS to decode it for you) is the format of the GPS coordinate. It’s not user-friendly. It’s expressed in DDMM.MMMMM; degrees, minutes and decimal minutes. To display the coordinate in a different format, there’s a useful Excel spreadsheet published by the UK Ordnance Survey.
UK Ordnance Survey Coordinate Calculator
To use the spreadsheet, simply enter the GPS coordinate in the format you have, and the spreadsheet will calculate and display the GPS coordinate in the other two formats.
Click here to download the UK Ordnance Survey Excel spreadsheet coordinate calculator.
First of all, on behalf of all of us here at GPS World magazine, allow me to welcome you to 2015. We wish you a healthy and prosperous new year!
I’d like to start out the new year stating the obvious for some of you, maybe most of you…perhaps all of you: GNSS is the new GPS.
In the high-precision GNSS community, I think this is already our mindset, and has been for quite some time. The benefit of using signals from as many satellite navigation systems to the high-precision user is obvious. We saw this with the adoption of GLONASS more than a decade ago. It’s to the point now that even many consumer receivers (such as my Samsung Galaxy S5) utilize both GPS and GLONASS satellites.
I think it’s pretty obvious we’ll see the same phenomenon with Galileo (Europe) and BDS (China’s BeiDou system). It’s exciting to think about what high-precision GNSS positioning is going to look like just 2-3 years from now. Think about how much better RTK positioning will be with 30+ satellites in view. By the way, that’s already a reality in China where BDS has 14 regional satellites in addition to GPS and GLONASS. It’s the best place in the world for RTK positioning due to the number of satellites in view at any one time, and it might be the reason that China consumes more RTK receivers than the rest of the world combined.
BDS coverage area.
BDS satellite orbit map.
I certainly look forward to the deployment of Galileo and BDS. It will only make us more productive in accomplishing our work. Yet I’m reminded frequently when reading mainstream news headlines that Galileo, BDS, and GLONASS compete with GPS. Even some of those who hold GPS dear to their hearts, such as those who were involved in the development, promotion and deployment of GPS, view the other satellite systems as competition.
Maybe that’s not a bad thing because competitors push each other to perform better. However, where it might hurt is when it comes to support, such as funding. Galileo, in particular, because it’s funded with civil funds instead of defense funds like GPS and GLONASS, has been criticized as a wasteful use of resources because GPS already exists. What more can it add, they ask? The mainstream media doesn’t have a clue that the satellite navigation systems are complementary rather than competitive. You and I know that more satellites generally equates to increased productivity no matter who owns/operates the satellite that is sending the signal. I cringe when I read these headlines:
News Headlines
GPS and its Three Main Competitors: Galileo, Beidou, GLONASS
GPS vs. Galileo; Where Are They Headed?
China Spreads Alternative To U.S. GPS System
China’s Beidou Navigation Satellite System More Precise than GPS in Certain Areas
GPS vs GLONASS: Which Is Best for Tracking Applications?
Generally, I dismiss the mainstream media in the GNSS arena, but these misleading articles can have an impact on funding of the various GNSS, such as Galileo. Politicians and various purse-string holders can be influenced by these stories.
Galileo Moving Forward
The Europeans are pushing forward after the recent hiccup when the first two Full Operation Capability (FOC) Galileo satellites were inserted in the incorrect orbits due to an improper fuel line installation on the rocket launcher resulting in the satellites being inserted in an orbit far below its intended orbit (an elliptical orbit, 49.8 degrees at 26,200 km, vs. the intended circular orbit, 55 degrees at 29,900 km).
In October 2014, shortly after the faulty launch, the outlook for the two satellites was bleak. The consensus was that there was no feasible method to move the satellites to their intended orbits. The good news was that besides the fact that they were in New York instead of Los Angeles :-), they checked out healthy, were properly oriented to the sun, and were “thermally stable.” Would they join GPS SVN-49 in being demoted to permanent test mode status, never being allowed to join the operational constellation, further delaying the deployment of Galileo? Not so.
The first live test was completed on December 9, 2014, when the satellite was one of four Galileo satellites that delivered a position fix of better than two meters. Furthermore, in a January 1 article published on GPS World’s website, Peter Steigenberger and André Hauschild of the German Aerospace Center wrote that the rogue Galileo FOC satellites can likely be used by commercial, multi-frequency, high-precision GNSS receivers for carrier-phase positioning. One drawback is that because the satellite’s orbit doesn’t fall within the limits of the standard Galileo almanac, it may take receivers longer to begin tracking the satellite.
Flight engineers are now working on maneuvering the second rogue Galileo satellite in the same manner, hoping for the same result.
All in all, this is about as good of a result that could possibly be expected. My hat’s off to the folks who made this happen.
Meanwhile, the next four Galileo FOC satellites are moving through the production process. Originally slated for a December launch, I suspect last year’s launch anomaly had the Galileo folks double-triple-quadruple checking, dotting i’s and crossing T’s, so make sure the next launch has the best chance of success. They haven’t announced a new launch schedule yet, but I would guess it’s likely in the next six months, with quarterly launches resuming if things goes smoothly. If all goes well, we could be benefiting from 10 healthy Galileo satellites by the end of the year.
A little more than a year ago, Swift Navigation started a Kickstarter campaign for a $995 RTK GNSS receiver named Piksi. The goal was to raise $14,000 for the project. By the time the Kickstarter campaign was ended, Swift raised $166,097 from 303 backers.
I wrote an article about the Swift’s Kickstarter campaign in September 2013, a few days before the fundraising period ended. Following is a two-minute Kickstarter promotional video describing Piksi.
At the time, the $995 price for the Piksi raised a lot of eyebrows and generated conversations in the high-precision GNSS user community about inexpensive RTK technology.
This week, I took the opportunity to catch up with Swift to see how the project is going and where the company is headed.
I spoke with Tim Harris, CEO of Swift, and Fergus Noble, chief technology officer. Tim said they delivered most systems to their backers in April (2014) as well as issuing “very few” refunds, which is not unusual for Kickstarter projects. As of today, Tim said there are about 1,000 units in the field among 350+ users. He said they still consider the Piksi to be in beta testing phase and expect production units to start shipping next year.
“We are in RTK reliability mode now” — Fergus Noble
Fergus said that the hardware design is stable and hasn’t changed since they shipped Piksi last spring. He said the focus has been on refining the RTK firmware to make it robust and reliable for professional use. He admits that “covering all corners” has been a challenge and, according to Swift’s blog, “The RTK software has been a tougher nut to crack than we originally anticipated.” This is especially ambitious, since Swift said they developed the RTK firmware based on published academic technical papers, combined with their own techniques (preparing patent applications) to run on a low-power hardware platform. Basically, they started from scratch, and without any RTK veterans on their team. You gotta love their guts.
Apparently, the venture capital world liked what they saw. A few months ago, Swift, based in San Francisco, secured $2.6m in seed funding from Fall Line Capital, Felicis Ventures, Kal Vepuri, Lemnos Labs, Qualcomm Ventures and VegasTechFund. With the additional funding, Swift has increased its headcount from 4 to 9.5.
The Devil Is in the Details
Swift Navigation Lodestar. Photo: Swift Navigation
Start-ups like these are a cool story, but at some point the rubber has to hit the road. Piksi is a long way from being an RTK product that you and I use for GIS and surveying, and Tim/Fergus are humble enough to admit this. The founders see the Unmanned Aerial Systems (UAS) market as their key customer base due to Piksi’s micro-size and low cost. They say this is reflected in their current customer base, which is comprised largely of UAS users, then surveying and agriculture, with the remaining 20 percent being a variety of apps like augmented reality. I agree that UAS is the right segment for them to focus on. At this point, the limitations for general surveying and GIS use are too great:
Must use a Piksi RTK base due to the proprietary protocol. In other words, not compatible with existing RTK base stations or networks.
Baseline length (distance between the RTK base and rover) must be very short. They haven’t done much testing even with a one-mile baseline.
RTK initialization is spec’d at 15 minutes, although Fergus said it’s typically 7-10 minutes.
Since Piksi is only a single-frequency receiver (L1), it has the same limitations as other L1 RTK systems we’ve seen marketed over the past few years — short baselines and long initialization times compared to dual-frequency receivers. While these limitations are significant for surveying and GIS users, they aren’t as significant for UAS users, since UAS users require line of sight to the aircraft at all times and UAS are operated in a clear-sky environment.
Communications
If you’ve used RTK, you are aware of the importance of data communications between the base and rover. As part of its kit, Swift supplies license-free, 915MHz frequency-hopping radios to facilitate communications between the base and rover for testing. However, the founderse make it clear that they aren’t in the data radio business. They expect that most Piksi users will incorporate their own communications technology. Swift is discussing incorporating GSM (mobile phone) modem capabilities in the future.
Future
In 2015, Swift plans to roll out the Piksi in production quantities, so Fergus said their focus is on creating a robust and professional RTK system. Of course, the burning question is if/when Swift will incorporate a second GPS frequency into its receiver to take advantage of the faster RTK initialization and longer baseline distances that a dual-frequency receiver offers. Swift isn’t willing to talk about that right now. Said Tim, “We’re holding our plans close to our chest.”
The 2014 Trimble Dimensions User Conference is being held in Las Vegas this week. Photo: Trimble
With more than 4,000 attendees, this year’s Trimble Dimensions User Conference was the largest ever and, I must say, a well-organized event chock full of technical content — enough to squelch the most intense geospatial hunger pangs you might have.
One could write a book on all the technology and market segments that Trimble is pursuing and offering solutions for. In addition to a wide range of GNSS, geospatial, construction, control, and data management systems previously offered, Trimble boasted a USB stick full of press releases with new product and service announced at Dimensions. So, the challenge is deciding what to write about without writing a little bit about everything.
After my first day at Dimensions, it became clear to me what I needed to do. Among the many product and service announcements was a new GNSS correction service named Viewpoint RTX. While I’ve tried to stay up to speed on Trimble’s various GNSS real-time correction services, this one was the straw that broke the camel’s back for me. I decided I needed to get a solid grip on the range of real-time GNSS correction services that Trimble offers because the picture was getting fuzzier, at least to me, with each new real-time correction service introduced. It used to be pretty simple to decipher; not so much any longer. So I had a conversation with Patty Boothe, general manager of Positioning Services at Trimble. Patty, a 15-year Trimble veteran, was appointed GM of the newly formed group three years ago. Here’s the low-down on the services.
Remember, Trimble acquired the land portion of OmniSTAR’s business a few years ago. For years, OmniSTAR has been one of the two dominant commercial satellite-based, real-time GNSS correction services (the other being John Deere’s Starfire service, as well as new entrant Terrastar). The OmniSTAR acquisition was Trimble’s entry into the satellite-based, real-time GNSS correction services business. Since then, Trimble has introduced the RTX (not to be confused with RTK) range of GNSS correction services. You might say that OmniSTAR and RTX are competitive services within Trimble. They are, to a certain extent, and I’ll attempt to clarify that below.
Following is a list of Trimble’s real-time GNSS correction services, starting with the OmniSTAR services:
OmniSTAR VBS: Satellite-based, real-time submeter service. The VBS service has been made obsolete largely by free public satellite-based augmentation systems (SBAS) such as WAAS/EGNOS/MSAS/GAGAN/SDCM. It is still used in geographic regions where free public SBAS don’t exist, primarily South America, Central and Southern Africa, and Australia. GPS-only service. Requires single-frequency receiver (L1).
OmniSTAR XP: Satellite-based, real-time 15-cm service based on Jet Propulsion Lab (JPL) technology and delivered to users on the ground via OmniSTAR’s geosynchronous satellite network. GPS-only service. Requires dual frequency (L1 and L2).
OmniSTAR HP: Satellite-based, real-time 10-cm service based on OmniSTAR’s reference station network and delivered to users on the ground via OmniSTAR’s geosynchronous satellite network. GPS-only service. Requires dual frequency (L1 and L2).
OmniSTAR G2: Satellite-based, real-time 10-cm service based on Jet Propulsion Lab (JPL) technology and delivered to users on the ground via OmniSTAR’s geosynchronous satellite network. GPS+GLONASS service. Requires dual frequency, dual constellation (L1 and L2).
To use OmniSTAR services, one must have an OmniSTAR-enabled GNSS receiver. There are a several receiver manufacturers that support OmniSTAR GNSS correction services, such as NovAtel and Hemisphere GNSS, in addition to Trimble.
After, or at nearly the same time, Trimble acquired OmniSTAR, the company launched its RTX GNSS correction service. RTX’s infrastructure consists of ~110 GNSS reference stations around the world working to create high-precision corrections on a near global scale. The first significant differentiator is that Trimble RTX services are only offered on Trimble GNSS receivers, so you’ve got to be “all in” with Trimble to utilize RTX.
Viewpoint RTX: Internet-based (notice I didn’t write satellite-based), real-time submeter service. This is a new service introduced this week at Dimensions for the new Leap GNSS receiver and the Geo7 GNSS handheld. GPS+GLONASS service. Requires single-frequency receiver (L1).
The above are the three RTX services. There are some options for the above, but let’s talk about satellite-based GNSS correction services for a minute.
The advantage of satellite correction services is that, because GNSS corrections are delivered via satellite, your receiver doesn’t need to be connected to the Internet or have any other sort of terrestrial radio communications to receive data from the GNSS reference station(s). Because delivery is by satellite, you could be in the middle of a desert with no mobile phone coverage within 100 km, and you could still use OmniSTAR or RTX services. The only requirement is that your receiver needs to have direct, continuous line-of-sight to the OmniSTAR/RTX geosynchronous satellite (both services use the same geosynchronous satellites to broadcast the corrections).
The primary disadvantage of OmniStar and RTX services is the “convergence” time required to achieve the stated accuracy service levels. With the exception of OmniSTAR VBS (sub-meter), Viewpoint RTX (sub-meter) and Rangepoint RTX (50-cm) services, the OmniSTAR and RTX centimeter and decimeter services require tens of minutes of initialization time to converge to the stated accuracy. For example, if you want to use the 4-cm Centerpoint RTX service, you may have wait up to 30 minutes for it to converge to 4-cm accuracy.
Now, there are a couple of ways to reduce the convergence time:
Start on a known point. For example, if you’re using Centerpoint RTX on a tractor for planting and you shut down for the evening, you can start it up the next morning (assuming you didn’t move the tractor), and it will converge nearly immediately.
Trimble offers a fast convergence option ($) in some geographic areas where it augments RTX with local RTK reference stations. Currently, Trimble offers this service in five U.S. “corn belt” states.
For OmniStar XP, HP and G2 services, the only way to reduce convergence time is number one above, start on a known point.
It’s important to note that all of the centimeter and decimenter satellite-based services described above are based on real-time Precise Point Positioning (PPP) technology, which is different than RTK technology. The fundamental difference is that real-time PPP technology relies on a global, distributed network of reference stations. For example, Trimble has ~110 reference stations to cover the globe (mostly) with its RTX service. On the other hand, RTK requires a much more dense network of GNSS reference stations. For example, in Washington State there are ~100 GNSS reference stations that comprise the state-wide RTK network.
Lastly, Trimble offers a hybird RTK/RTX service called XFill. The idea is that for RTK users who lose communications to their RTK base or RTK network can use the Centerpoint RTX as a “seamless” back-up, maintaining RTK-level accuracy (1-2cm) for the first five minutes of RTX service, and then degrading to Centerpoint RTX accuracy after 20 minutes. Trimble reports there is no convergence time when transitioning from RTK to RTX, like you would if you were starting RTX right away. Standard XFill is included with certain Trimble RTK receivers and allows up to five minutes of RTX satellite time. Last month at the INTERGEO conference, Trimble introduced Expanded XFill which is a subscription service for those users who want more than five minutes of RTX time. For those users, Patty said that users can buy blocks of RTX time starting at 10 hours.
So, you might ask how Trimble handles the horizontal datum differences between RTK and RTX since they are likely not referenced to the same horizontal datum. For example, in the US, Trimble VRS RTK infrastructure is typically referenced to NAD83/2011 while Trimble RTX is referenced to ITRF08. There’s about 1 meter difference between the two. After finding the correct Trimble person, he said that Trimble does a 3-parameter local shift (dX, dY, dZ) on the fly when in RTK mode so that when there’s a transition from RTK to RTX, the horizontal datum difference is already resolved.
A by-product of Trimble’s ~110 global GNSS reference station network is a real-time, world-wide TEC (Total Electron Content) map. Since real-time PPP GNSS correction services (and public SBAS like WAAS/EGNOS/MSAS/GAGAN) rely on accurate models of the TEC in the ionosphere to account for the GNSS measurement delay, real-time TEC maps give users an indication of how the ionosphere’s TEC is behaving. This sort of map is particularly useful in attempting to predict the understand single frequency receivers using services such as public SBAS, OmniStar VBS, and Viewpoint RTX. The next time you here about an impending solar storm, take a look a the map using this link and see the TEC hotspots around the globe. Notice the more intense activity near the geomagnetic equator.
TEC map from Trimble’s ~110 global GNSS receivers. Photo: Trimble
Shifting gears slightly, at the conference, Trimble also introduced a new mobile phone GNSS add-in product called Leap, which uses the Viewpoint RTX service.
Trimble Leap GNSS Receiver with a Samsung Galaxy Phone. Photo: Trimble
In September, I attended the Institute of Navigation (ION) GNSS+ conference in Tampa, Florida.
Downtown Tampa, location of the 2014 ION GNSS+. Photo: GPS World
The ION GNSS+ conference is a gathering where many of the GNSS scientists from around the world come to share their successes, trials and tribulations. It gives one a view into the future of where GNSS positioning might go. Granted, most of the ideas and concepts presented won’t ever be introduced in a commercial product, but it’s great to see that engineers are pushing the technology envelope to see how much they can squeeze from receivers.
As I was perusing the ION GNSS+ conference agenda, I was looking for presentations and other subject matter relevant to RTK GNSS technology. (Yes, I’ve been obsessed with low-cost RTK receivers this past year, if you haven’t been following).
I’d like to tell you about two presentations I attended. The first was sort of unexpected, and the second was every bit of what I hoped it would be.
The first was a presentation by SubCarrier Systems Corp (SCSC), a small consultancy focused on ITS (Intelligent Transportation Systems) technology. It just so happens, according to David Kelley of SCSC, that RTK receivers and RTK networks will play a critical role in the future of ITS and, as a result, help drive down the cost of RTK technology.
How is RTK relevant to ITS?
In ITS, I’ve been told there are three levels of accuracy that drive particular ITS applications. The accuracy terms are expressed in transportation terms:
Which Road?, Which Lane? and Where in the Lane?
Translated into GPS accuracy terms:
Which Road? = Autonomous GPS — 5-meter accuracy
Which Lane? = WAAS (or SBAS)-corrected GPS — 1-meter accuracy
Where in the Lane? = RTK — 2-cm accuracy
Mr. Kelley further presented that transportation applications of RTK technology will drive mass-market adoption (commoditization) of RTK technology and into the millions of units sold.
Lastly, he discussed the strain that such massive deployment of RTK technology in transportation might place on existing RTK networks run by state agencies.
To view the entire presentation from Mr. Kelley, you can click here.
The second RTK-centric presentation I attended at the conference was a moderated discussion panel entitled “High-Precision GNSS — What Will It Look Like in 2020?”
Discussion Panel Members: High Precision GNSS – What will it Look Like in 2020? Photo: GPS World
The discussion panel members were (from right to left):
Gian Gherardo Calini – European GNSS Agency
Ivan Di Federico, Chief Strategy Office and EVP, Topcon Positioning
Bernhard Richter, GNSS Business Director, Leica Geosystems, Switzerland
Elmar H. Lenz, General Manager – Geospatial GNSS, Geospatial Division, Trimble Navigation Ltd.
Jan Van Hees, Director of Business Development, Altus Positioning Systems
Shaowei Han, Co-founder and CEO/President, Wuhan Navigation and LBS, Inc., China
The discussion began with a short presentation by Gavin Schrock, who, among other things, administers the Washington State Reference Network, a state-wide RTK network, to frame the discussion.
Next, each panel member commented on the presentation and provided some of their own thoughts. The thoughts by the mainstream manufacturers were largely what you’d expect, since they do not look forward to the day that RTK technology becomes a commodity.
I’ll cut to the chase and just say that the gentleman from China, Dr. Han, stunned the audience with his claim that RTK GNSS chips will eventually be sold for $20 each. OK, to be fair, he also said RTK GNSS modules (an RTK GNSS chip on a circuit board with supporting components) will sell for $100. At first, these numbers seemed somewhat shocking to the audience, and one might dismiss it as being a speculative pipe-dream to disrupt the current RTK receiver competitive landscape. But then, when questioned, he dropped the reality bomb with a sort of puzzling look at the audience, being a little surprised why they didn’t understand. He said, and I’m paraphrasing, that $100 for an RTK module in 2020 doesn’t seem to be a stretch at all if you consider that RTK GNSS modules in China are selling for only $400 today. BOOM! He dropped the hammer. I admit, the $400 number even surprised me a bit. I thought it was more like $800.
The reason for the low price is the number of RTK GNSS receivers sold in China is more than 100,000 per year now, and growing. That’s more than the rest of the world combined. What’s driving the demand for RTK GNSS receivers? You guessed it — transportation. While the mainstream RTK GNSS manufacturers are still talking about RTK GNSS technology for niche markets like surveying, engineering, GIS, construction, and agriculture, Dr. Han was talking about RTK GNSS technology being used by everyday consumers for everyday activities. He’s talking about the commoditization of RTK GNSS, and he’s right. The only question that remains is how soon it will arrive.
