My previous column provided an update on the current set of published orthometric heights in the southeast Texas region and rules by the National Geodetic Survey (NGS) for estimating and publishing GNSS-derived orthometric heights using OPUS Projects. It also highlighted my personal crusade, that is the United States geodesy crisis. The Geodesy Crisis white paper can be downloaded from the American Association for Geodetic Surveying website.
This column focuses on potential errors in orthometric heights using a digital barcode leveling system with multi-piece leveling rods. Every business makes decisions based on expenses and ultimately on the profit margin. That said, making a business decision that results in a bad technical outcome may lead to issues that cost the company more than expected.
I have been involved with establishing orthometric heights, both leveling-derived heights and GNSS-derived heights, for most of my career. On a personal note, the digital bar code leveling system is important to me because I was the lead author of the document by the Federal Geodetic Control Subcommittee (FGCS) to incorporate the digital barcode leveling systems “Specifications and Procedures to Incorporate Electronic/Digital Barcode Leveling Systems (2004).” Recently, a colleague brought to my attention that many surveyors are using the digital barcode leveling system (which wasn’t a surprise to me), but they are not using the one-piece, single-scale, invar rod. They are using the multi-piece rods, either fiberglass or wooden. Surveyors can use any type of instrument to perform their project, but it will not meet the Federal Geodetic Control Subcommittee specification and procedures for leveling unless they use a one-piece leveling staff. This not a new requirement; it has been a requirement since the first publication of the FGCS specifications and procedures. Surveyors have always requested to use multi-piece rods but the potential errors associated with them were considered too large to be incorporated into the specifications and procedures to meet first-, second-, or third-order U.S. federal accuracy standards.
In 2018, NGS documented a study to evaluate the use of multi-piece rods using the digital barcode leveling system. I first saw a draft of this report in 2011 and forgot that it existed. A colleague of mine recently provided me with the 2018 report. In my opinion, anyone who uses the digital barcode system and multi-piece rods should read this report and, of course, the rest of this column.
The image below provides the important summaries of the tests. I highlighted the statement “In the field, these errors were as high as 1.5 mm per setup and up to 7 mm for the entire 180-meter section.” In my opinion, an error of 7 mm in a 180-meter section is too large for any order and class of leveling. Also, the results of the study support the current FGCS specifications and procedures requirement of the use of one-piece, calibrated rods.
The 2018 report states that multi-piece level staffs are popular among the surveying community because of their ability to break down into an easily transportable unit and they are relatively inexpensive and readily available. They may also be extended to reduce the number of leveling setups required over sloping terrain. This makes good business sense but surveyors should not make technical decisions based solely on business costs. That said, the FGCS “Specifications and Procedures to Incorporate Electronic/Digital Barcode Leveling Systems (2004)” prohibit the use of multi-piece rods for any order/class of leveling based on technical decisions, not on business expenses. To evaluate the multi-piece barcode rods, NGS developed and implemented laboratory and field tests designed to detect and quantify possible loss of precision in multi-piece leveling staffs. All their tests were conducted at the NGS Testing & Training Center located in Woodford, Virginia, in December 2011. The report was published in 2018.
The report did note that only a small sampling of instrumentation, three multi-piece leveling staffs comprising two separate models from two separate manufacturers, were included in the NGS study. Therefore, the results found within the report evaluate the accuracy and precision of the specific staffs tested. As the report states: “The tests are qualitative in nature with respect to bifurcation and non-Invar construction. Similar results are expected for similarly designed level staffs; nevertheless, the results should not be considered precisely valid for all types or models of multi-piece leveling staffs.”
Users should download the document and read it but I’ll highlight a few results. First, the report made the following statements about the plumbing of the rods:
- “With the Leica GKNL4M level staff carefully plumbed, the section directly above the bottom section housing the level vial was visually slightly out of plumb. No correction was made for this effect in the lab or field tests. No measurements were made to the top third section of this level staff during this evaluation.”
- “With the Trimble LD23 level staff carefully plumbed, the sections directly above and below the middle section housing the level vial was visually bowed and slightly out of plumb. No correction was made for this effect in the lab or field tests.”
Obviously, having a correctly plumbed rod is extremely important.
To estimate the potential scale error in a controlled environment, NGS performed a special test where they set up the level instrument 5 m from the leveling rods inside their building and made a measurement every decimeter on each rod. To perform this, the level instrument was moved upward one-decimeter after each measurement and the measurement was repeated. Figure 9 from the 2018 report depicts the results of the process. The first thing to notice is that the two calibrated, invar rods indicate very small errors. The other thing to notice is that there is a change in scale error at the section breaks of the multi-piece staffs. I highlighted the section breaks in the image below. The plots in Figure 9 indicate that the upper section of the rod is different from the lower section. This may result in a large error when going up (or down) an incline; that is, when leveling up an incline the upper section of a rod would be read in the back-sight reading and the lower section of a rod would be read in the foresight reading. The opposite would occur going down the incline.
NGS also performed a small field test. The height difference was only 10 m over about 180 m distance. Figure 15 from the report provides the results of their field test. Some of the rods showed an error of almost 7 mm in the 180-meter section. Obviously, this is a significant error over such a short leveling distance, especially since it appears to be systematic. The report made a note about the systematic error; it noted that the height differences between the forward and backward runs were similar, but they were different from the standard. In other words, the forward and backward runs may meet a FGCS section misclosure but the mean difference would still have the accumulated systematic error. This means that following double-run procedures will not account for the systematic error. As a side note, according to FGCS specifications and guidelines, for establishing a height of a new bench mark, double-run procedures must be used. Single-run methods can be used to re-level existing work provided the new work meets the allowable section misclosure.
As stated in the 2018 report, errors as large as 6.8 mm over a 180-meter sloping section were due to using the multi-piece leveling rods. This is unacceptable for meeting FGDC specifications and procedures for leveling surveys.
The NGS report was based on a small sample and over a very small project area. It provides a compelling argument for requiring one-piece leveling rods. Now, for a real-world example that supports the results of NGS’s study. I received a report where the results of repeat leveling surveys using the multi-piece fiberglass rods over the same basic route indicated a large systematic height error. The figure below provides the difference in heights between the two surveys. As indicated in the figure, the data indicates a height dependent error of -0.43 mm/meter between the two surveys. In this example, the difference approaches 200 mm. Clearly this type of systematic error needs to be accounted for when multi-piece barcode rods are used in a survey.
As previously stated in the conclusion of NGS’s 2018 report, “Calibration of these type survey instruments provides a means of quantifying these type error sources, thus providing a mechanism for ‘correcting’ for them during post processing of data sets.” If a company or agency created a calibration process similar to NGS’s test site, then surveyors could use the site to evaluate their multi-piece barcode rods. In my opinion, until users account for the index and scale error in multi-piece barcode leveling rods, they should not be used to perform leveling surveys to compute orthometric heights with any expected accuracy value.
Clearly, it is more cost effective to use multi-piece rods instead of single-piece invar rods because of the increase in expenses for the single-piece invar rods. However, making a business decision that results in a bad technical outcome could lead to lawsuits, professional liability issues, and/or additional expenses for having to resurvey projects. Enough said.
