Oh My Foot, Another Change! – Photogrammetry & Survey Engineer
Oh My Foot, Another Change!
Editor’s Note: Retired after nearly 43 years with the Wyoming Department of Transportation as State Photogrammetry & Surveys Engineer, Curtis Clabaugh is currently with ESP Associates doing project liaison and educational awareness. He is currently the chair of the Transportation Research Board subcommittee for small unmanned aerial systems.
Sometimes I wonder if I should be worried about unanswered problems especially with GNSS surveying. Maybe my concerns are not actually a problem or insignificant in the bigger picture. It could be that somebody already knows the answer and I am just unaware. Either way, sometimes just taking a wait and see position will determine if there is a problem and it has solution. Such is the case with the upcoming reference frames, along with deprecation of the U.S. survey foot (1 ft = 1200/3937 m exactly) and solely using the international foot (1 ft = 0.3048 m exactly).
I have been in several meetings and conferences and raised my concerns about these topics. I feel there are plenty of problems that could arise. After a lengthy career with the Wyoming Department of Transportation as the State Photogrammetry and Surveys Engineer responsible for providing all mapping for highway design, I found there were no easy answers. A common response seems to be that the survey equipment and software can address the change. There might be some procedural checks to validate your work if this is correct. GNSS vendors are assuring the survey community the datum’s geodetic criteria and state plane zones will be integrated into their technology. That should be easy enough on the equipment side of the issue but what about actual practices used in the field?
One of the things I was reminded of in the coming changes is to not refer to the new format as the new datums as but rather the are National Spatial Reference System. After watching several presentations by NGS and their willingness to incorporate each State’s desires, I do believe improvements are ahead.
I also recall hearing similar statements when we went to metric and then back to U.S. customary. Knowing the correct conversion factors was all you needed. As a surveyor with the project COGO files you were usually capable of resolving issues. Unfortunately, many projects involve a large team taking the survey mapping forward with design and construction to follow. Many just converted the data back to the unit system they were more comfortable with. Once you start changing or manipulating data, you immediately increase the likelihood that errors will occur when many hands touch the data.
At the time of these U.S. customary to metric conversions I had 274 projects either underway or completed that wound up requiring corrective actions. These corrections often turned into major efforts and often require almost starting over with processing survey data. While the measurement data could be easily converted the problem came up when the plan sets were produced from the survey data. The line styles, fonts, symbology, cells, etc. are completely different when plotting the plan in English or metric scales. When that need to move to metric arose, a complete new set of CADD standards and templates had to be developed including symbols, cells/blocks, and fonts for instance were needed to be created to publish metric plans.
After several years of producing metric plans, we finally gave up on the metric system and returned to U.S. customary. Well that would seem to be a much easier task to roll things backwards and just grab the prior CADD standards and charge forward. Such wishful thinking. The mapping software and design software would no longer work with the newer computer workstations as they had all moved forward and there was no backward compatibility.
We had used the survey foot for our work so it would seem like an easy conversion for survey measurements. At the beginning of the digital survey era in data collection there were numerous issues involving the compatibility of the survey equipment. Electronic data collectors and the total stations not only had communication issues as they were often not from the same manufacturer. One of the problems that occasionally occurred is when the data collector and the total station both used the value of the survey foot to produce the coordinate values. It needed to only be assigned to one piece of equipment so both pieces of equipment were not applying the conversion factor. Eventually our data collectors and the EDMs came from the same manufacturer so this error was eliminated.
As Dr. Michael Dennis of the National Geodetic Survey pointed out, having two standards for defining a foot is not good. NGS therefore took the opportunity with the modernization of the National Spatial Reference System slated originally for 2022 to help move the U.S. toward a single, uniform definition of the foot to restore order from chaos. Some States already use the international foot. But there is a need to understand both for items such as retracement surveys.
While the case can be made that the difference is small over the distance of a mile using the mathematical conversions, the issue is when using actual State Plane Coordinates that are values extremely larger than 5280 feet even though they may inverse to be a mile.
The lesson to be learned from these past experiences is that incompatible data may continue to be an issue in the future. Design plans produced from surveys using the survey foot may be staked out with an erroneous value using the new international foot. Pay items were calculated using one criteria and possibly staked out using different parameters. While the difference may seem small in value such that they could be ignored, I think it depends on the accuracy standards for the item being surveyed. I always like to use the example of a sidewalk ramp for ADA accommodations where the thickness of a dime when checking slope could be the difference between acceptance or rejection.
I have heard numerous comments that the differences are so small between the two types of feet that they are hardly noticeable. When applying the factor to project coordinate systems that have values larger than some arbitrary 10,000, 5,000, 1,000 value, that is not the case. I bring this issue up as it has been suggested to truncate the project coordinate values. For the prior reason I stated I would think that is a poor solution. Large projects adjoining other large projects typically have a combined scale factor that is different depending on the span of a project and changes in elevation. Additionally, coordinate value differences between county or State Plane zones do not allow the survey to overlap without additional computations.
When I have asked many people how they will handle the possible problems I get several answers. From the equipment manufacturers I am told their gear will have all the geodetic parameters necessary to handle the new reference system. I believe that is true once NGS provides them the State Plane Coordinate specifics and the surveyors and engineers understand these local project considerations.
When I ask how they know that there are no issues between staking old plans with the new system I routinely hear “as long as you perform a site calibration, your survey should be good to go”. A question I have not resolved is when there are shifts that occur with the new reference system and positional values are being derived from the new criteria, what happens? It would be wonderful to have a sample set of data from and old project to see what occurs. With the origin of each of the surveys being different and resulting in a three-dimensional shift, does site calibration resolve the problem?
To paraphrase Michael J. Olsen, Professor at Oregon State University, the scale factor could “eat” much of this error and would certainly be an ad hoc approach. It is risky to rely on the site calibration to solve these issues. If there is a problem with one of the control coordinates to begin with that has not been tracked down, particularly when there are just a few points used in the calibration, or the geometry is poor, such an approach is not acceptable, but unfortunately it is very common. Likely the operator might mistakenly believe the residuals or larger than normal scale factor is a result of this problem rather than a bad control point.
It has been suggested that if the surveyor holds the original survey datums and criteria, there should not be any issues. When using real time GNSS reference networks will this option still exist? One item that was proposed as a solution in the past was stakeless construction layout. In reality, I saw too many examples of projects where people also assumed stakeless to mean that monuments were not necessary. I think most have learned over time that good control that adequately covers the project limits for site calibration is critical.
After hearing about these changes coming to the survey profession, I support the idea of the opportunity to do better work which we all should welcome. However, if the need to change comes via better technologies or methodologies, we will need to be aware of the potential pitfalls. Coordination between surveys, engineers, and contractors must occur to eliminate or mitigate problems.
I personally would rather see a proactive approach of testing procedures and determining ways to mitigate any issues. Ignorance might be blissful and maybe the problems are small. The reality check of what to do is always a function of what amount of error can we live with and what is the cost if we cannot live with that amount of error. Unfortunately, many of the work around “solutions” people will do are just sweeping it under the rug and may not fully address the problems. Once we resolve how to move forward, the future should be less of a concern knowing that the tectonic change will be handled easier with the new approach. Since part of the proposal is a new reference epoch on a five-year cycle, I cannot help but hope that we do not go back to metric or decide the survey foot was not such a bad solution.
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