Where’s the surveyor?
I recently read the project case study in the July 2004 issue of CE News that deals with "Measuring moving targets," written by Howard James. I applaud CE News for ongoing involvement of the land surveying profession in your publication and appreciate articles such as this. I do, however, have some concern about the information presented, as it tends to give a simplified or unrealistic vision of land surveying.
[The author] used these projects to demonstrate that, as he summarizes at the end of his article, "automatic" bridge monitoring can be accomplished "more accurately and faster with automatic laser total station technology." While some of the data collection for such a task can be automated, this is a high-end surveying function that most surveyors do not have the education or experience to execute. There is a great deal of painstaking work that must be done manually to allow such "automatic" monitoring to be done accurately and reliably.
The land surveyor is not discussed at all in this article. In fact, there is no reference at all to who collected or interpreted the data. There is an illusion created that an engineer could now contact the surveyor they might normally use and, with this wonderful technology, they will be able to reproduce such results. Or better yet, just rent this marvelous equipment and you’re in business!
My second point is one that I am constantly faced with as the head of a surveying group in a large engineering firm. This article serves to perpetuate the ignorance that engineers (and many surveyors for that matter) have with regard to attainable and reliable accuracies that come from surveying observations. The Leica 2003 instrument that is referred to in the article has a published accuracy of 1 millimeter, plus or minus 1 ppm. That means that with multiple observations under optimum conditions on short measured lengths, you could claim that you are obtaining a mean of the observations that is within 1 millimeter.
This is almost impossible to actually accomplish in the real world. The error sources due to setup of the instrument, setup of the targets, the atmospheric influences, and the dynamic nature of bridges create an observation environment that is challenging, at best.
If the observed distances are more than 500 meters, as they could be to the center of a 3,900- meter-long bridge, you have a published instrument error of almost 2 millimeters to start from as your best possible achievable accuracy. This does not include the error due to the angle observations. The error in position due to a 0.5-second error in angle observation at 500 meters is more than a millimeter alone.
In addition, the inset article "Precision instruments monitor Millau Bridge," suggests that the Leica System 500 RTK GPS system is providing "millimeter-level accuracy" for this project. As someone who purports to specialize in GPS technologies, Mr. James should know better than anyone that this is not possible. Leica’s stated accuracies for RTK work is plus or minus 1 centimeter, not 1 millimeter. The static accuracies are only 3 millimeters (plus or minus 0.5 ppm).
I really am happy that you continue to publish articles such as this. But, my experience has shown that there is an ever-widening gap in the understanding of surveying in the engineering community. The absence of the land surveyor, coupled with the accuracy claims, are, at best, perpetuating this, and, at worst, are widening the gap even further. The land surveyor should be included in any discussion of such a project to help readers understand their role in this great project and the diligence, care, hard work, and high cost of such a monitoring system should be presented to avoid creating an illusion regarding the implementation of this wonderful technology from which we all benefit.
Andrew Clarke, P.L.S., P.E. via e-mail
As a writer, it’s always flattering to know that a reader is paying careful attention to one’s copy, and I am grateful to Mr. Clarke for his perceptive and informed critique. His point is very well taken that large-scale civil engineering tasks, such as the bridges in China and France, are highly complex and demanding jobs, which require careful and thorough preparation on the part of engineers and surveyors. I did not mean to imply that this was easy and effortless, or that this technology would somehow eliminate the need for land surveyors. Indeed, this technology requires even closer collaboration between the surveying and engineering communities.
Unfortunately, the space requirements for the article left little room for lengthy explanations, and as a result, it may have appeared to oversimplify the complexity and difficulties of the job. While Mr. Clarke is indeed correct about the single measurement specifications of the Leica GPS and TPS equipment mentioned, what was not clearly communicated in this article is the fact that in complex projects such as these, multiple sets of measurements always are required. To reach the standard deviations as stated in the article, many measurements were taken, sometimes over periods of weeks or months. The planning for and execution of precise control and monitoring networks requires highly skilled personnel, using the best equipment available, who understand all the complexities of measurement and error theory.
In writing, as in surveying, it always pays to be precise about these things, and I stand corrected.
Cathy Bazán-Arias, Ph.D., has written a thoughtful, expository commentary on the subject of design codes. Not many experienced structural designers would disagree with Bazán- Arias’ comments, particularly with regard to the AASHTO Code.
With the introduction of LRFD procedures and the accompanying parameters and modifiers, one often asks, "Where has all the judgment gone?" This was the title of a paper given by Ralph B. Peck, Ph.D., (see Judgment in Geotechnical Engineering, John Wiley & Sons, 1984; pages 149-155). We can only hope that judgment has not gone to codes and computer software. Of course, we need computers and codes, but the input and answers must be checked by human neurocomputation and judgment.
Walter E. Hanson, P.E., S.E. Springfield, Ill.