Technology is a double-edged sword, especially when it comes to skilled application of technology to solve problems. In surveying, the preoccupation with "toys" sometimes flirts with the childish.
But the push to provide clients with better, faster, cheaper products and services is going on across the broad spectrum of business. From plane table to stadia to electronic distance measurements (EDMs) to total stations, time in the field constantly has been reduced. With plane tables, data reductions were done in the field and plotted. Stadia enabled faster accumulation of data for mapped points, but it took more time in the office to process the information, and required greater communication between the field and office personnel and greater skill to represent the topography properly.
With EDMs and total stations, the strain on the observer was reduced, and if an electronic data collector was used, mistakes in transcribing information in the field and in the office were practically eliminated. In fact, with electronic data collection, the map could be drawn with little human intervention. While optical instruments with all the bells and whistles made field operations much faster, they still were limited by the necessity of a clear line of sight between the instrument and the prism pole. However, GPS put an end to that.
But just as computers have enabled us to make more mistakes, faster, and with more disastrous results, the new technologies also can lead to more flawed results and incorrect conclusions at a faster rate.
I encounter proud reports of the prowess of high technology-equipped surveyors: 1 part per million results with GPS observations, or 1:100,000 traverses with modern total stations. But usually in the same conversations, I hear talk about the unreliability of state plane coordinates, or the control monuments of the national, regional, and state networks.
The conversation goes something like this: "Well, I connected between a first order and third monument, running a 1:100,000 traverse. The total traverse was about 20 miles, so why did I misclose by a foot-and-a-half?"
Usually, these high-tech surveys are accomplished in record time, and they often are tied into the state plane coordinate system or some other wide-ranging datum. The cost to accomplish the task, the amount of human resources used, and the turnaround time seen by the client is short. But the ability to deliver quality work comparable to or better than the old paradigm can be frustrated by three scenarios.
1. The advertised performance for the new technology is taken out of context. Generally, the manufacturer of a total station or GPS receiver does not take responsibility for the failure of other components in the surveying system to deliver comparable results. Optical plummets, prism poles, prisms, even tripods can be the equivalent of bald tires on a high-performance racecar. The emphasis on quantity and speed often means less attention to critical tasks, such as measurement of the height of antennas and prism poles. Potential problems can be resolved by a high degree of care in determining total system performance and surveying system component inspection, testing, adjustment, and calibration. The surveyor risks a sub-par survey by mistaking the performance of the central, high-tech system component for the performance of the surveying system.
2. Surveyors misunderstand the new technology. The physics of the new measurement systems are much harder to integrate into operations than the transit and steel tape of yesteryear. I often suggest (humorously, I hope) that to be positive of the results, surveyors should use the steel tape for distance measurement, rather than EDM or GPS. It is not obvious when a malfunction of the latter two occurs, if they continue to report apparent results. It is obvious when the steel tape is broken.
In addition to such technology idiosyncrasies as EDM modulation, wavelength errors, and GPS multi-path, the effect of the environment can be significant. Surveyors commonly remark that, "I checked the effect of my EDM PPM correction by changing it from —20 to +50. I only saw a difference of one or two hundredths of a foot." The 70 PPM difference introduced by the surveyor trying to "check out" what the PPM correction does would only show a 0.02-foot discrepancy in a measurement of about 275 feet. That doesn’t make it negligible. A consistent error of 70 PPM makes the survey’s accuracy no better than 1:14,000!
Pay attention to the instruction manuals, invest in training, and spend time understanding the technology. By overlooking the physics affecting measurements made with high-tech instruments, the surveyor can overlook error contributions that would not have been missed in the days of the transit and steel tape.
3. Surveyors think that precision is equivalent to accuracy. Precision, which has to do with repeatability and fineness of the measurements, relates to methods. But the surveyor also must report results that are correct. That’s where accuracy comes in: it only pertains to the "truth" of the resulting measurement, and does not reflect methods.
It is quite common to have traverses with an internal precision of 1:150,000 that are not worth the silicon they are stored in. Without accounting for systematic and random errors, the surveyor is in the dark about the accuracy of those traverses. Mistaking precision for accuracy is a sure road to providing incorrect results.
The bottom line with high technology is that brainpower content has to rise in proportion to the reduction in time, operational effort, and muscle power in the measurement.
Joseph V.R. Paiva, Ph.D., P.S., P.E., is a geomatics consultant. He can be reached at firstname.lastname@example.org.