Back to Basics
The foundation of our method of course measurement is the steel tape. We use it in a process that multiplies its length until we reach a race course of a desired length. We establish a calibration course, we calibrate a bicycle, and lay out the course. Each step in the process contains some error. Still, at the end of the day we believe we are achieving accuracy in the range of 0.1 percent.
In Measurement News #111, January 2002, the results of a comparison of 33 tapes from 21 measurers were presented. Tapes from USA, Britain, France, Canada and Brazil were included. Draft specifications from NIST were cited in the study. According to NIST “The specification gives a general tolerance of +/- 0.009 percent of nominal length, for the range of lengths tested. Tapes that range from 99.991 to 100.009 percent of nominal length thus match the draft spec.”
Mr. Charles Fronczek of NIST reported “In general, the accuracy of steel measuring tapes we receive here at NIST for calibration is fairly good. Usually we see typical accuracies of ¼ inch in 100 feet. In fact, it is seldom that we see one longer than that.”
The worst tape we tested had a length of 30.01 meters for a nominal 30 m length. The error is about 1/3 of our short course prevention factor. For most competent measurers its use would lead to an acceptably accurate course.
So, we have a standard which we can buy in a hardware store and trust its accuracy. We can hold it in our hand. We understand how it works.
By contrast we look at GPS, and see that it is based on measurements involving a group of satellites, the measurements being tweaked by a priesthood of government experts. The accuracy can be varied at will by these operators to suit the desire of the US government. The general principles are understood by many, but the details are not. If we are to consider it as a reliable form of measurement we are forced to take it or leave it.
The surveying profession, and manufacturers of surveying equipment, have devised equipment and procedures to use GPS with accuracy comparable to the older accepted methods. The equipment is costly and requires special training to use.
Cheap commercial GPS units generally used by outdoorsmen and runners are not built to the surveying standard. Few verifications of their accuracy exist, and those that do indicate that they perform to a far lesser standard than does the method based on the steel tape.
Given the above, what do we trust? Do we trust what we can hold in our hand and understand, or do we take it on faith that the magic electronic readout is true? Reasonable skepticism requires that we hold off jumping on the GPS bandwagon until it’s proven to be better than it seems to be today.
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