Letters to the Editor
Professional Surveyor Magazine - January 2004
In "GPS Made Simple" (December 2003), Matthew Mokanyk describes the benefits of using stop-n-go GPS techniques for improvement locations on an ALTA survey. While I believe this is an appropriate use of the technology, the author makes a statement near the end of the article about the use of two base stations that should be interpreted with caution: "… for every shot, we were creating a triangle, and we were able to check the integrity of every triangle with the blunder-detecting and loop-closing features of the software." The implication is that each rover position is being independently determined with from the two base receivers, thus adding valuable redundancy to the position.
However, each of the two vectors being generated for the rover position relies upon the same data from the rover receiver. Since stop-n-go GPS typically involves very short observation times—often only 10 to 20 seconds per point—the redundancy offered by the use of a second base receiver is illusory. The two-base procedure cannot mitigate the effects of GPS error sources to which stop-n-go observations are susceptible, such as poor satellite geometry, ionospheric signal delay, and multipath reflection. In my opinion, the use of a second base receiver for stop-n-go operations is unwarranted unless one of the base positions is likely to become unsuitable for observations during the session.
Via the Internet
Jim is totally correct with regards to the second receiver station in conjunction with stop-and-go observations. However, we did not observe the boundary control points with the stop-and-go method. Instead, to assure accuracy, we used static GPS methods on all boundary control points. The static method increases observation times to 12-40 minutes depending on satellite geometry and baseline length. The second base receiver becomes invaluable when observing a static network.
Jim also pointed out the EXACT reason why we used the second base receiver during stop-and-go surveying: "unless one of the base positions is likely to become unsuitable for observations." Since the mall was surveyed by one man, it was difficult to "guard" both base receivers. Using only one base receiver would be disastrous if that receiver became inoperable (during the course of the stop-and-go survey). For example; strong winds may topple a base receiver antenna, curious people may bump the base receiver antenna out of position, or satellite geometry may become undesirable at the base receiver location. During stop-and-go surveying we used the second base receiver to safeguard against the above mentioned scenarios. If our base receiver fell over, we had a second base receiver collecting good data. So the second base receiver does prove valuable during a stop-and-go survey. Jim's comment (above) reinforces this idea.—M.M.
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