Last night I went to the monthly North Puget Sound – Land Surveyors of Washington (NPLS-LSAW) meeting. After all the chapter business was conducted, Bobby Wildrick, PLS (i.e. professional land surveyor) gave a presentation about GPS accuracy.
Mr. Wildrick conducted an experiment, in which seeking to hold all other variables constant, he used high-end Leica survey equipment, WAAS enabled Magellan, and a recreational Garmin to identify a few points. Now, though I didn’t catch the specific makes and models the analysis was pretty interesting.
Mr. Wildrick took five points – the four bridge seats and a monument at the southern mid-span of what was then the NE 12th Street Bridge in Bellevue which – R.I.P. – by the magic of YouTube can be remembered here.
As expected the survey equipment was spot on. The Magellan deviated by roughly five feet in the same northeasterly direction at all points. Whereas the Garmin … well, it was off by as much as 30 feet in most points – but not in the same direction. The end result was a trapezoidal figure which was “rather” — that’s code for “completely” — misfigured disfigured.
So, what’s the upshot for owners of GPS units? Answer: Go ahead and have fun geocaching if you like, but for important matters take it a step at a time with a knowledgeable professional behind good equipment.
In other words, don’t take your Garmin, or even Magellan, out to the the edge of your lawn and then perhaps into your neighbors garden and think that your land holdings have magically been enlarged. Though some of you may not be able to refrain from thinking it, make sure you double check with a surveyor first and an attorney second, before you decide to take and make a bouquet of roses from that garden. What do you think?