Written by Plantman

I am certain we have all questioned the accuracy of our GPS at one time or another. This article provides details on how to determine the accuracy of your unit. This article explains how to test your unit if you live near, or are visiting Victoria.

Take your GPS unit to the top of Mt. Douglas. Beside the lookout there is a concrete pillar with a brass plate on it. This plate is an official geodetic survey reference point. The exact location of this point is 48 29 35.05139 X 123 20 48.28241 and the elevation is 225.073 m. This data is good to 0.001 m. The Mount Douglas geodetic location data is in degrees, minutes and seconds to 5 decimals. It requires a little arithmetic to get it into the more familiar geocaching format of degrees and minutes to 3 decimals. The accuracy of the equipment doesn’t justify more than 3 decimals. I make it 48 29.5842 X 123 20.8047 that you want to get within + or – 0.003. You should be using Map Datum WGS 84.

Lay your gps unit on the brass plate and turn it on and leave it there for 5 minutes then record the reading. You will not get exactly the same reading but you should be within + – 0.003 which is about 20 feet at this location.

You will not get it exact since there are some intrinsic errors in the whole process. One technical error source is how many and specifically which satellites are in view and take part in the calculation at any given time. The same machine will likely give a different reading on different dates. A more esoteric error source is the speed of light through the column of atmosphere under the particular conditions in effect that day. This is sensitive to the density of charged particles and thus to solar activity.

The arithmetic is Euclidian, however, post Newtonian, Einsteinian and Quantum physics are in effect. This cheap, handheld gps unit we all take for granted is actually a high tech toy.