More about GPS: Test for Position

Those of you who have taken GPS measurements to characterize your GLOBE observation site know that the “elevation” on the GPS unit often varies a great deal. That’s one of the reasons the GPS protocol requires that you take five measurements instead of just one.

In tracking the rock layer (see last blog), we didn’t stay in one place very long, and we averaged two hurried measurements at each location. So why were we so successful in tracking the rock layer? To find out more about the accuracy of the GPS elevation, I did an experiment, which I will now describe.

We live on the top of a mesa, inside an oval formed by the street and sidewalks. One trip (circuit) around the oval on the sidewalk is about 500 meters. I picked a point at the west end of the oval (the Western Point), where there was an oil stain just large enough for me to put both feet on). I picked a second point at the east end of the oval (the Eastern Point, where there was a distinctive set of cracks in the surface). Thus I could repeat measurements at exactly the same two points.

To do the rock layer observations, we walked and stopped only long enough to get the two GPS readings. So I decided I would do the same thing in the oval. The next three mornings, I got up early and walked around the oval six times. I stopped at the Eastern Point and the Western Point, just long enough to take two GPS readings of latitude, longitude, and elevation. On the third day, I took the same measurements in the afternoon. Two days after that, I stayed at the Eastern Point and took position measurements using the GLOBE protocol (except with six measurements instead of five, to have the same number as the other days). Then I went to the Western point to obtain six positions and elevations the same way.

How did I do with GPS position?

Figure 4. GPS positions for Eastern Point. On the left, each point is plotted relative to the average on that day. On the right side is plotted the daily average positions relative to the overall average position (+ sign).The green figures show how the positions varied at the Eastern Point. The figure on the left shows how the positions scattered relative to the mean latitude and longitude. We converted the numbers to meters. (One degree latitude equals about 111 kilometers; for longitude you have to reduce this number to allow for the longitude lines getting closer together toward the Poles. For those of you who know trigonometry, this is done by multiplying 111 by the cosine of the latitude. Thus at 40 degrees North, one degree longitude is about 85 kilometers).

Notice that the positions on the last two days (white and yellow) changed by over 10 meters if you use only one point to define your position.

The second figure shows the average for each day at the Eastern Point. the “+” marks the average position for all the days. Three points — for two of the “morning” circuits (black triangles) and for the GLOBE averages (white triangle, upside down so that you can see the black triangle underneath), were close to the average position. The farthest point from the average is only about 3 meters away. This is about as good as I think I can do with my GPS unit.