Another issue is that in determing x-y position (lat-long) the satelites are able to determine position better because of the geometry of the situation. A good anlogy is VOR triangulation. Lets say VOR equipment an accuracy of 2 degrees. If you were in Denver using two VOR's to find your position, you'd get the most precise location by using two nearby VOR's that are, lets say, to your north and southeast. You can imagine, though, if you used 2 VOR's hundreds of miles away that were very close to each other, or almost due north and south of you, your area of uncertainty would be quite large. This is sort of how it is with GPS, but GPS uses timing instead of radials. in a map view, the satelites are all spread around you, and at greatly varying distances, which gives more opportunities to average out the errors. In an elevation view, however, all the satelites are a relatavely similar distance from you, and therefore there is a greater uncertainty/error associated with that measurement.