I used GPS Essentials on my Android phone to track myself as I took a walk around the city. I purposely walked long straight distances as well as some zig-zag stretches to see how accurate the track would be. Turns out it was really accurate as it correctly laid out my entire walk. I did notice that the path often strays from the street grid, instead appearing to pass through or over buildings. GPS Essentials also gives altitude data and I saw that there were a lot of sharp inclines and declines of hundreds of feet in the data as if I had been scaling buildings. One possible source of error is poor satellite geometry, either due to a lack of satellites or uneven distribution of the satellites relative to the location of the subject. Also, the tall buildings can reflect the radio waves, which causes the GPS receiver to think the satellite is further away than it really is.
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