GPS accuracy

A GPS receiver estimates its position from satellite signals, and every fix has some error, often a few meters with a phone and less with a dedicated logger. Trees, grandstands, buildings and a poor antenna position all make it worse. Accuracy also depends on how often you get a fix: at once per second, a car at 100 mph travels about 45 meters between points.

For lap timing, good software does not just pick the nearest point to the line. It interpolates between fixes to estimate when the car crossed, and a good system tells you how confident it is. For line analysis, a few meters of error matters a lot when the whole track is only 12 meters wide.

Practical tips: mount the phone or receiver with a clear view of the sky, let it get a solid fix before you head out, and be cautious about drawing conclusions on line choice from low-rate data. Speed and time are usually more trustworthy than exact position.