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Reading the result: the ± and the quality
How far a figure can be trusted, and how the app gets it.
The ±
"1.24 km ± 18 m" means the real length is very likely between 1.22 and 1.26 km. An enclosed area gets a percentage the same way. A drawn measurement has no ±: its precision is that of your taps on the image.
The quality
It sums up a GPS measurement: Good under 3 % of uncertainty, Fair under 10 %, Poor above. To do better: keep the phone in the open, walk steadily, or measure corner by corner.
How we measure
- Filtering: a fix with an accuracy worse than 30 m is ignored, and so is a jump that would mean travelling faster than 200 km/h (a tunnel exit, a reflection off a building).
- Standing still: when the GPS chip reports a speed below 0.5 m/s, no distance is counted. Without this, GPS jitter would add phantom metres at every stop.
- Smoothing: positions go through a Kalman filter weighted by their accuracy, and a new segment is only counted once it exceeds 2 m or three quarters of the current accuracy.
- Uncertainty: each accepted segment carries half its fix accuracy as an error; those errors are added in quadrature.
- Short distances: below about 50 m, the uncertainty is as large as the measurement itself. Measure on the map instead.
Signal gaps
After 30 seconds without a position (a tunnel, a building), the straight line across the gap is not counted. The saved measurement says how many gaps there were and how long they lasted.
Checking
Compare to a known length, in the map tools, measures something whose length you know and shows the gap.
