Guide
How accurate is GPS area measurement?
A phone's GPS is good to a few metres. What that means for a field, a fence or a garden, and how to know how far your own result can be trusted.
What a phone GPS can do
Under a clear sky, a phone places you within about 3 to 5 metres. Under trees, between buildings or in a valley, it can be 10 to 30 metres off. On a small garden, a few metres of error is a large share of the area; on a field of several hectares, it is a small one.
Why a walked outline is larger than it should be
A GPS position wanders around the true one. Walking adds up these wanders as extra distance, and an app that counts every fix inflates both the length and the area. Distance Measure Pro filters fixes that are too imprecise, rejects impossible jumps, counts nothing when you stand still and smooths the track before measuring it.
Read the ± and the quality
Every GPS figure comes with its margin: "1.24 km ± 18 m", or "0.87 ha ± 4 %" for an area. The measurement is also rated Good (under 3 %), Fair (under 10 %) or Poor, with what to do to improve it.
Five ways to do better
- Let the GPS settle in the open for a minute before starting.
- Keep the phone in the open, not deep in a bag.
- For a plot with few corners, measure corner by corner: the averaged corners beat a walked outline.
- For land you can see from above, draw it on the satellite map: the result no longer depends on the GPS.
- Under about 50 m, the GPS error is as large as the distance itself: measure on the map.
Check it yourself
Draw a sports pitch or a fence whose length you know, then use Compare to a known length: the app shows the gap and whether it is within the expected precision.
When it is not enough
For a legal boundary, a sale contract or a dispute, only a licensed surveyor's measurement counts.
