Coordinate Precision Calculator

In short Every decimal place of latitude is worth a fixed distance almost anywhere on earth, because a degree of latitude is about 111 km at every latitude. One place is 11.1 km, two is 1.1 km, three is 111 m, four is 11.1 m, five is 1.1 m, six is 111 mm and seven is about a centimetre. That gives a simple rule: four places locates a building, five locates a room, six locates a doorway. Longitude behaves completely differently, and this is the part that produces real bugs. A degree of longitude is 111 km at the equator, about 78 km at 45°, 56 km at 60° and zero at the poles, because meridians converge. Any code that treats a degree of longitude as a fixed distance is correct at exactly one latitude and wrong everywhere else. The practical conclusion is that most published coordinates carry more digits than they deserve. Consumer GPS is accurate to roughly three to five metres in the open and considerably worse among tall buildings, so five decimal places already exceeds what the measurement supports and six is beyond it. Coordinates with twelve or fifteen decimal places are showing the binary representation of a floating-point double, not a better position — which is why you see values like 51.507400000000004. Writing trailing zeros is itself a claim: 51.500000 asserts you know the position to about ten centimetres, and if it came from a map click, three places is the honest way to write it.