Measuring Distance and Area Correctly on a Map
Why geodesic measurement beats pixel measurement, how projections distort area, and how to get trustworthy distance and area numbers from GeoJSON.
Straight lines on a screen are not straight on Earth
Measuring in screen pixels or in raw degrees produces numbers that drift badly with latitude — a degree of longitude is about 111 km at the equator and under 30 km at 75° north.
Geodesic measurement computes across the ellipsoid instead, so the answer stays right whatever the map projection looks like.
Projection choices change the answer
Web Mercator, the projection behind most tile maps, is conformal but wildly inflates area away from the equator: Greenland looks larger than Africa. Never compute area from Web Mercator coordinates.
For local work use an equal-area or UTM projection; for global work use a geodesic algorithm.
Practical checklist
Before you quote a number in a report:
- Confirm the input coordinate system and datum
- Close every polygon ring — an unclosed ring silently changes area
- Watch for self-intersecting geometry, which can produce negative or doubled area
- Report units explicitly (m², hectares, km²) and keep sensible precision
Doing it in the browser
The Distance and Area tools use geodesic maths on your GeoJSON and report results in metres, kilometres, hectares and square kilometres, so you can sanity-check a dataset in seconds without opening a desktop project.