Latitude/Longitude to UTM: How Coordinate Conversion Actually Works
Understand WGS84 latitude/longitude, UTM zones, eastings and northings — and how to convert between them without breaking your data.
Two ways to describe the same place
Latitude and longitude are angles on an ellipsoid. UTM coordinates are metres on a flat grid. Both can describe your front door; they are just different languages.
Angles are perfect for storage and interchange. Metres are what you want the moment you need to measure something.
How UTM zones work
UTM slices the world into 60 north-south zones, each 6° of longitude wide, and projects each zone separately to keep distortion small. Zone number = floor((longitude + 180) / 6) + 1.
Within a zone you get an easting (metres east of a false origin, so values stay positive) and a northing (metres from the equator, with a 10,000,000 m offset in the southern hemisphere).
The mistakes that bite
Nearly every coordinate bug traces back to one of these:
- Coordinate order — GeoJSON and WKT are longitude, latitude; most humans say latitude first
- Wrong zone — data spanning a zone boundary must be handled per-zone, not averaged
- Missing hemisphere — the same easting/northing exists north and south of the equator
- Datum mismatch — WGS84 vs a local datum can shift positions by hundreds of metres
When to convert
Convert to UTM before computing distances, areas or buffers over a local project area, and convert back to WGS84 before publishing to a web map or sharing GeoJSON.
GeoToolBox picks the correct zone from your longitude automatically and lets you override it when your project mandates a specific zone.