GIS Coordinate System Reprojector — Free WGS84, Mercator & UTM Tool

This tool converts coordinates between the three systems most web maps and field datasets use: WGS84 latitude/longitude (the GPS system), Web Mercator (the x/y system behind web map tiles), and UTM (the metre-based grid surveyors and field teams use, divided into 60 zones). You enter a point in one system and get it in another. The supported set is stated plainly: WGS84, Web Mercator, and UTM only. National grids, older datums, and vertical datums are not supported, because each needs its own transformation parameters and a small browser tool that pretended otherwise would quietly misplace points.

How to use it

Choose the source system.
Choose the target system.
Enter the two values (and the UTM zone when UTM is involved).
Click Reproject.
Check the result against a known point before batch use.

Worked example

A field team records the Colosseum area in Rome as latitude 41.9028, longitude 12.4964. Converting to Web Mercator gives x around 1,391,000 and y around 5,148,000 — the coordinates a web map uses to place the tile. Converting to UTM zone 33 gives easting around 292,000 and northing around 4,641,000 — metres from the zone’s central meridian and the equator, the form a surveyor’s GPS and a printed grid prefer. The same place, three addresses: degrees for humans, Mercator metres for web maps, UTM metres for the field.

Honest limits

UTM input (easting/northing back to lat/lon) is not supported in this version: enter WGS84 or Web Mercator and convert to UTM. The UTM forward conversion uses the standard transverse Mercator series, accurate to well under a metre near a zone’s centre but degrading toward zone edges, so always use the correct zone for your longitude. Web Mercator also distorts badly near the poles and is undefined at exactly ±90 degrees latitude. For legal boundaries, construction staking, or any work where metres matter legally, verify coordinates in professional GIS software against ground control.

Who it is for

This tool is for students learning coordinate systems, researchers preparing field points for web maps, and field teams converting GPS readings for a report. It teaches the core idea every GIS user needs: a coordinate is meaningless without its system, and the same point wears different numbers in different systems.

Going further

Always record the coordinate system alongside the coordinates in your dataset — a column of bare numbers with no system note is a classic way to lose a field season’s work, because WGS84 degrees and Mercator metres look nothing alike yet both appear as plain numbers in a spreadsheet. A data dictionary entry like “coordinates in WGS84, decimal degrees, EPSG:4326” prevents the next analyst from guessing. Also watch the latitude/longitude order trap: some systems list lat first, others lon first, and swapping them can place a point in the wrong ocean. When sharing points, state the order explicitly, and test with a known landmark before trusting a bulk conversion. Test every conversion with a landmark you can verify independently — a city centre, a survey monument, a well-known peak — before trusting it on field data. If the landmark lands where you expect in the target system, the parameters are right; if it lands kilometres away, you have the wrong zone, the wrong order, or the wrong datum, and no amount of precision in the output will save the result. Keep a one-line test record with the landmark, its source coordinates, and its converted values, so the next person to use your conversion can repeat the check in seconds rather than rediscovering the error in the field. When working across UTM zones, remember that a single project spanning a zone boundary needs a decision: reproject everything into one zone (accepting distortion at the far edge), use a custom transverse Mercator centred on the project, or keep zones separate and never mix coordinates across them. Mixing eastings from two zones in one calculation is a silent error that looks like ordinary numbers. State the chosen approach in the project notes before any analysis, and check it with a point near the boundary, where the cost of a wrong choice is largest.

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