Coordinate systems are one of those topics that every GIS professional understands they should know well, but which trips up practitioners at all levels — even experienced ones — in ways that are silent, hard to detect, and potentially serious. The two systems you will encounter most often in web GIS work are WGS84 and Web Mercator. They look similar on a map, they are often confused with each other, and mixing them up causes spatial analysis errors that can be very difficult to diagnose.

This guide explains what each system is, when you will encounter each, what goes wrong when you confuse them, and how to convert between them when needed.

WGS84 — The Geographic Coordinate System

WGS84 (World Geodetic System 1984) is a geographic coordinate system. It defines positions on the Earth’s surface using latitude and longitude in decimal degrees. The coordinate (51.5074, -0.1278) is London in WGS84 — 51.5° north of the equator, 0.128° west of the prime meridian.

WGS84 is the coordinate system used by GPS satellites. Every GPS receiver — your phone, your Garmin, a survey-grade GNSS unit — reports positions in WGS84. It is also the coordinate system required by the GeoJSON standard (RFC 7946 mandates WGS84 for all GeoJSON data) and used by virtually every global dataset: OpenStreetMap, GADM administrative boundaries, NASA raster products, and UN humanitarian datasets all distribute data in WGS84.

The EPSG code for WGS84 is EPSG:4326. If you see that code in a dataset’s metadata, you have geographic coordinates in decimal degrees.

Web Mercator — The Projected Coordinate System

Web Mercator (also called Pseudo-Mercator or Spherical Mercator) is a projected coordinate system. Unlike WGS84, which uses angular units (degrees), Web Mercator uses linear units (metres). A position in Web Mercator looks like (−14210.939, 6711546.953) — those are X and Y values in metres, measured from a defined origin point.

Web Mercator was introduced by Google Maps in 2005 and rapidly adopted by every major web mapping platform — Bing Maps, OpenStreetMap, Mapbox, Esri — because it made tile caching mathematically simple (tiles are square at every zoom level). The EPSG code is EPSG:3857.

You will encounter Web Mercator coordinates when:

• Working with map tile services (XYZ, WMTS)
• Exporting data from ArcGIS Online or ArcGIS Server in their default projection
• Receiving data from systems that store coordinates as metres rather than degrees
• Working with web APIs that return X/Y values instead of lat/lon

The Key Difference: What Can Go Wrong

The most common mistake is treating Web Mercator coordinates as if they were WGS84 latitude/longitude. If you receive a dataset with X value 13,400,000 and attempt to use that as a longitude, your data will appear somewhere in the Pacific Ocean. The values are in different units and different ranges: WGS84 longitude ranges from -180 to 180; Web Mercator X values range from approximately -20,037,508 to 20,037,508 (metres).

The second common mistake is performing area or distance calculations in Web Mercator. Because Web Mercator distorts area (significantly so at higher latitudes — Greenland appears the same size as Africa on a Web Mercator map), any area calculation performed in this projection will be wrong. Area analysis must be done in an equal-area projection or in WGS84 using the haversine formula or similar spherical geometry approach.

How to Convert Between Web Mercator and WGS84

If you have data in Web Mercator and need it in WGS84 (or vice versa), the Web Mercator Converter on Quick Map Tools handles this directly in the browser. Paste your EPSG:3857 X/Y values and get back the WGS84 latitude and longitude. This is useful for one-off coordinate lookups — debugging a coordinate that looks wrong, checking a coordinate from an API response, or converting a single point from a colleague’s data.

For bulk conversion of datasets, QGIS (Project → Properties → CRS, or the Reproject Layer tool) and gdal/ogr2ogr are the standard tools.

Bearing and Azimuth: A Related Conversion

A closely related source of confusion in GIS and survey data is the difference between compass bearing and azimuth. Both describe direction, but use different conventions:

• Bearing uses cardinal direction notation: N45°E, S20°W. It is common in navigation, surveying, and legal land descriptions.
• Azimuth measures clockwise from north in degrees from 0° to 360°. N is 0°, E is 90°, S is 180°, W is 270°. It is standard in GIS, military, and meteorological contexts.

Survey data, property boundary descriptions, and older datasets often use bearing notation. GIS software expects azimuth. The bearing to azimuth converter on Quick Map Tools handles this conversion instantly for individual values. This is a small but frequent point of friction when ingesting survey data into a GIS workflow.

Choosing the Right Coordinate System for Your Project

A practical decision framework:

• Storing and sharing data: Use WGS84. It is the universal standard for data interchange, required by GeoJSON, and understood by every tool.
• Web map display: Use Web Mercator for your map tiles, but keep your data in WGS84 and let the mapping library handle the projection on-the-fly.
• Distance calculations: Use a local projected coordinate system appropriate to your region (UTM zones, national grids) or perform calculations in WGS84 using spherical geometry.
• Area calculations: Use an equal-area projection. Never calculate area in Web Mercator.

Coordinate Systems and Downloaded Boundary Data

When you download administrative boundary data — from GeoBoundaries, GADM, or the Quick Map Tools boundary download tool — it will always be in WGS84. This is what GeoJSON requires and what global datasets distribute. If your analysis environment uses a different projection, the reprojection step is your responsibility. See our guide on downloading free administrative boundary data for more on working with these datasets.

Summary

WGS84 is the geographic coordinate system of GPS and global data interchange — decimal degrees, EPSG:4326. Web Mercator is the projected coordinate system of web mapping — metres, EPSG:3857. Keep your data in WGS84, display it in Web Mercator if needed, and never perform area calculations in Web Mercator. When you need to convert individual coordinates between the two, the Web Mercator Converter handles it in seconds. For bearing-to-azimuth conversions from survey data, use the bearing and azimuth converter.