WGS84 vs UTM vs Web Mercator: CRS Explained for GIS Beginners

The core problem

The earth is roughly an oblate spheroid — slightly squashed at the poles, slightly bulging at the equator. Maps are flat. Every map projection lies about the earth in some specific way: distorting distance, area, angles, or shape. A coordinate reference system (CRS) is the agreement about which lie a particular file is telling.

A CRS has two parts:

1. A datum — a 3D model of the earth (which ellipsoid, where its centre is).
2. A projection — the math that flattens the ellipsoid onto a plane (or the agreement that you're staying in 3D / spherical coordinates).

EPSG codes — EPSG:4326, EPSG:3857, EPSG:25832 — are a registry that pairs a datum and a projection (and a unit) and gives that combination a number. The number is shorthand for "this exact CRS."

WGS 84 (EPSG:4326): the global reference

WGS 84 (World Geodetic System 1984) is the datum that GPS uses. EPSG:4326 is WGS 84 expressed as latitude and longitude in decimal degrees — no projection at all, just position on the ellipsoid.

When you see coordinates like (52.5163, 13.3777) for the Brandenburg Gate, that's WGS 84 lat/lon. Every smartphone GPS chip reports in WGS 84.

Strengths:

• Global. Works anywhere on earth.
• The default for GeoJSON (RFC 7946 mandates it).
• Universally readable.

Weaknesses:

• Degrees are not metres. One degree of longitude shrinks as you head poleward. Distance calculations require great-circle math, not Pythagoras.
• Useless for any computation that depends on metres (area, length, buffer).

Use it for: storage, exchange, web-map source data. Avoid it for analysis that needs metric units.

UTM (EPSG:32xxx, 25xxx): the metric workhorse

The Universal Transverse Mercator system slices the globe into 60 longitudinal zones, each 6° wide, and projects each zone independently. Within a single zone, the projection is nearly conformal (preserves shape) and distances are accurate to about 1 part in 2,500.

EPSG codes for UTM follow conventions:

• EPSG:32601 through EPSG:32660 — UTM zones 1–60 Northern Hemisphere on WGS 84.
• EPSG:32701 through EPSG:32760 — same zones, Southern Hemisphere.
• EPSG:25828 through EPSG:25838 — ETRS89 / UTM zones 28–38 (European reference frame).

The coordinates are in metres, expressed as easting (x) and northing (y) within the zone.

Strengths:

• Metres are metres. Pythagoras works for short distances.
• Areas and lengths are computed with normal geometry.
• Standard for survey, cadastral, and engineering work.

Weaknesses:

• Zone-local. Data crossing zone boundaries gets ugly.
• Not suited to global datasets.
• Not what web maps render in.

Use it for: surveying, cadastral data, anything that needs accurate distance or area at a regional scale.

Web Mercator (EPSG:3857): the tile grid

The Spherical Mercator projection, popularised by Google Maps in 2005 and codified as EPSG:3857, is what every slippy tile map renders in. It assumes the earth is a sphere (not the WGS 84 ellipsoid), projects it as a cylinder, and uses metres as the unit.

The catch: it distorts area massively as you move from the equator to the poles. Greenland looks roughly the size of Africa on a Web Mercator map. Africa is actually 14 times larger.

Strengths:

• It's what tile servers and browser map libraries use natively. Z/X/Y tile addressing assumes Web Mercator.
• Square pixels at the tile level — clean rendering.
• Reasonably accurate north-south for navigation use.

Weaknesses:

• Area is wildly wrong away from the equator. Choropleths shaded by Web Mercator-derived area are nonsense.
• Not defined above ~85° N/S latitude (the projection blows up at the poles).

Use it for: rendering web maps. Never for area calculations. Never for analytical work.

The practical rule

• Store your data in WGS 84 (EPSG:4326). It travels well and every tool reads it.
• Compute area, length, buffers, and other metric operations in a local projected CRS — UTM if your data is regional, a national grid (e.g. ETRS89 / LAEA Europe, EPSG:3035) if you need a continent-scale equal-area system.
• Render in Web Mercator (EPSG:3857) because that's what the tile servers speak.

Converting between them

# WGS 84 to Web Mercator (for tiling)
ogr2ogr -t_srs EPSG:3857 out.geojson in.geojson

# WGS 84 to UTM Zone 33N (for German metric analysis)
ogr2ogr -t_srs EPSG:25833 out.geojson in.geojson

QGIS does the same job through Layer → Save As → CRS dropdown.

A note on accuracy

Reprojecting introduces small errors. WGS 84 → Web Mercator and back is essentially lossless because both use the same datum. WGS 84 → an older national grid (NAD27, OSGB36) requires a datum shift, which is centimetre-to-metre level depending on the grid file used.

For survey-grade work, always note the source CRS and don't reproject more than once.