EPSG Lookup

How the EPSG Lookup works, and what it assumes

An EPSG code is shorthand for a complete coordinate reference system: a datum, which fixes where the coordinate origin sits relative to the earth; an ellipsoid, which is the shape the datum is measured on; and, for a projected system, a projection method with its parameters and a unit. Two systems can share a projection and differ by datum, or share a datum and differ by projection, and neither pair is interchangeable.

Searching matches the code, the name, the datum, the projection method and the area of use, so 4326, WGS 84 and British National Grid all find their system, and Finland finds the systems used there. An exact code always comes first. Where several systems match, select one to see its definition; UTM zones are ranked below the named systems so a general search is not buried under sixty of them.

Both the PROJ string and the WKT are generated from the same stored parameters, which means the two can never disagree with each other. WKT is produced only for the projection methods whose WKT 1 parameter mapping is unambiguous. For the handful where more than one mapping is in circulation — Krovak and the Swiss oblique Mercator among them — the tool says the WKT is not included rather than presenting a guess as authoritative.

Using the EPSG Lookup

• Working out what the SRID on a geometry column means before trusting the coordinates in it.
• Finding the right UTM zone code for a country or a longitude band.
• Getting a PROJ string to hand to ogr2ogr, gdalwarp or pyproj without leaving the browser.
• Settling an argument about whether a dataset is in degrees or metres by reading the units the code declares.

A worked example

Searching for 27700 returns:

Code        EPSG:27700
Name        OSGB36 / British National Grid
Type        Projected coordinate reference system
Projection  Transverse Mercator
Datum       Ordnance Survey Great Britain 1936
Ellipsoid   Airy 1830 (a = 6377563.396 m, 1/f = 299.3249646)
Units       metre
Area of use United Kingdom, onshore: Great Britain and the Isle of Man

with the PROJ string ready to paste into ogr2ogr or pyproj:

+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000
+ellps=airy +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489 +units=m +no_defs

Those seven towgs84 numbers are a Helmert transformation onto WGS 84, accurate to roughly a metre. Survey-grade work in Great Britain uses the OSTN15 transformation grid instead, which is accurate to a few centimetres and cannot be expressed as seven numbers.

Where this comes up in GIS work

1. Type a code such as 27700, a name such as British National Grid, or a place such as Finland.
2. If several systems match, select one from the list to see its full definition.
3. Read the table for the datum, ellipsoid, projection, units, axis order and area of use.
4. Select Copy beside the PROJ string or the WKT to take the definition into GDAL, PROJ, QGIS or your own code.

When the output looks wrong

Nothing in the offline dataset matches your search

This lookup ships a curated subset, not the whole registry, so a perfectly valid code can be absent. Try the number on its own in case you searched a name that is spelled differently here, and otherwise look it up at epsg.org or in the PROJ database.

WKT is not included for this projection method

A few methods, among them Krovak and the Swiss oblique Mercator, have more than one WKT 1 parameter mapping in circulation. Rather than pick one and present it as authoritative, the lookup gives the PROJ string and points you at the registry for the WKT.

The coordinates do not line up with the area of use

Then the code on the data is wrong, which is common and is worth catching early. An easting of 400,000 is British National Grid; a value near 500,000 with a seven-digit northing is UTM; a value between -180 and 180 is degrees whatever the metadata says.

Two codes look like the same system

They usually are, on different datums or at different epochs: 4326 and 4258 are within a metre of each other in Europe but are not the same system, and GDA94 and GDA2020 differ by about 1.8 m. Check the datum row, not the name.

Questions about the EPSG Lookup