What is the current Web Mercator projection code?

Question

The web mercator projection, popularised by Google Maps, seems to be given a new EPSG code every couple of years.

EPSG:900913 (Google in calculator text) was an unofficial code

EPSG:3785 - is the projection I currently use for my datasets

However this blog post suggests that the code is now EPSG:3857.

This projection can be found on the EPSG site but it seems to also use the code SR-ORG:6864 and claims "it is not a recognized geodetic system: see WGS 84 / World Mercator (CRS code 3395)."

So what is the official code to use?

Answer 1

This has been an annoying problem for a while, and hopefully will no longer be an issue.

3857 looks to be the current and correct code (I hope, that's what all my tile caches are in!).

Update 9/7/11 - as noted by Vadim below in comments, Esri did in fact revert back to 102100 from 3857 at Service Pack 1. Oddly, ArcGIS Server with SP1 applied returns a WKID of 102100 for a web mercator map service, but in the Services Directory, a web mercator map service has a spatial reference of '102100 (3857)'. EPSG has no entry for 102100. Not sure why Esri has chosen this route, but Esri's 102100 and 3857 are treated as equivalent by their products.

EPSG - (no direct link)

Code: EPSG::2008.114 Reporter: OGP Request: Revisit spherical mercator used for some web mapping applications Actions Taken: Deprecated ellipsoid 7059, datum 6055, method 9841, projection 19847, CRSs 4055 and 3785, tfm 15973. Added methods 1024 and 1026, proj 3856 and projCRS 3857. Entity Types Affected: Ellipsoid; Datum; Coordinate Operation Method; Coordinate Operation; Coordinate Reference System Codes Affected: 7059; 6055; 9841; 15973 19847; 4055 3785 Report Date (UTC): 2008-12-11 Closed Date (UTC): 2009-02-10

OpenLayers -

Today, there is an officially registered EPSG code 3857 whose projection is identical to EPSG:900913. (http://www.epsg-registry.org/export.htm?gml=urn:ogc:def:crs:EPSG::3857). So, if you need to combine overlay layers that are using either an alias or the official EPSG code with an OpenLayers SphericalMercator layer, you have to make sure that OpenLayers requests EPSG:3857 or other alias in stead of EPSG:900913.

ESRI -

@jamie - For a long time EPSG refused to assign a code to this coordinate system; therefore ESRI created the WKID codes 102113 and 102100. When EPSG did assign a code, they used 3785, but later changed it to 3857. ArcGIS 10 will follow ESRI practice of using an EPSG code when one exists, and will advertise the coordinate system of the service as 3857. ArcGIS 10 and all Web APIs are being designed to recognize EPSG 3857, ESRI WKID 102113, and ESRI WKID 102100 as equivalent.

Answer 2

Here's a very nice article by Alastair Aitchison describing the history of Web Mercator projection, together with all the EPSG code changes and reasons for them: The Google Maps / Bing Maps Spherical Mercator Projection. Highly recommended read.

Answer 3

The authoritative online source for information on EPSG codes is the EPSG Geodetic Parameter Registry which is powered by the EPSG dataset but is often more current than the most recent version available for direct download.

According to this source, the EPSG::3785 projected CRS was initially created in response to Change Request EPSG::2008.016. This initial code was deprecated by way of Change Request EPSG::2008.114 which also introduced EPSG::3857 (and EPSG::3856).

You're not alone in the use of this deprecated code though; I have seen 3785 in the wild many times.

SpatialReference.org is FAR more user-friendly than the EPSG Registry (and actually linkable) but can not provide authoritative answers due to the lack of timely database releases by EPSG.