Just a tip for peoples writing documentation with links to on-line Coordinate Reference Systems (CRS) definitions. There is various web sites that allow to browse CRS on-line, but only two of them (as far as I know) are authoritative references. We often see (in Wikipedia and elsewhere) URLs like below, where 3395 can be replaced by any other EPSG code:
* http://spatialreference.org/ref/epsg/3395/ * http://epsg.io/3395 Those two sites offer nice interfaces, but their definitions differ from official OGC and EPSG definitions in at least two aspects: axis order and units of measurement when those units are not degrees or meters. Nevertheless those sites are useful if the goal is to link to the definitions as used by MapServer or PostGIS for instance. If the goal is to link to official definitions, then the authoritative source of EPSG definitions is http://epsg-registry.org. But we do not see many links to that site maybe because the way to forge URL for a given EPSG code is not well know. There is the recipe (just replace "3395" by the desired CRS code): * http://epsg-registry.org/?display=entity&urn=urn:ogc:def:crs:EPSG::3395 For a definition in GML, two sources are available: * http://epsg-registry.org/export.htm?gml=urn:ogc:def:crs:EPSG::3395 * http://www.opengis.net/def/crs/EPSG/0/3395 Those two sources differ in two aspects: the epsg-registry GML uses "xlink" extensively: their GML provide only the main information, then all sub-components are specified by "xlink" to other GML documents. At the opposite, the opengis.net site provides the same GML except that all "xlink" have been expanded in-line (no need to search for separated documents). The epsg-registry approach is much more compact but the OGC approach is easier to parse (at the cost of a lot of duplication, especially if we read more than one CRS). Another difference is that the OGC registry is currently behind the EPSG one. For the most up-to-date information, epsg-registry still the source. Martin
