add EPSG:900913 to location wizard

[timmie]

The Google Mercator Projection sometimes referred to as ESRI-EPSG code 900913 while it does not have an official code?  http://spatialreference.org/ref/sr-org/6/

Even in the GRASS docs, 900913 is used:  http://grass.fbk.eu/gdp/html_grass64/g.proj.html

In GRASS location wizard (version 6.4.2RC2) the code 900913 is not present. But g.proj -c epsg=900913 loc=google works.

How does this fit together?

Could the code officially be added?

Thanks.

[hamish]

after originally reacting with horror, AFAIU the epsg people have given up the fight and issued the spherical mercator with wgs84's major ellipsoid radius its own epsg code. So this will be fixed in time with a new version of the epsg file and the only action needed is to wait. it is not recommended to use spherical merc for real analysis, only data transfer and matching visual cartography. it is designed to be very fast on their servers, not to seek the ultimate perfection in geodesy. (fair enough for their needs)

I think g.proj works but it isn't listed in the wizard as the wizard just parses the main epsg file while g.proj uses the PROJ4 library which also checks the esri.extra file that contains the "Chris's funny" projection.

be careful calling it the 'google' projection, their google maps slippy tile projection is a slightly different thing again (and even more distorted). each row of tiles has its own slightly different projection! the two are related but get confused a lot.

Hamish

[neteler]

I would note that it is not the official code:

tail -3 /usr/share/proj/esri.extra 
#  Chris' funny epsgish code for the google mercator
#
<900913> +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs <>

The fact that 900913 exists is AFAIK historically: to get +nadgrids=@null into a code. For subsequent history, see Hamish's comment above.

See also:

 http://trac.osgeo.org/openlayers/wiki/SphericalMercator "SphericalMercator? and EPSG aliases

The SphericalMercator? projection in OpenLayers? uses code epsg:900913. When Google started to use the SphericalMercator? projection, a corresponding EPSG code did not yet exist, so a temporary code '900913' (notice the resemblance to the word 'google') was invented. Many other services, such as OpenStreetMap?, Bing and Yahoo are now also using the same projection, but are not necessarily supporting the use of code EPSG:900913. Other codes, such as EPSG:3857 and EPSG:102113 were invented. 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)."

I would suggest to support the official code rather than the inofficial one which may disappear since superceeded.

[timmie]

thank you very much for your explanantion.

Is there a consensus in OSGEO apps what to use?

QGIS definately used 900913 for the Google sperical Mercator.

[neteler]

Replying to timmie:

thank you very much for your explanantion. Is there a consensus in OSGEO apps what to use?

I have cited the Openlayers comment, hence they consider it a temporary and unofficial code number.

QGIS definately used 900913 for the Google sperical Mercator.

... which does not make it ufficial nor correct :)

[timmie]

Should this be then brought to the attention of:  http://trac.osgeo.org/metacrs/ or all  http://wiki.osgeo.org/wiki/Project_Steering_Committees

[hamish]

is the real problem that the other epsg sibling files are not considered? Should the location wizard concatenate them all, if found?

/usr/local/share/esri, esri.extra, other.extra

Hamish