I’ve been starting to use GeoServer caching (geowebcache) more often and have a question regarding the caching for the web mercator or Google projection.
The “official” EPSG code for the web mercator projection is EPSG:3857. GeoServer does support this by default. However, geowebcache uses 900913 as a default gridset. Perhaps that should be 3857?
As well, the default definition for 3857 in GeoServer is missing the datum transformation parameters. Adding something like the following should work.
TOWGS84[0,0,0,0,0,0,0],
(In PROJ.4 format, “+nadgrids=@anonymised.com” could also be added to not use any grid shift fileand apply a zero shift.)
Should GeoWebCache use 3857 rather than 900913 as the default web mercator gridset?
Should the definitions of 3857 and/or 900913 be changed in GeoServer? or maybe entries can be added to override these reference systems in \geoserver_data\user_projections?
Would anyone recommend building cache tile sets for both 3857 and 900913 (since clients may be use either one)?
Thanks for your help/.
John
John Callahan
Research Scientist
Delaware Geological Survey
University of Delaware http://www.dgs.udel.edu
a truly academic question. Yes - in theory. However, the widespread use of
900913 is a result of the Cinderella treatment of Google by the spatial
fraternity and the past confusion with EPSG:3587 and EPSG:3857.
What geowebcache uses as a default is entirely up to you because in the
geowebcache-core-context.xml is the following tag:
Part of my question may be academic but, like many technical things, it becomes practical when getting into the details. I currently have 20 - 30 GB used by geowebcache for SRS 900913 (total for about 12 raster layers). When a request comes in using 3857, it will not use the 900913 cache. To use cache for 3857, I would need to duplicate the cache for each layer. It comes down to which is used more often and if I want to support caches for both.
I agree that I can alter the default SRS for geowebcache anytime. I would think 3875 should be the default and we should move away from 900913, unless everyone is still using 900913 much more than 3857.
···
John
John Callahan
Research Scientist
Delaware Geological Survey
University of Delaware http://www.dgs.udel.edu
a truly academic question. Yes - in theory. However, the widespread use of
900913 is a result of the Cinderella treatment of Google by the spatial
fraternity and the past confusion with EPSG:3587 and EPSG:3857.
What geowebcache uses as a default is entirely up to you because in the
geowebcache-core-context.xml is the following tag:
Question 3: No why, they are the same projections.
Cheers
Christian
Dr Christian Maul
Project Manager
Information Services Branch
Department of Sustainability and Environment
Level13, Marland House, 570 Bourke Street
Melbourne 3000
just an idea which wouldn't work in windows but I would like to know what
would happen if you have the other (empty) directory as a symbolic link
pointing to the directory that has the tiles. Haven't tried that but it
should work in the Unix/Linux world.
Cheers
Christian
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____________________________
Dr Christian Maul
Project Manager
Information Services Branch
Department of Sustainability and Environment
Level13, Marland House, 570 Bourke Street
Melbourne 3000
On Wed, Sep 11, 2013 at 8:04 AM, John Callahan <john.callahan@anonymised.com>wrote:
Part of my question may be academic but, like many technical things, it
becomes practical when getting into the details. I currently have 20 - 30
GB used by geowebcache for SRS 900913 (total for about 12 raster layers).
When a request comes in using 3857, it will not use the 900913 cache. To
use cache for 3857, I would need to duplicate the cache for each layer. It
comes down to which is used more often and if I want to support caches for
both.
I agree that I can alter the default SRS for geowebcache anytime. I would
think 3875 should be the default and we should move away from 900913,
unless everyone is still using 900913 much more than 3857.
Meh, my current perception is that both are used, 900913 more used by the
pratictioners, 3857 (not 3875!) more by people paying more attention to OGC
details
(just made a search with google for EPSG:900913, returns 133k results,
EPSG:3857 returns 149k results).
It would be nice to have them both and have some internal transformer that
"canonicalizes" one onto the other to avoid
duplicating the tile storage. Who's going to develop/sponsor it?
