[geowebcache-users] Forcing GeoWebCache TMS to use top left as origin

[Harv H. <har...@gm...>]

Hi,

I've asked this question in the GeoServer mailing list, but its
perhaps belongs here instead.

I have set up GeoServer with the integrated GeoWebCache. I am
accessing my "mn2gt" layer-group (gridset EPSG:900913) tile server
service as below:
http://localhost:8080/geoserver/gwc/service/tms/1.0.0/mn2gt@EPSG:900913@png/1/0/0.png

It seems that by default the y-alignment of tiles in geowebcache is
from left bottom origin. My client which uses these tiles expects the
origin to be left top. I see that there is an xml element
<alignTopLeft> which can be set to true in geowebcache.xml for a given
gridset. However since, EPSG:900913 is defined by default, I cannot
seem to redefine it in my geowebcache.xml file with the <alignTopLeft>
tag set to true.

I then tried using the stand-alone GeoWebCache instead of Geoserver's
integrated GeoWebCache. I created a copy of gridset EPSG:900913 named
"my_EPSG:900913" in my geowebcache.xml (the srs number for
"my_EPSG:900913" is still 900913). I also defined my "mn2gt"
<wmsLayer> to use "my_EPSG:900913" as the <gridSetName> in
geowebcache.xml. My geowebcache.xml file is attached for reference.

It should be easy enough to use my geowebcache.xml and then try to
fetch the following tiles:
http://localhost:8080/geowebcache/service/tms/1.0.0/mn2gt@my_EPSG:900913@png/1/0/0.png
and
http://localhost:8080/geowebcache/service/tms/1.0.0/mn2gt@my_EPSG:900913@png/1/0/1.png
The first tile should be North America, and the second tile should be
South America, but it is reversed, regardless of <alignTopLeft> being
set to true or false.

Any application using Google or OpenStreetMaps tile service expects
the tile coordinate origin to be be top left. Its a bit surprising to
me that GeoWebCache would not be able to offer a tile service
compatible with these applications. Perhaps it is because GeoWebCache
is standards-based and the default for a standard TMS service is to
use the bottom left as the tile coordinate origin. If the
<alignTopLeft> element in geowebcache.xml worked, then it would have
not been an issue, but since its not working, I'm kind of stuck.

Any help with this is appreciated.

-- 
Harv Hundal