James Mapserver wrote:
Projection problems with Google maps / Virtual Earth
I don't really understand whats happening here - so would be grateful
for any suggestions.
I have 2 .map files serving out WMS data:
- BNG.map
- WGS84.map
James,
Are you building MapServer from source yourself? If so, could you possibly
try upgrading to PROJ 4.6.0? This release makes a fundamental change in
how changes of ellipsoid that lack datum shift parameters are done that
may be at the root of your problem. Alternatively, if you stick with an
older PROJ.4 please read the last question in the PROJ FAQ:
http://proj.maptools.org/faq.html
Hmm, on reflection the problem is more likely that *no* datum shift
is being done when going from the OSGB 1936 datum underlying the BNG
projection. Ideally what you would like it to do is apply a datum
shift from OSGB 1936. The EPSG lookup file does not have a datum shift
for EPSG:27700 but I think a reasonable good one is:
+towgs84=446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894
Consider adding that into the <27700> entry in the EPSG init file.
I'm sorry, but I lack time for a deeper analysis. I hope this will give
you something to work with.
Best regards,
Both these map files are displaying the same data - but one has the raw
data as British National Grid (epsg 27700) and the other as WGS84 (epsf
4326)
When tested in a client such as MapInfo - alignment is exact for both
However when displayed on virtual earth / google maps (using the
projection <54004> +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0
+lon_0=0.0 +x_0=0.0 +y_0=35 +k=1.0 +units=m +no_defs <>)
if using the WMS service from the WGS84.map file - alignment is fine
if using the WMS service from the BNG.map file - the overlay is about
100m off to NW
What I don't understand is why these both work fine in a client such as
MapInfo, but only one fails in Virtual Earth / Google maps - as one
works presumably I've got this working OK ?
The difference is both that when being called from MapInfo the request
uses SRS=27700 so no conversion is required
It seems as if the conversion from 27700 to 54004 is messing up - but
4326 to 54004 works OK ?
OK - looked a bit deeper in MapInfo - and if I change the WMS request
from the BNG.map file to return data as SRS=4326 (as in the query below)
- I get similar miss alignment
GET
/scripts/mapserv.exe?map=C:/Inetpub/wwwroot/MapserverService/Map/WMSTest.map&LAYERS=OSBUILD&VERSION=
1.1.1&REQUEST=GetMap&LAYERS=OSBUILD&STYLES=&SRS=EPSG:4326&BBOX=-3.296092,51.945322,-
3.290018,51.949813&WIDTH=971&HEIGHT=718&FORMAT=image/png&TRANSPARENT=TRUE&BGCOLOR=0xFFFFFF&EXCEPTIONS=application/vnd.ogc.se_xml
HTTP/1.1
I don't remeber having these issues when using mapserver via CGI - it
only seems to relate to WMS
Obviously I can convert all my data to WGS84 and avoid the problem - but
thats not an ideal long term solution
Any ideas
Thanks
James
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