Projected layer misaligned with Mapserver

Question

I have a couple or raster files in epsg:23031 reference that i want to reproject to other reference systems (epsg:4326 and google projection) using the 'on the fly reprojection' of Mapserver.

No problems using the default projection. But when i try to overlap my layer (in epsg:4326) with google earth there is a 100m misalignment between the two of them.

Any ideas?

Here is the map file:

MAP
 NAME topo
 IMAGECOLOR 125 125 125
 SIZE 600 400
 RESOLUTION 96
 RESOLUTION 300
 MAXSIZE 4096 
 STATUS ON
 PROJECTION
 "init=epsg:23031"
 END
 OUTPUTFORMAT
 NAME png
 DRIVER "GD/PNG"
 MIMETYPE "image/png"
 IMAGEMODE RGB
 EXTENSION "png"
 END
 OUTPUTFORMAT
 NAME GEOTIFF_RGB
 DRIVER "GDAL/GTiff"
 MIMETYPE "image/tiff"
 IMAGEMODE RGB
 EXTENSION "tif"
 END
 EXTENT 443275 4647717 488074 4691799
 WEB
 IMAGEPATH "C:\OSGeo4W/tmp/ms_tmp/"
 IMAGEURL "/ms_tmp/"
 METADATA
 "wms_srs" "EPSG:4326 EPSG:900913"
 END
 END
 LAYER
 NAME orto
 PROJECTION
 "init=epsg:23031"
 END
 TYPE RASTER
 DUMP TRUE
 STATUS ON
 TILEINDEX "orto.shp"
 TILEITEM "LOCATION" 
 EXTENT 443275 4647717 488074 4691799 
 END
END

I'm using mapserver 5.6.4 that comes with osgeo4w tools

Answer 1

I believe that the issue that you are seeing is the result of proj4 (the projection engine behind gdal/mapserver) is not doing the datum transformation for you. This is likely because there are several options for data transforms between 23031 and WGS84 and proj.4 no longer defaults to one of them.

Take a look at this explanation: http://trac.osgeo.org/proj/wiki/GenParms#towgs84-DatumtransformationtoWGS84

If you specified a towgs parameter, you may get a more accurate transformation. If you are requesting the map from MapServer using a normal URL call, you could define your output projection using the proj4 text instead of an EPSG code, something like:

PROJECTION proj=utm zone=31 ellps=intl units=m towgs84=-87,-96,-120 no_defs END

I came up with the towgs84 parameter based on the table here: http://earth-info.nga.mil/GandG/coordsys/onlinedatum/CountryEuropeTable.html You might be able to come up with a better one yourself.

I believe that you are requesting the image to overlay in Google Earth as a WMS, so you will need to modify the epsg database to include the towgs parameter for epsg 23031.

Your EPSG database should be at: C:/OSGeo4W/share/proj/epsg

• DavidF: Or, as Roger said, the basemap data in your area might not be all that accurate...
• Nidleb: I made the changes on the file but there's still the same misalignment. But i think that has to be something like that. I just install geoserver on a test machine with the same raster files, and there is no misalignment so i'm sure that i am doing -somewhere- something wrong
• Nidleb: ehem... the error was tree spaces in the towgs84 line... so this is it, thank you very much!

Answer 2

As crazy as this may sound, maybe the problem is Google Earth and not MapServer. The horizontal accuracy of some Google Earth data in my area is often poor - even by as much as 100m. Hopefully that's not the case in your area of interest, but it's something I'd look into as part of a process of elimination.

• Nidleb: Hey Roger, thanks for your suggestion, i haven't think it, but unfortunately it's not the case. I just check it with an "official" web service imagery source and there's still the 100m misalign