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Appendix A: Converting coordinate system in data files

As mentioned in the documentation on setting up the WMS, the OneGeology project requires that your WMS can serve data in latitude-longitude coordinates with the WGS84 ellipsoid and datum (EPSG:4326). If your data files are stored in a different coordinate reference system, MapServer can convert the coordinates to EPSG:4326 or other client requested coordinate reference systems on-the-fly. However, to reduce the load on your server, as we can expect that a substantial proportion of requests to OneGeology servers will be for EPSG:4326 then we suggest that you convert your underlying data sets to this coordinate reference system so that the conversion won’t have to be carried out on every request. The same tools that MapServer uses internally are available with command line programs bundled in the MS4W package and can be used to convert your underlying data sets as follows.

For shapefiles the main program you will want to use is ogr2ogr.exe which is located in ms4w\tools\gdal-ogr where ms4w is the top-level folder of your MS4W installation. The easiest way to use the programs is to run the batch file ms4w\setenv.bat from a DOS window which will set up your path. (You may need to edit the setenv.bat file to reflect the location where you have installed MS4W.) Next you need to find out whether your current data set has a coordinate reference system assigned to it. If you have, for example, a dataset in a shapefile called datafile.shp you would issue a command like:

:\ > ogrinfo -so datafile.shp datafile

(The first datafile.shp refers to the file name; the datafile afterwards is a layer name which is redundant in the case of shapefiles which only have one layer but is the way ogrinfo works.) You should get some information including something like that below:

Layer SRS WKT:
PROJCS["British_National_Grid",
 GEOGCS["GCS_OSGB_1936",
 DATUM["OSGB_1936",
 SPHEROID["Airy_1830",6377563.396,299.3249646]],
 PRIMEM["Greenwich",0.0],
 UNIT["Degree",0.0174532925199433]],
 PROJECTION["Transverse_Mercator"],
 PARAMETER["False_Easting",400000.0],
 PARAMETER["False_Northing",-100000.0],
 PARAMETER["Central_Meridian",-2.0],
 PARAMETER["Scale_Factor",0.  999601272],
 PARAMETER["Latitude_Of_Origin",49.0],
 UNIT["Meter",1.0]]

The details do not matter as long as you don’t get the below:

Layer SRS WKT:
(unknown)

In this situation you will need to find out what coordinate system your data is in. If the data has a coordinate system assigned you can issue a command like that below to convert the data (note that the destination file is specified before the source file):

:\ > ogr2ogr -t_srs EPSG:4326 new_datafile.shp datafile.shp

If your data set does not have a coordinate system assigned to it but you have found out what it is you can specify the source coordinate system on the command line with the parameter -s_srs, for example:

:\ > ogr2ogr -s_srs EPSG:27700 -t_srs EPSG:4326 new_datafile.shp datafile.shp

For GeoTIFF files the utilities you will want to use are gdalinfo.exe and gdalwarp.exe. Issuing a command like: gdalinfo imagefile.tif will result in some information including projection information like that below:

Driver: GTiff/GeoTIFF
Size is 522, 252
Coordinate System is:
GEOGCS["WGS 84",
 DATUM["WGS_1984",
 SPHEROID["WGS 84",6378137,298.2572235630016,
 AUTHORITY["EPSG","7030"]],
 AUTHORITY["EPSG","6326"]],
 PRIMEM["Greenwich",0],
 UNIT["degree",0.0174532925199433],
 AUTHORITY["EPSG","4326"]]
Origin = (-180.,000000000000000,83.879999999999995)
Pixel Size = (0.690000000000000,-0.690000000000000)
Metadata:

You can transform an image in a similar way to the ogr2ogr utility for shapefiles but unlike ogr2ogr the source and destination files are specified in the more common source then destination file order so typical command lines would be:

:\ > gdalwarp —t_srs EPSG:4326 imagefile.tif new_imagefile.tif

or

:\ > gdalwarp -s_srs EPSG:27700 -t_srs EPSG:4326 imagefile.tif new_imagefile.tif

Section last modified: 19 January 2010

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