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Using geoserver I have some layers in CRS EPSG: 20935 (Arc 1950 UTM 35S). In order for them to work with base map layers (in leaflet) I set the native projection to 20395, the declared projection to 4326 (WGS 84) and ask geoserver to reproject to declared. This works and I can see my layers on leaflet map. The base maps are in web mercator but as I understand it 4326 will work.

However, using ArcGIS doing the same reprojection there are ~9 options as to which transformation to use. Given the location of my data number 7 would be most appropriate. Testing ArcGIS's "layer to kml" tool I discovered that ArcGIS defaults to reproject using the first transformation (ARc1950 to WGS84(1)) which leads to an East West shift of 30 - 70 metres compared to transformation(7). So in Arc it is necessary to reproject to WGS84 first (selecting the desired transformation). How can I find out what transformation geoserver is using?

  • Answering the ArcGIS question, In ArcToolbox, open the environment, Output Coordinates. You can set a default transformation there. – mkennedy Aug 24 '16 at 21:10
  • Thanks mkennedy but that is not true for this particular tool. I've tested it on several layers. For some reason the environment settings get overriden when using layer to kml in 10.4 – Oliver Burdekin Aug 24 '16 at 22:27
  • Next possibility. In the ArcGIS install, find the gtdefault.json file in the pedata folder. It's text; open in Notepad. Find the line that has "4209, 4326," at the start, you'll see that it's using 1113 which is the #1 variant. Edit it to use 1119, which is the #7 variant. – mkennedy Aug 24 '16 at 23:40
  • True, about environment setting, it's not listed as supported in the tool help. I should have checked that first. – mkennedy Aug 24 '16 at 23:52
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You should get some details of the projection by going to the "Demos" section in the admin utility and selecting "SRS List". Your GeoServer may even respond to the following URL:

http://localhost:8080/geoserver/web/wicket/bookmarkable/org.geoserver.web.demo.SRSDescriptionPage?3&code=EPSG:20935

This is what my GeoServer 2.10 (master) reports:

PROJCS["Arc 1950 / UTM zone 35S", 
  GEOGCS["Arc 1950", 
    DATUM["Arc 1950", 
      SPHEROID["Clarke 1880 (Arc)", 6378249.145, 293.4663077, AUTHORITY["EPSG","7013"]], 
      TOWGS84[-138.0, -105.0, -289.0, 0.0, 0.0, 0.0, 0.0], 
      AUTHORITY["EPSG","6209"]], 
    PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]], 
    UNIT["degree", 0.017453292519943295], 
    AXIS["Geodetic longitude", EAST], 
    AXIS["Geodetic latitude", NORTH], 
    AUTHORITY["EPSG","4209"]], 
  PROJECTION["Transverse_Mercator", AUTHORITY["EPSG","9807"]], 
  PARAMETER["central_meridian", 27.0], 
  PARAMETER["latitude_of_origin", 0.0], 
  PARAMETER["scale_factor", 0.9996], 
  PARAMETER["false_easting", 500000.0], 
  PARAMETER["false_northing", 10000000.0], 
  UNIT["m", 1.0], 
  AXIS["Easting", EAST], 
  AXIS["Northing", NORTH], 
  AUTHORITY["EPSG","20935"]]
  • Thanks. I'm more interested in the transformation. Presumably the "TOWGS84" line gives some clue. Further investigation leads me to believe it is using transformation (2) cross reffing with the GeoRepository site (georepository.com/transformation_1114/Arc-1950-to-WGS-84-2.html). This is stated as being useful for Botswana. Why would Geoserver choose that one and can I add my own?.... AND... how many web maps are wrong? – Oliver Burdekin Aug 24 '16 at 9:16
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You Can go to the demo section, reprojection console, and get the WKT for the transformation. Check the GeoServer documentazione in hiw to customize the transformation.

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