MapServer: using a CRS for which there is no EPSG code

Question

I am developing a MapServer map file for a map created with Goode's Homolosine projection. The PROJ string is rather simple:

+proj=igh +lat_0=0 +lon_0=0 +datum=WGS84 +units=m +no_defs

The IOGP (former EPSG) never assigned a code to this CRS, meaning that it falls into the special case identified in the MapServer manual:

However, it is often impossible to find an EPSG code to match the projection of your data. In those cases, the “wms_srs” metadata is used to list one or more EPSG codes that the data can be served in, and the PROJECTION object contains the real PROJ4 definition of the data’s projection.

Therefore the map file contains the following segments:

WEB
  METADATA
    "ows_enable_request"    "*"
    "wms_srs"               "EPSG:4269 EPSG:4326"
  END # METADATA
END # WEB

# [...]

LAYER
    NAME 'coutries'
    TYPE RASTER
    STATUS DEFAULT
    DATA Countries_Goode.tif
    PROJECTION
        "+proj=igh +lat_0=0 +lon_0=0 +datum=WGS84 +units=m +no_defs"
    END
END # LAYER

So far so good, however, the GetCapabilities request only lists the CRSs declared in the METADATA section:

<Layer>
    <Name>sample</Name>
    <Title>sample</Title>
    <Abstract>sample</Abstract>
    <SRS>EPSG:4269</SRS>
    <SRS>EPSG:4326</SRS>
    <LatLonBoundingBox minx="-1" miny="-1" maxx="-1" maxy="-1" />

And from this I am not able to issue a GetMap request with the original CRS. What is needed to make the original CRS available to the GetMap request (and listed in the GetCapabilities document)?

Answer 1

I would have a try by creating a fake EPSG code into the lookup table "epsg" that is located under the proj directory. On Linux it may be found as share/proj/epsg, in MS4W installation on Windows as ms4w\proj\nad\epsg.

The epsg file has records like

# WGS 84
<4326> +proj=longlat +datum=WGS84 +no_defs  <>

Add your own code as

 # Goode's homologine
 <200200> +proj=igh +lat_0=0 +lon_0=0 +datum=WGS84 +units=m +no_defs" <>

Now it may be possible to refer to this tailored CRS as EPSG:200200 both in your mapfile and by your WMS clients. However, too clever clients like QGIS may fail because they can't initialize their map into EPSG:200200 because they do not know what it is.

You can even create your own namespace by creating a file of that name (for example gisse) in the same location as the epsg file on your system, and reference that instead, so your map file can have:

 "OWS_SRS" "EPSG:4326 EPSG:21095 EPSG:21035 EPSG:21036 EPSG:32635 EPSG:32735 EPSG:32736 EPSG:3857 EPSG:4210 EPSG:4258 ESRI:102022 CRS:84 GISSE:1"

and the GetCapabilities response can be:

...
<CRS>EPSG:3857</CRS>
<CRS>EPSG:4210</CRS>
<CRS>EPSG:4258</CRS>
<CRS>ESRI:102022</CRS>
<CRS>CRS:84</CRS>
<CRS>GISSE:1</CRS>
<EX_GeographicBoundingBox>
    <westBoundLongitude>29</westBoundLongitude>
    <eastBoundLongitude>37</eastBoundLongitude>
    <southBoundLatitude>-2</southBoundLatitude>
    <northBoundLatitude>4.5</northBoundLatitude>
</EX_GeographicBoundingBox>
<BoundingBox CRS="EPSG:4326"
            minx="-2" miny="29" maxx="4.5" maxy="37" />
<BoundingBox CRS="EPSG:21095"
            minx="721844" miny="-224185" maxx="1.61841e+06" maxy="505393" />
<BoundingBox CRS="EPSG:21035"
            minx="721844" miny="9.77582e+06" maxx="1.61841e+06" maxy="1.05054e+07" />
<BoundingBox CRS="EPSG:21036"
            minx="54455.4" miny="9.7787e+06" maxx="945380" maxy="1.04989e+07" />
<BoundingBox CRS="EPSG:32635"
            minx="721913" miny="-224492" maxx="1.61848e+06" maxy="505089" />
<BoundingBox CRS="EPSG:32735"
            minx="721913" miny="9.77551e+06" maxx="1.61848e+06" maxy="1.05051e+07" />
<BoundingBox CRS="EPSG:32736"
            minx="54535.7" miny="9.7784e+06" maxx="945464" maxy="1.04986e+07" />
<BoundingBox CRS="EPSG:3857"
            minx="3.22827e+06" miny="-222684" maxx="4.11882e+06" maxy="501454" />
<BoundingBox CRS="EPSG:4210"
            minx="-1.99745" miny="28.9993" maxx="4.50312" maxy="36.9992" />
<BoundingBox CRS="EPSG:4258"
            minx="-2" miny="29" maxx="4.5" maxy="37" />
<BoundingBox CRS="ESRI:102022"
            minx="29" miny="-2" maxx="37" maxy="4.5" />
<BoundingBox CRS="CRS:84"
            minx="29" miny="-2" maxx="37" maxy="4.5" />
<BoundingBox CRS="GISSE:1"
            minx="29" miny="-2" maxx="37" maxy="4.5" />
...

Proj 6 and later

With Proj 6 and later the CRS database is a Spatialite file. It is still possible to create a custom CRS digestible by MapServer, with a bit of SQL adding a new record to the projected_crs table. The example below copies an existing record with the ESRI authority to a custom with EPSG.

INSERT INTO projected_crs (
       auth_name , code ,
       geodetic_crs_auth_name ,
       geodetic_crs_code ,
       text_definition , deprecated )
SELECT " EPSG " , 54052 ,
       geodetic_crs_auth_name ,
       geodetic_crs_code ,
       text_definition , deprecated
  FROM projected_crs WHERE code = 54052;