[mapserver-users] PostGIS data in EPSG:31300 using for Google Maps overlay

[Brent Wood]

--- Steven De Vriendt <gisaalter at gmail.com> wrote:

> Hi list,
> 
> I have some data in PostGIS I wish to overlay in Google Maps using WMS
> I'm using the same data in a mapserver application, where EPSG code
> 31300 is used.
> Now for overlay in Google Maps, 4326 is required.
> I have made a copy of my map-file and specified 4326 as projection
> system.Yet, non of my data is showing up.
> 
> Do I need to reproject my data for this use ? I thought PostGIS was
> capable of doing reprojection on the fly ?

Hi Steven,

Both mapserver & POstGIS can reproject the data on the fly, but they need to be
told the input & output projections.

As your data is all displayed via mapserver, the instructions either way will
be in your mapfile. 

To reproject using PostGIS, edit the SQL command in your data statement which
retrieves the PostGIS data. You need to tell PostGIS to return a virtual table
to mapserver, and transform the data to the required projection in this. Edit
the data line in the layer something like this:

DATA "new_geom from (select gid, transform(the_geom,4326) as new_geom from
table) as mytable using unique gid using srid=4326" 


To reproject in mapserver, you need to set the main mapfile projection to
EPSG:4326, using, in the global header part of the mapfile:

UNITS DD
PROJECTION
  "init=epsg:4326"
END

In the METADATA section, you'll need to set the output projection for WMS, eg:
METADATA
  "wms_title" "My WMS Server"
  "wms_srs"   "EPSG:4326"
END

Then in the layer definition, you specify the layer's native projection, so
mapserver can reproject from one to the other:

LAYER
  ...
  PROJECTION
    "init=epsg:31300"
  END
  ...
END

Note that to reproject in PostGIS, Proj.4 support must be compiled into
PostGIS, and similarly for mapserver if you do the reprojection there. If you
compiled these from source, this is not done by default, & you'll need to run
configure with the appropriate flag. If you installed from a package, or on
Windows, proj support is generally built in.


HTH,

  Brent Wood