[MAPSERVER-USERS] how mapserver handles different datum projections

[Frank Warmerdam]

DNC wrote:
> Hi all.
> 
> Can anyone clarify how mapserver handles different datum in the map
> projection and in the layers projection when serving a wms service?
> Does MS apply datum transformations? If not, can it be done somehow without
> converting the layers?
> 
> In my case I'm working with vector data, and have defined a map projection
> using "init=epsg:code" syntax. Then at the layer level I use the same syntax
> to declare the projection the data is in. 
> And finally I want my WMS service to be published in 3 different datum and
> for that I advertise several epsg codes in the "wms_srs" metadata entry.
> 
> I'm noticing a shift in images produced by MS that indicate a datum shift is
> not being applied. What can be done?

Duarte,

In general MapServer will apply a datum shift if you have datum definitions
for the source and destination coordinate system.  But often the definitions
in the EPSG init file lack datum shift info and so nothing is done.

So, start by examining the PROJ.4 definitions in the "epsg" init file and
see if they have +towgs84= or +nadgrids directives.  You might also want to
skim the material in the PROJ.4 FAQ about datums.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org