[Mapserver-users] World map projections without inverse projection

[Frank Warmerdam]

Jan Hartmann wrote:
> Hi,
> 
> I have been working on something educational with projections, using a 
> small world map in latlong and projecting it to all supported 
> projections in MapServer 3.7 CVS and Proj4.4.6. Some twenty projections 
> crash MapServer, but work fine when used from command-line Proj. Most of 
> these are from the "Misc" section of the Proj manual, like Apian or 
> Ortelius . What unites them is that they all lack support for inverse 
> projections. Can anyone explain why this is so (or even better, suggest 
> a way to get them working with MapServer)? I can't understand why 
> MapServer would need an inverse projection just to project a map from 
> latlong to these projections.

Jan,

MapServer needs to convert both ways.  It converts the extents rectangle from
the map projection to the layer projection in order to establish a query
rectangle in that projection for which features are required.  Then it needs
to project the feature geometries from the layer projection to the map
projection.

In theory MapServer could be modified to skip the extents rectangle inverse
projection and just fetch all features but I am dubious about the value of
that.

You mention that it crashes MapServer.  Do you mean an actual seg fault, or
just that things fail with an error?  If you can submit a simple case of
mapserver crashing with un-invertable projections into bugzilla (and assign
it to me) I would be willing to fix up mapserver to at least return a
meaningful error.

Best regards,

-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent