[mapserver-dev] MS RFC 37: AUTO Projection Support

Howard Butler hobu.inc at gmail.com
Mon Feb 22 11:44:07 EST 2010


On Feb 22, 2010, at 10:18 AM, Jeff McKenna wrote:

> Howard Butler wrote:
>> All,
>> I would like to put forward MS RFC 37 to provide a mechanism for layer drivers to be able to provide their projection information automatically.  This feature would be opt-in, and it would require users to set the following in their mapfile:
>> PROJECTION
>>    AUTO
>> END
>> <http://mapserver.org/development/rfc/ms-rfc-37.html>
>> The other items that were once in this RFC, like allowing users to set projection information using data types other than proj.4 have been removed.  It is now simply an implementation for AUTO support and the virtual table methods required to allow drivers to optionally implement support for it.  
> 
> Howard,
> 
> I have some general comments:
> 
> - I have never used this existing 'auto' ability of OGR/raster, mostly because I usually see a blank SRS section from ogrinfo or gdalinfo.
> 
> - for that reason I worry that users will assume that MapServer will magically read the projection info from any source file, when using the PROJECTION AUTO parameter.

If the driver does not implement AUTO support through the virtual method, and the user set PROJECTION AUTO on the layer, an error will be raised that complains that the driver does not support PROJECTION AUTO.

> 
> - I'm not sure if this is the same question as Frank's, but, what happens when no projection info can be read from the source file, when using PROJECTION AUTO?

It will be an unfilled projectionObj, which is what a layer with no PROJECTION object currently ends up with.

> 
> - I think this should be documented with examples (I know, I'm looking in the mirror here)...to make sure users don't expect wonders to happen from PROJECTION AUTO

As Daniel and I were discussing, the first or second thing the performance FAQ will talk about explicitly setting PROJECTION objects on layers a user wants projected, and setting them to nothing in cases where it doesn't matter.  This is opt-in convenience functionality at the cost of performance.  


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