[MetaCRS] A Question About The Typical Method Of CRS Transformations

[Frank Warmerdam]

Landon Blake wrote:
> I had a question about the typical method used by existing CRS code in 
> different projects to perform transformations from one CRS to another.
> 
>  
> 
> Does most code search for the definition of a transformation algorithm, 
> with no real knowledge of the projection geometry of the source and 
> destination CRS, to perform the transformation?
> 
>  
> 
> Or does most code, on the contrary, use knowledge of the projection 
> geometry of the source and destination CRS to determine the 
> transformation algorithm on its own?

Landon,

I'm afraid I don't really follow the essence of your question.  As others
have pointed out it is typical to implement a bunch of particular projection
methods (ie. Transvere Mercator, Lambert Conformal Conic), that take a set of
parameters, and then use various sorts of configuration files, databases, etc
to identify the parameters and method for a particular CRS.

Another thing that *seems* to be suggested by your question is whether some
systems need to know the extents of the input and output to setup reprojection.
Generally this is not the case, though in some cases knowing the area being
operated on could be helpful in automatic selection of the most appropriate
datum shift (for instance).  I'm generally against this sort of automatic magic
because it can cause very hard to understand differences, but it is possible.

But perhaps you were getting at something else?

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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