[postgis-devel] Issue 89 in postgis: transform() grid-shift 2nd chance logic defective

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Comment #7 on issue 89 by fwarmerdam: transform() grid-shift 2nd chance  
logic defective
http://code.google.com/p/postgis/issues/detail?id=89

It is my opinion that a more appropriate location to adjust behavior is the  
+nadgrids
specification for the coordinate system.

The default +nadgrids setting for NAD27 is (with +datum=NAD27):

   +nadgrids=@conus, at alaska, at ntv2_0.gsb, at ntv1_can.dat

The @ prefix means no error is reported if the files are not present, but  
if the end
of the list is reached with no file having been appropriate (ie. found and
overlapping) then an error is issued.

If, conversely, you wanted to ensure that at least the standard files were  
present,
but that if all files were scanning without a hit a null transformation is  
applied
you could use:

   +nadgrids=conus,alaska, at ntv2_0.gsb,ntv1_can.dat,null

The null grid shift file is a valid grid shift file covering the whole  
world and
applying no shift.

I still feel the default behavior (report an error if no covering grid  
shift file is
found) is reasonable, but the behavior is fairly easily modified by  
changing the grid
shift string.  In the case of postgis, this could be done by replacing  
+datum=NAD27
with the above +nadgrids directive in the spatial_ref_sys table.

I'm not too keen on the patch approach.





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