[postgis-users] NAD conversion problem

[Frank Warmerdam]

Peter N. Schweitzer wrote:
> So my hypothesis--please help me understand how wrong this is--is that the
> @null has the effect of keeping proj going when I project a point that is
> not in the areas covered by the nad27-to-nad83 conversion tables.  

Peter,

Yes, this is the expected behavior of including null in a list of
nadgrids. It is a fallback for quietly doing nothing if no other matching
grid is found.  BTW the @ prefix means it's ok for the grid to not be
found at all.   So if you wanted conus to be used for the continental
US, and do nothing outside that region you could use +nadgrids=conus, at null
while +nadgrids=@conus, at null means if the conus grid does not exist it is
ok to do nothing everywhere.

 >I have
> to admit, as you probably suspect already, that I really don't know this
> stuff very well.  And since the data are not highly accurate in any case,
> the difference won't matter much in any analysis of the converted data.
> But this is what I'm thinking.  Your counsel would be welcome!

OK, as long as you understand the potential for error involved.

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