[OSRS-PROJ] NTv2 Datum transformation

[Frank Warmerdam]

Brent Fraser wrote:
> I get it now.
> 
> The way I set up my functions was to define a "from" and "to" coordinate
> system definition (here's my VB):
> 
>     sToCSD = "+proj=latlong +datum=NAD27"
>     hToCSD = GPinitializeProjection(sToCSD) ' a VB friendly wrapper to
> pj_init
> 
>     sFrCSD = "+proj=latlong +datum=NAD83"
>     hFrCSD = GPinitializeProjection(sFrCSD)' a VB friendly wrapper to
> pj_init
> 
>     status = GPdatum2datum(hFrCSD, hToCSD, -122, 49, newLon, newLat) ' a VB
> friendly wrapper to pj_transform
> 
> 
> It never occured to me to simply add the nadgrids keyword to the NAD27
> coordinate system def:
>     sToCSD = "+proj=latlong +datum=NAD27 +nadgrids=NTv2_0.gsb"

Brent,

I would add that you should not include a +datum and a +nadgrids clause
as it becomes ambiguous whether the +nadgrids from the +datum will be used
or the standalone one you include. So the better forumulation would
be "+proj=latlong +ellps=clrk66 +nadgrids=NTv2_0.gsb".

Best regards,

-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent


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