[OSRS-PROJ] NTv2 Datum transformation

Brent Fraser bfraser at geoanalytic.com
Thu Nov 20 13:24:30 EST 2003


  I'm trying to work through a problem using proj4 to do datum conversions
using NTv2 (the Canadian NAD83/27 datum shift grid file).  It seems that
Proj4 goes through the list of U.S and Canadian grids as specified in
pj_datums.c (line 75):

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

and uses the first one that returns a reasonable result.  So when I do a
conversion on 49N 122W, it chooses the conus grid instead of the ntv2_0.gsb

I see that with cs2cs I can set the shift file by specifying
"+nadgrids=ntv2_0.gsb". Is there a way I can set the shift file to use using
a proj4 function call (I'm making a VB-callable DLL) ?

Brent Fraser
GeoAnalytic Inc.
Tel:(403) 213-2700
bfraser at geoanalytic.com

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