[gdal-dev] C# Nad27 to WGS 84

[Frank Warmerdam]

Tomas R wrote:
> Hi again,
> 
> New problems arises as the world of Gdal is quite complex to grasp.
> 
> Have understood that the transformation of coordinates from Nad 27 datum 
> to WGS 84 requires some look up tables to be done correctly, tables that 
> differ with the are the Nad 27 is valid in.
> 
> Simple question is, how to set up a general solution in C# that uses 
> these shift tables.
> I have in code a simple test:
>           SpatialReference spatNad27 = new SpatialReference("");
>            spatNad27.SetWellKnownGeogCS("Nad27");
>            SpatialReference spatNad83 = new SpatialReference("");
>            spatNad83.SetWellKnownGeogCS("Nad83");
>                      CoordinateTransformation trans = new 
> CoordinateTransformation(spatNad27, spatNad83);
>            double[] p = new double[3];
>            trans.TransformPoint(p, -117, 30, 0);
> 
> that gives back
> -117, 30,0000000008175
> which I believe is not correct ( see http://proj.maptools.org/faq.html )
> 
> I have Proj4.dll, I have set an environment path to PROJ_LIB where the 
> nad shift tables should exist. Must I do something else in the setup or 
> is the call to the transform different? or have I done correctly but 
> misinterpreted the results?
> 
> What I in reality is interested is transforming whatever coordinate in 
> whatever projection/datum to and from WGS84 without any extra hassle 
> fast and easy via the C# interface. That should not be to hard? Or?

Tomas,

If I test with the testepsg program (using GDAL services) with
CPL_DEBUG and PROJ_DEBUG defined ON I get the following output.

warmerda at gdal64[264]% setenv PROJ_DEBUG
warmerda at gdal64[265]% testepsg -t NAD27 NAD83 -117 30
OGRCT: Source: +proj=longlat +ellps=clrk66 +datum=NAD27 +no_defs
OGRCT: Target: +proj=longlat +ellps=GRS80 +datum=NAD83 +no_defs
pj_open_lib(conus): call fopen(/wrk/home/warmerda/bld/share/proj/conus) - succeeded
Ctable Conterminous United States 273x121: LL=(-131,20) UR=(-63,50)
pj_open_lib(alaska): call fopen(/wrk/home/warmerda/bld/share/proj/alaska) - 
succeeded
Ctable Alaska 529x249: LL=(-194,46) UR=(-128,77)
pj_open_lib(ntv2_0.gsb): call 
fopen(/wrk/home/warmerda/bld/share/proj/ntv2_0.gsb) - failed
pj_open_lib(ntv1_can.dat): call 
fopen(/wrk/home/warmerda/bld/share/proj/ntv1_can.dat) - succeeded
NTv1 393x177: LL=(-142,40) UR=(-44,84)
pj_open_lib(conus): call fopen(/wrk/home/warmerda/bld/share/proj/conus) - succeeded
pj_apply_gridshift(): used Conterminous United States
(-117.000000,30.000000,0.000000) -> (-117.000806,30.000113,0.000026)

I don't know if it is practical for you to enable debugging and see
stderr output, but if you can it might help you deduce what is
or is not working.

It *sounds* like you have done what is necessary, but it is hard
for me to know if you have made some mistake, such as preprocessing
the grid shift file into binary form.

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