[Gdal-dev] converting from NAD 83 to NAD 83 HARN

[John Hayes]

Frank,
  Thanks for the ideas.  I found some HARN grid shift files the other day so I will give this a try in the next few days.

Thanks,
-john

-----Original Message-----
From: Frank Warmerdam [mailto:fwarmerdam at gmail.com]
Sent: Wednesday, July 06, 2005 5:09 PM
To: John Hayes
Cc: gdal-dev at xserve.flids.com
Subject: Re: [Gdal-dev] converting from NAD 83 to NAD 83 HARN


On 7/6/05, John Hayes <jhayes at lizardtech.com> wrote:
> Hi
> 
> I'm trying to reproject an image from "Arizona Central State Plane NAD 83" to "Arizona Central State Plane NAD 83 HARN".  This should shift the image by (X=-0.100, Y=0.182) meters according to http://www.ngs.noaa.gov/cgi-bin/nadcon2.prl.  PROJ4 does not seem to be applying the shift.
> 
> Does PROJ4 support HARN corrections?
> 
> I have the "normal" NADCOM grid shift files installed and a 2005-04-25 snapshot of GDAL.

John,

PROJ/GDAL is not distributed with any HARN related grid shift files
as far as I know, and in any event, the OGRSpatialReference needs
to have special cases in the WKT<->PROJ.4 translation to use grid
shift files, which it does not have for HARN. 

How are you specifying the HARN coordinate systems?  By EPSG
code?  

I think the solution to this issue would be for GDAL to support a WKT
extension for specifying grid shift files, as opposed to the TOWGS84
method of defining a datum.   I think ESRI WKT already has this feature,
so I could do it similarly.  However, this isn't really high on my 
priority list at this time. 

If you just wanted to hack something in, you would need to look in the
gdal/ogr/ogr_srs_proj.cpp code circa line 1670 (search for 
+datum=nzgd49).   You would also need to read up on use of
grid shift files on the PROJ.4 web site (the +nadgrids option) and
dig up suitable HARN grid shift files. 

All in all quite alot of work!

Best regards,
-- 
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