[Gdal-dev] gdalwarp reprojection shift

[chalmet at telusplanet.net]

Hi Matthew, I replied yesterday, but apparently only sent it to you, so here is 
a resent of my problem.

I actually am using FWtools (windows version)- gdalwarp is part of that. So, 
according to what you wrote, this version should handle the datum shift 
correctly.

Is there anybody who could help me (step by step instructions would be 
extremely appreciated, as I am rather new to gdalwarp and proj4 and such) 
handle this datum shift correctly.

Thanks!
Wim.

Quoting Matthew Perry <perrygeo at gmail.com>:

> Wim,
> 
> On 8/16/06, chalmet at telusplanet.net <chalmet at telusplanet.net> wrote:
> > If I do this using gdalwarp, I have a shift of about 70 meters compared to
> my
> > reprojected tiles in PCI and ERmapper. I use the following command for
> gdalwarp:
> >
> > *******************
> > gdalwarp -s_srs "+proj=tmerc +datum=NAD83 +x_0=500000 +lon_0=-115 +lat_0=0
> > +k=0.9992" -t_srs "+proj=latlong +datum=NAD27" -rc -tr 0.000035 0.000035
> > 400113.tif 400113_latlong.tif
> > *******************
> >
> > Does anybody know why this is happening. And more importantly, how could I
> make
> > the gdalwarp tiff line up with my other tiffs, meaning avoid the 70m
> shift?
> 
> 
> Most likely your version of proj4 (which gdal uses for reprojecting
> coordinates) is not built with support for datum shifts. The proj4 faq
> is pretty helpful for this issue...
> http://proj.maptools.org/faq.html.
> 
> You might have to rebuild proj4 with the datum support files or you
> can download FWTools (for win and linux) that has proj4 precompiled
> with the datum shift files.
> -- 
> Matt Perry
> perrygeo at gmail.com
> http://www.perrygeo.net
>