[Gdal-dev] gdalwarp reprojection shift

[chalmet at telusplanet.net]

Hi,

I am at a loss. Not being the most experienced user, I was hoping if somebody 
could shed some light at the following issue.

I have some data in tiff format. The data is located in Alberta, Canada. It is 
projected in "provincial 10tm" which is a Transverse Mercator projection based 
on NAD83 with the following parameters:
- central meridian: 115W
- false easting: 500000
- false northing: 0
- scale factor: 0.9992

I am trying to reproject this data to latlong, using a different datum: NAD27. 
I have done this in PCI and in ERmapper, and the resulting tiffs line up 
perfectly.

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?

Thanks for any input.
Wim.