[GRASS-user] ECW Import not matching up

[Frank Warmerdam]

Adam Dershowitz wrote:
> So it seems that the import is NAD83.  Could that be the problem?  
> Shouldn't GRASS correct this on import?  If not, how can I get GRASS to 
> correct this?
> If this is not the problem, does anyone have any suggestions for what 
> else it could be?

Adam,

The errors you see are typical of NAD27/NAD83 differences.  You could
import the NAD83 data into a new location in GRASS with a corresponding
coordinate system, and then reproject it to your working location.  Or
you could pre-warp the imagery before importing it using a command like:

   gdalwarp -t_srs '+proj=utm +zone=13 +datum=NAD27' in.ecw out.tif

Note, r.in.gdal was never intended to support reprojecting of imagery
on input.  It is unfortunate that the coordinate system comparison did
not notice the difference in datum, but comparing coordinate systems is
difficult.

Are you really confident that you want to be doing your project in NAD27?
Depending on your needs, it might be better to do everything in NAD83
(essentially the same as WGS84), instead of converting into NAD27.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent