[Gdal-dev] GMT & GDAL Geotiff grid registration

[Rich Signell]

Frank Warmerdam wrote:

 > Why the terminology of "LINE" vs. "PIXEL"?

Frank,

Only because "LINE"/"PIXEL" is the terminology GMT uses.
(while "POINT"/"AREA" is the terminology GeoTIFF uses)

LINE is the same as POINT
PIXEL is the same as AREA

I'll stick with "Area" and "Point" terminology here, since GeoTIFF 
is more universal that GMT.

> You are suggesting the node_offset on the "z"
> variable should be examined to determine if the x and
> y locations are relative to the top left corner of the pixel or
> the center?  

Yes.  The question is what to do with that information, I guess.

Since GeoTIFF can be specified as  "Area" or "Point", one approach 
would be to write the appropriate value for the GeoTIFF based on 
the "node_offset" attribute on the "z" variable.

But if applications that deal with GeoTIFF are always expecting 
"Area" and don't generally handle "Point", then it may better to 
just expand the range by 1/2 a grid cell when "Point" GMT grd 
files are found.

I've place examples of "Point" and "Area" GMT grd files at:

http://stellwagen.er.usgs.gov/rps/share/point.grd
http://stellwagen.er.usgs.gov/rps/share/area.grd

They are EPSG:32618, just in case that's useful.

-- 
Richard P. Signell
U.S. Geological Survey       Phone: (508) 457-2229
384 Woods Hole Road          Fax:   (508) 457-2310
Woods Hole, MA  02543-1598