[GRASSLIST:2155] Re: Where's the data within a cell?
glynn.clements at virgin.net
Wed Jul 25 20:10:20 EDT 2001
Rich Shepard wrote:
> > The location produced by d.what.rast is the cell center. The location
> > produced be d.where is something more like the pixel location. The
> > coordinates bounding a region covered by a raster map are the coordinates
> > at the outside edge of the bounding row or column of the raster map.
> Thank you, that's very good to know. I wonder where the point is for USGS
> DEMs imported via r.in.gdal. Perhaps Frank will answer this for us, or I
> need to dig into the SDTS DEM metadata and see if it's defined there
I'm not quite sure what the question is, so I'll just dump some info.
A typical USGS DEM covers 40°x50° (lon x lat), tiled into 4800x6000
cells, each 1/2 minute square. The edges are all at exact 10°
multiples. The UL[XY]MAP values in the .hdr file are of the centre of
the top-left cell.
E.g. w020n90 covers 20W to 20E and 90N to 40N; the top-left cell
covers 20:00:00W to 19:59:30W and 90:00:00N to 89:59:30N, with its
centre at 19:59:45W and 89:59:45N.
This gives the .hdr values:
Glynn Clements <glynn.clements at virgin.net>
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