[GRASSLIST:2155] Re: Where's the data within a cell?

[Glynn Clements]

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.
> 
> Roger,
> 
>   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
> somewhere.

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:

ULXMAP        -19.99583333333333
ULYMAP        89.99583333333334

-- 
Glynn Clements <glynn.clements at virgin.net>