On Sun, 16 Nov 2008, Kurt Springs wrote:
Hi folks,
I am trying to convert ascii files that I bought from the Irish Ordinance
Survey in to GRASS vector format. These are digital terrain models. I want
to eventually make a raster surface map and a 3d raster map out demonstrating
where various archaeological sites are.
The files the OSI has sent me have a heading set off with "#" on each line,
then the data. There are three columns, eventually x, y, z. These are
separated by a space.
Some years ago I obtained gridded DEM data from the Ordnance Survey of
Northern Ireland, and I wonder if this was the same format. The two
Ordnance Surveys in Ireland do seem to do a reasonable amount of
cross-border co-operation. Anyway (I didn't actually use the gridded DEM
data; they just sent me a sample to see if it would be useful for our
purposes) the data I obtained was gridded at 50m resolution, i.e. it was
already in raster format so as Hamish said could be directly imported as
a raster. Could you maybe post a short sample of a few lines of the data
format so that we can see the format exactly? ISTR (this was perhaps 6
years ago so memory a little hazy) that I wrote a Perl script to process
the DEM format - if it *is* the same format I could try digging it out for
you.
What is the resolution of the data? There are global free elevation
datasets with a better resolution than 50m these days. Also with the OSNI
data I realised that it had just been interpolated from contours with some
other GIS, and I decided I was much better interpolating my own DEM
directly from vector contour maps using one of the superior algorithms in
GRASS. I got the vector contour maps from OSNI in DXF format and imported
them into GRASS with v.in.dxf. I would recommend this approach over
pre-gridded DTM data if possible, as it gives you more control over
interpolating a DEM that is most useful for your purposes.
Paul
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