[GRASS-user] Using v.in.ascii to create elevation vector

[Paul Kelly]

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