[GRASS-user] LIDAR Data

[Hamish]

Dr. Manuel Seeger wrote:
> we have obtained huge airborne laserscanner data, and want to try out 
> the modules in GRASS.
..
> Another question. How many points can be handled with grass and these 
> modules (or grass vector files in general)?


For vector point data loaded with v.in.ascii & the default settings you
can probably get up to ~1 million points before running out of memory.
(happens in the "building lines" step of building topology, watch memory
grow with `top`, M)

see the NOTES section,
 http://grass.ibiblio.org/grass63/manuals/html63_user/v.in.ascii.html

The first thing you do can do to save memory is -t to skip creating a DB
table. If data is just x,y,z you don't need one, just use -z and make a
3D vector map. (no DB table is the default if no extra columns besides
xyz?) This will get you up to ~1 million(?) points before running out of
memory.

Next you could try the new (6.3 only) v.in.ascii -r flag to only import
points which are in the current region (area of interest). [g.region]

The final v.in.ascii flag to try is -b, which skips building topology
altogether. This means that the new vector can't be used for much, but
the max point limit becomes 25+ million (as high as Helena's has tested
I think). Then you start to get into LFS 2gb file size problems, ..?
While there isn't much you can do with a vector map without topology,
one important thing you can do is run v.surf.rst to make a surface.
It is hoped that v.surf.idw(2) and v.univar can be updated to work with
them as well.


The "other way" is to use r.in.xyz to create a raster directly from
the points. This is very fast and the only limit is from the LFS
issues, but I'd guess you could load 4gb of data without a problem.
It is hoped to one day update r.in.xyz to optionally read data from
stdin, in which case you can avoid and file system issues*. Then AFAIK
there would be no limit, as no file to open. LFS support is much better
for rasters than it is for vectors currently.

[*] would need to disable scan mode if input=stdin

 http://grass.ibiblio.org/grass63/manuals/html63_user/r.in.xyz.html
 http://hamish.bowman.googlepages.com/grassfiles#xyz


Hamish