[GRASS-user] v.rast.stats and Lidar data

Hamish hamish_b at yahoo.com
Wed Jun 18 13:44:12 EDT 2008

Hi Wesley,

> I have some LiDAR data of a forested area and I am trying
> to calculate tree heights / canopy heights based first and
> second return points (pre-filtered ground and non-ground).
> I have two questions that I hope some of you may be able to
> help me with. Firstly I have windowed / clipped my data to a
> small 'test' region with about 150 000 non-ground
> and 100 000 ground points.
> 1. I have interpolated the ground points using idw in grass
> and have taken that as my ground surface. I would now like
> to assign an extra column to my non-ground vector file and
> populate it with the raster values of the DSM to calculate
> actual point height above ground (points are currently in
> meters above sea-level). Traditionally, one would just
> create a digital surface model and a canopy model and
> subtract the two to get tree canopy height. I would like to
> do the subtraction in the database and then interpolate the
> result. I am trying to do this with v.rast.stats and it is
> taking a really long time. How can I speed this up? Reading
> the help page I also see that the vector data are rasterised
> and values are assigend based on categories. I dont want
> categories, I am just interested in the raster value
> directly below each point? Is v.rast.stats the correct
> application to use?

No, r.in.xyz is. ;)  Well at least it is much less trouble to work with.

(or if you want to stick with your current method, use v.what.rast instead of v.rast.stats to pick off single points at the data point; and v.surf.rst might make a nicer surface than IDW, at the risk of overshoots)

> 2. While the above mentioned data set is fairly small my
> largest data set is around 700MB with 
> wc -l  =  20602625 AreaThreeNonGround.xyz
> If I import using v.in.ascii without a db and topology will
> I still be able to manipulate the vector data as described
> above

without a DB you can not populate a DB..
v.surf.* have been modified to work even without topology, just for this case.

r.in.xyz should handle 700MB input|20m points without complaint.

> and will I be able to use R scripts and functions on
> the data set?

don't know.
> Apologies for the long mail, I hope it is not too
> confusing.
> Many thanks and looking forward to your comments and
> suggestions.

hopefully r.in.xyz solves your problems, it is updated with more statistics and features in GRASS 6.3.0 if you can get your hands on that.



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