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

Wesley Roberts wroberts at csir.co.za
Wed Jun 18 12:00:38 EDT 2008

Dear mail list

I am using Grass 6.2.2 on Ubuntu 7.10

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?

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

Apologies for the long mail, I hope it is not too confusing.

Many thanks and looking forward to your comments and suggestions.

Wesley Roberts MSc.
Researcher: Forest Assessment (Remote Sensing & GIS)
Forestry and Forest Products Research Centre
Tel: +27 (31) 242-2353
Fax: +27 (31) 261-1216

"To know the road ahead, ask those coming back."
- Chinese proverb

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