[GRASS-user] Appropriate use of v.surf.rst (lidar data)

Wesley Roberts wroberts at csir.co.za
Wed Feb 11 04:22:02 EST 2009

Dear Grass Users,

I am having trouble with interpolating a canopy height model using v.surf.rst and some lidar data. The lidar point density ranges from 5 - 8 points per meter squared. I know I can import the data using r.in.xyz but I would prefer to use an interpolation technique as opposed to a spatial average. I chose r.surf.xyz based on the paper written by Helena Mitasova, Lubos Mitas, and Russell S. Harmon. While the applications differ the use of a large data set with many points means dealing with the same issues. I ran the algorithm using default values and the result was a smooth moon-like surface not really approximating a plantation forest canopy (see link 2)

v.surf.rst input=AreaOne_4pts at Wesley layer=1 zcolumn=dbl_3 elev=Area1_4_def tension=40. segmax=40 npmin=300 dmin=0.050000 dmax=0.250000 zmult=1.0

Given that I am working with very dense data with a high number of points I then ran the algorithm with a smaller tension value (roughly 2.5) with an incrase in the maximum number of points per segment (decrese computation time) and I find that the interpolation is blocky in nature. (see link 3)

v.surf.rst input=AreaOne_4pts at Wesley layer=1 zcolumn=dbl_3 elev=Area1_4_def6 tension=2.5 smooth=0.0001 segmax=100 npmin=300 dmin=0.050000 dmax=0.250000 zmult=1.0 

Is the blockiness related to the increase in the maximum number of points in a segment i.e. segmax value?

I have also been looking at using R to do the interpolation using gstat or automap. Does anyone on the list have any 
advice with regards to methods to use for the interpolation of lidar canopy returns, with a view to creating a 3-D model
of the canopy. The website spatial-analyst (http://spatial-analyst.net) suggests the use of regression-kriging but 
I don't have any ancillary data besides height and intensity.

I have uploaded pics of the study area and interpolation results to flickr (everyone should have access to these)

Link 1
Link 2
Link 3

Many thanks for taking the time to read my post and for your valuable contributions
Wesley (I am using Grass 6.3.0 on Ubuntu Hardy Heron)

Wesley Roberts MSc.
Researcher: Earth Observation (Ecosystems)
Natural Resources and the Environment
Tel: +27 (21) 888-2490
Fax: +27 (21) 888-2693

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

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