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