[GRASS-user] v.variogram: maxdist and nlag

[Wesley Roberts]

Dear Grass users

I am using grass 6.2.3 and have downloaded the v.variogram module from the Grass_AddOns website (http://grass.osgeo.org/wiki/GRASS_AddOns#Vector_add-ons). I wish to run the v.variogram module on a number of forest compartments for which I have LiDAR canopy heights. I have tried various combinations of maxdist and nlags but cant seem to find the 'sweet-spot' so to say. 

A colleague of mine suggested that the lag distance multiplied by the number of lags should equal about half the largest distance within a particular compartment. Is this a suitable 'rule-of-thumb' to use? 

I am interested in identifying the range of the semi-variogram as this value will determine the width of pseudo-flight lines I intend to use to sample the lidar data. Our point density is upwards of 5 points per square meter captured over even-aged managed Eucalyptus plantations with an espacement of 2 meters between trees and 3 meters between rows. Given the structural characteristics one would assume that semi-variance should reach a plateau (sill) at around 6m (canopy width), however my preliminary research indicates that the location of the plateau varies based on maxdist and nlags, how does one manage this to effectively quantify the distance at which spatial auto-correlation no longer applies?

I hope this all makes sense and that the question is appropriate for this mailing list.

Many thanks and looking forward to your responses.
Wesley


Wesley Roberts MSc.
Researcher
Earth Observation Research Group (Ecosystems)
Natural Resources and the Environment
CSIR
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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