Michael Perdue wrote: > > > I read through the one of the papers today ( M. A. Brovelli , M. > Cannata and U. M. Longoni, 2002. Managing and processing LIDAR data > within GRASS, Proceedings of the / Open source GIS - GRASS users > conference 2002) and the authors recommended a step value or 3 to 4 > time the average point spacing of the data set. So 4m sen & see values > seems appropriate for a data set that has an average coverage of 1 > return/m^2. Unfortunately, the authors didn't really explain why it > should be 3-4 times and not say 10. I can only guess that a smaller > step size would produce finer details in the interpolation and better > feature isolation for the classification./ > > There definitely seems to be something funky going on between the > spline step size and the division of the region into subregion. I ran > tests using v.lidar.edgedetection on tiles of 500x500m, 800x800m , > 801x801m and 1000x1000m. Times for each run > were 1m54.065s, 6m35.420s, 22m16.708s & 68m25.265s respectively. There > was a ~3 fold increase in processing time when I grew the data set by > 1m from (800m)^2 (what I understand to be the maximum size of the > region before it is broken up into subregions) to (801m)^2 and > it tripled again when I jumped to (1000m)^2. I also recompiled grass > with NSPLX_MAX and NSPLY_MAX set to 1500 instead of the current 200 > and the time to process a (1000m)^2 tile dropped from 68 minutes to 12 > minutes. > > My understanding is that a region is broken up into smaller subregions > to avoid memory limitations while processing. I can't see any reason > why the division of the region into more smaller subregions should > increase the resolution of those subregions. I'm far from an expert on > these programs, and I'm not skilled enough to read through the code > and know what it is doing, but if I understand you correctly, your > arguments make sense to me. If you're kind enough to share your > changes I'll apply them and do some more testing. I didn't get to look into that in more detail, stopped at edgedetection.c without getting a working solution. The general idea is either to use the current geographic region resolution or to use point density for estimated resolution, then use sen and see as step size. I can not possibly say if that makes sense, I have to read the references, but will not get to it soon. How about contacting the original authors?
Markus M _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
