FYI -------- Messaggio originale -------- Oggetto: Re: [postgis-users] How do you use PostGIS Raster? Data: Mon, 29 Aug 2011 09:52:44 -0400 Mittente: Mathieu Basille <basi...@ase-research.org> Rispondi-a: PostGIS Users Discussion <postgis-us...@postgis.refractions.net> A: PostGIS Users Discussion <postgis-us...@postgis.refractions.net> CC: marie-...@lists.refractions.net, Pierre Racine <pierre.rac...@sbf.ulaval.ca>, ude Labbé <marieclaude.m.la...@gmail.com>
Dear list, I am resurrecting this thread to honour an old promise to Pierre. This might be of interest for those interested in using PostGIS for the study of wildlife movements. First, a summary of our use of PostGIS: We are using PostGIS to analyze large datasets combining rasters and vectors. Basically, we are looking at movement and habitat selection in a predator-prey system, using GPS-collar data over large areas. The major use of PostGIS Raster is for the intersection of prey steps (line segments or buffers around these segments) with different raster: Landsat (landcover type), slope, road density, relative probability of occurrence of the predator, etc. Note that we first tried to use ArcGIS for this step, without success, due to many bugs and the need to correct them by hand (which was largely too time-consuming). We also used PostGIS Raster to intersect predator locations with the Landsat map to estimate Resource Selection Functions (RSF) in order to build maps of relative probability of occurrence. In the end, information of the intersections were used in R to characterize movements of the preys on the landscape. Second, some numbers: we generally work on rasters (LandSat, road density, DEM) of approximately 150e6 pixels (25m×25m), i.e. 12600×11900, and occasionally on a much larger raster at the scale of Québec (LandSat) of approximately 1700e6 (20000×850000) pixels, same resolution. We are intersecting these maps with a total of 850 000 GPS relocations of four species, i.e. slightly less line segments between each relocation (so-called steps). We generally contrast each step with 10 randomly generated steps, which means that we are working on hundreds thousand steps at a time (we generally process them by season/species). Here are some processing times on a Intel Xeon station (4 cores @2.67 GHz) running Debian Linux, with 12 GB of RAM: - Intersection of 1 000 000 steps (mean=200m) takes 7.3 h on the most complex rasters (i.e. rasters which vary a lot along the steps, hence increasing computation times). Speed seems fairly constant, with approximately 130-140 thousand steps per hour. - On simpler rasters (i.e. less variation along the steps), computation time can be up to 10 times higher (1.5e6 steps per hour). - Intersection of 57 000 buffers (i.e. polygons around points, radius = 300m) takes almost 1h to get the composition of the buffer. Hope this is useful! Best, Mathieu Basille. _______________________________________________ postgis-users mailing list postgis-us...@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users _______________________________________________ AniMov mailing list AniMov@faunalia.it http://lists.faunalia.it/cgi-bin/mailman/listinfo/animov
