I figured out a solution to my problem using R, which is posted below for any future user's benefit (though this is specific to my dataset, some changes will be needed). I would still like to know if it is possible in GRASS, though. Thanks,
Adam ################### This code intersects a set of points with any number of polygons (which may be overlapping) library(sp);library(rgdal) fires=readOGR("/media/Data/Work/Regional/CFR/FireAnalysis/FireData/fires_15072008","all_fires_07_08") #read in shapefile d=as.data.frame(cbind(slot(fires,"data")[1,],point=1)) #use first row as template, add "point" as placeholder to be filled later nfires=nrow(slot(fires,"data")) #get the number of polygons for(i in 1:nfires) { #loop through each polygon one at a time d2=overlay(fires[i,],points) #do overlay of all points for each fire polygon (this may result in lots of NAs) d2$point=as.factor((1:nrow(d2))+100) #add point ID - my point IDs start at 101 and go up, you will have to adjust this d=rbind(d,d2) #bind this polygon's overlay to the previous one print(paste(i," out of ",nfires)) #print progress } d=d[-1,] #remove first line - used to start dataframe d=d[!is.na(d$FIREREFERE),] #get rid of all the NAs using a field that is always populated d2=merge(d,slot(points,"data"),by.x="point",by.y="Locality_n",all.x=T) #merge with point data to get point attributes for each point ######################################### On Wed, Dec 10, 2008 at 9:13 AM, Adam Wilson <[EMAIL PROTECTED]>wrote: > Greetings all, > > Summary: I would like to request that the v.distance function be updated > (with a flag?) to allow reporting of multiple categories in one layer. > > Example of why this can be important: > I am working with historical forest fire data and want to extract histories > for different points. I start with a shapefile that contains many (1000s) > of polygons, many of which overlap in x-y space but vary over time (time > information is in the attribute table). I could separate this into separate > shapefiles (one for each year) prior to importing to GRASS, but this would > result in almost 100 different layers and I would rather avoid it. When I > import it into grass using v.in.ogr, it is topologically cleaned and the > result is a layer of (intersected) polygons, many of which have multiple > categories that link to the attribute table. For example, a single polygon > could have burned in multiple years, so it is linked to multiple rows in the > attribute table. These multiple categories are visible with "v.category -g > input=fire option=print" which results in something like this: > 2452 > 2452/2540 > 2452/2526/2540/2543 > 2540/2575 > 2406/2420/2517/2563/2581/2584 > 2406/2420/2517/2563/2581 > 2416/2452/2536 > Where each row is a unique polygon and the different elements are the > various categories (rows in the attribute table) that are linked to it. So > far so good. But what I want to do is extract the fire history for a number > of points, but v.distance only reports the last category for each polygon > (which in my case is usually only the most recent fire) and reports "*WARNING: > more cats of to_layer*." So there seems to be a hidden ID value for each > polygon (which would correspond to the invisible row number in the output > above) but I cannot seem to access it directly. If I could, then it would > be possible to v.distance to that ID, then use the output above to link a > given point to several fire records. > > If v.distance was updated to include multiple categories in the same layer, > I would be able to do this easily. This has been proposed before: > http://www.mail-archive.com/grass-user@lists.osgeo.org/msg02056.html. I > would like to encourage this revision (though maybe with a -m flag so you > could turn this feature on if wanted). It would ideally (for me) return a > table with multiple records for each point, each with an attribute from the > polygon layer. For example, something like this: > > point | fireyear > 1 1950 > 1 1975 > 1 2002 > 2 1960 > 2 1972 > 3 1954 > > In the short term, does anyone have any ideas on how I can get extract > this data? > > > Thanks for any ideas, > > Adam Wilson > > -- Adam Wilson __o http://hydrodictyon.eeb.uconn.edu/people/wilson/ _`\<,_ Department of Ecology and Evolutionary Biology (*)/ (*) BioPharm 223 University of Connecticut Tel: 860.486.4157 [EMAIL PROTECTED]
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