v.distance enhanced to report multiple categories

[wilsonadam]

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 is 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

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 a different category from the polygon layer. For example, something like this:

point | fireyear

1 1950

1 1975

1 2002

2 1960

2 1972

3 1954

For reference, I have been able to do this quite easily with a loop in R with the following code: (though this is specific to my dataset, some changes will be needed).

################### 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 with overlapping polygons 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

#########################################

[mlennert]

This is still an issue in 7.2 and trunk and it would still be nice to have this...

[mlennert]

For the record: v.distance actually reports the last of the multiple category values of the 'to' feature.

[neteler]

Ticket retargeted after milestone closed

[martinl]

All enhancement tickets should be assigned to 7.6 milestone.

[martinl]

Ticket retargeted after milestone closed

[martinl]

Ticket retargeted after milestone closed