On 11/12/08 14:56, Adam Wilson wrote:
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.
At this stage, I don't think it is, but it shouldn't be too hard to add
'to_id' as one of the upload options to v.distance. Please file a
feature request to the bug tracker, so we keep that in mind.
Moritz
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$point=as.factor((1:nrow(d2))+100) #add point ID - my point IDs
start at 101 and go up, you will have to adjust this
d=rbind(d,d2) #bind this polygon's overlay to the previous one
print(paste(i," out of ",nfires)) #print progress
}
d=d[-1,] #remove first line - used to start dataframe
d=d[!is.na <http://is.na>(d$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
<adammichaelwils...@gmail.com <mailto:adammichaelwils...@gmail.com>> 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
adam.wil...@uconn.edu
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