[GRASSLIST:6385] Computing polygon area after/during v.overlay

Jesse Hamner jhamner at emory.edu
Sat Apr 9 19:12:42 EDT 2005

Hi folks,

I have not been able to compute the area of vector polygons after a 
v.overlay operation. Here's what I've done in the 6.0.0 release of 
GRASS, running OS X 10.3.8:

1. Import ESRI GRID data (2.5-degree pixels, r.in.arc input=name.txt 
output=name1 type=CELL title=name1 mult=1.0 )
2. Import ESRI shapefile (global political boundaries, v.in.ogr 
dsn=cntry92.shp output=esri92shp layer=cntry92 min_area=0.0001 snap=-1 
location=esri92shp -o )
3. Convert the raster file into vector polygons (r.to.vect input=name1 
output=name2 feature=area )
4. Overlay the two vector files (v.overlay ainput=name2 atype=area 
alayer=1 binput=esri92shp btype=area blayer=1 output=name2_overlay 
operator=or olayer=1,0,0 )

All the data are lat-long format, as is the location. After the 
v.overlay, I get topology and the .dbf file shows that I've 
successfully merged the data, but I haven't found a way to compute the 
areas of each clipped polygon. Ultimately, I need to create weighted 
averages (first by area, later by other variables) for each country.

I have 1608 GRID files to import, convert, and overlay. So, it is less 
desirable to convert the files in Maptitude or ARC/INFO and then 
re-import the data, as I can script in Perl/bash but have not learned 
Maptitude's GISDK or the Arc Macro Language (or whatever it's called 
these days--I quit using ARC 6 around 1998...).

I've bought Neteler & Mitasova (2004), I've read through the tutorials 
I could find, and I have searched Google, groups.google, and the gmane 
archive of the GRASS list. Am I missing something? Is there another 
manual I should read for the answer?

Many thanks,


Jesse Hamner
Department of Political Science
Emory University
Atlanta, Georgia USA

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