[GRASS5] attributes using v.overlay

Otto Dassau otto.dassau at gmx.de
Mon Sep 6 11:13:26 EDT 2004

Dear list, 

I used the module v.overlay with operator "or" -> union, but I guess it`s the same for all operators. I have following problems with the attributes.

### Example:
v.overlay ainput=area1 binput=area2 output=area3 operator=or
the attribute table for area3 looks like this:

cat | cata | catb
1   |   2  |  4 
2   |   4  |  7 

AFAICS these columns can be connected to the attribute tables from ainput and binput using different fields. 

It seems that this is not done automatically, because in file 
vectors/area3/dbln only one link to column "cat" is set

-> 1 area3 cat $GISDBASE/$LOCATION_NAME/$MAPSET/dbf/ dbf

anyway now I can add further links to the other attribute tables via fields manually or by using v.db.connect

-> 2 area1 cat $GISDBASE/$LOCATION_NAME/$MAPSET/dbf/ dbf
-> 3 area2 cat $GISDBASE/$LOCATION_NAME/$MAPSET/dbf/ dbf

When I now query the map I get all attributes from the two maps I overlayed (unioned) before - this is nice and the result I was expecting.

Now my questions:

a) Why are the two attribute tables of the input maps area1 and area2 not automatically linked in vectors/area3/dbln? 

b) How is it possible to query two attributes represented by different fields? E.g. show all areas where area1 connected through field 2 has value 1 and area2 connected through field 3 has value 2.

c) When I remove the overlayed map area3, also the basemaps area1 and area2 are deleted, which is really bad :-(. How can I prevent this without removing the "dbln" entries? 

 thanks a lot 


More information about the grass-dev mailing list