[GRASS-user] Problem with v.to.db after v.dissolve and v.category

[Mira Kattwinkel]

Dear list,

I dissolve a vector map of lakes based on the names of the features 
(v.dissolve). As many of the features do not have names, I get, among 
the others, one new feature built up by many parts.

v.dissolve input=lakes at PERMANENT column=name output=lakes_diss
I do get a warning during the dissolve:
...
WARNING: Number of centroids exceeds number of areas: 44345 > 43502
WARNING: Number of duplicate centroids: 1237

This results in 932 features with one containing the majority of lakes 
(those without a name). Then, I want to calculate the area (v.to.db) but 
this gives a wrong result because I need the area of every single lake.

This is the same problem as described here 
https://lists.osgeo.org/pipermail/grass-user/2011-January/059282.html 
and I followed the advice given by Markus. However, I still get one big 
feature without a name and get error messages when calculating the area. 
Can anybody please give me an hint what's going wrong?

Here are the steps:
1. delete categories:

v.category input=lakes_diss at PERMANENT output=lakes_diss_wocat option=del 
cat=-1
Processing features...
Copying attribute table(s)...
Building topology for vector map <lakes_diss_wocat at PERMANENT>...
Registering primitives...
88399 primitives registered
1409269 vertices registered
Building areas...
43502 areas built
43426 isles built
Attaching islands...
Attaching centroids...
Number of nodes: 45215
Number of primitives: 88399
Number of points: 0
Number of lines: 0
Number of boundaries: 45291
Number of centroids: 43108
Number of areas: 43502
Number of isles: 43426
v.category complete. 43108 features modified.
(Mon Aug 20 15:59:30 2018) Command finished (3 sec)
--> still 932 features in attribute table

2. add category
v.category input=lakes_diss_wocat at PERMANENT output=lakes_diss_wcat 
option=add
Processing features...
Copying attribute table(s)...
Building topology for vector map <lakes_diss_wcat at PERMANENT>...
Registering primitives...
88399 primitives registered
1409269 vertices registered
Building areas...
43502 areas built
43426 isles built
Attaching islands...
Attaching centroids...
Number of nodes: 45215
Number of primitives: 88399
Number of points: 0
Number of lines: 0
Number of boundaries: 45291
Number of centroids: 43108
Number of areas: 43502
Number of isles: 43426
v.category complete. 43108 features modified.
(Mon Aug 20 16:02:07 2018) Command finished (3 sec)
--> 932 features in attribute table

3. add new column and populate with area

v.db.addcolumn map=lakes_diss_wcat at PERMANENT columns=area double

v.to.db map=lakes_diss_wcat at PERMANENT option=area columns=area
Reading areas...
Updating database...
WARNING: Record (cat 933) does not exist (not updated)
[a long long list...]
43108 categories read from vector map (layer 1)
932 records selected from table (layer 1)
932 categories read from vector map exist in selection from table
42176 categories read from vector map don't exist in selection from table
932 records updated/inserted (layer 1)

Thanks a lot,
Mira