[forwarding this to the list for Micha, as it ended up in my mailbox 
- Ben]
Thanks for the suggestions. Some additional comments below:
Benjamin Ducke wrote:
GRASS modules that create area type features should already be generating
centroids and adding categories to them automatically, shouldn't they?
As far as I am aware, e.g., v.in.ogr does this, so we are talking mainly
about adding centroid generation to the interactive digitizing tool, aren't we?
The GRASS Vector lib API should have a function that finds a good centroid automatically.
Or am I misled here (guess I am getting a bit confused myself, now)?
As far as I know, v.in.ogr creates centroids automatically when you
import a vector as type=boundary, and gives the centroids the same cat
values as the boundaries.
To be quite honest, I have always been a bit bewildered about the choice
of using a centroid point for linking attributes to area features.
Could anyone here fill me in on what advantage that has?Cheers (and a good New Year, btw.),
Ben
Michael Barton wrote:
Normally, the "best practice" is to digitize boundaries without category
values (unless you specifically want information concerning the
boundary, not the area it contains) and then digitize the centroids with
category values and relevant attribute data.
Yes, I understand that now.
In your case, the easiest way out would seem to be v.distance, i.e.:
- v.centroid in=YourBoundaries out=YourMap
- v.db.addcol YourMap column="b_cat int"
- v.distance from=YourMap from_type=centroid to=YourMap to_type=boundary
upload=cat column=b_cat #this should find the nearest boundary for each
centroid
- v.category in=YourMap out=YourMap2 option=del type=boundary
- v.reclass in=YourMap2 out=YourMap3 type=centroid column=b_cat
- db.copy from_table=YourMap2 to_table=YourMap3
- db.dropcol YourMap3 col=b_cat
- Now you should have a YourMap3 with centroids that are linked to the
correct attributes. If this is the case, you can safely do the next step:
- g.remove vect=YourMap,YourMap2This should be quite easy to make into a script module, or maybe extend
v.centroids to optionally do this for you.
I follow what you're suggesting, but as a general solution it seems like
quite a convoluted process to go thru just to get attribute values,
which were already entered, to display correctely.
I still think that when an area is created, a centroid should
automatically be placed at some standard and logical place for each
area. Area centroids should be an integral component of area topology,
not a separate point that must be manually placed and manipulated.Michael
Michael: I think you brought this up in the past? This seems to me the
most "natural" approach. So what would be needed is:
1- v.digit should automatically add a centroid in some reasonable place
for each closed boundary, and give it the boundary's cat value.
2- v.centroid should also automatically assign cat values identical to
the surrounding boundary's values.
Regards,
Micha
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