[GRASS-user] polygons and centroids redux

[Dave Roberts]

Thanks Hamish.  It seems only likely that such a routine would exist.

Here's the part where I need a little help though.  It appears that

v.distance from=pnt to=area dmax=0 upload=cat column=areacat

will find and report the cat number of the polygon that points lie 
within, and I can use that in a database operation which is extremely 
handy.  So, I have done that, and now the area cat for each point is 
available in the database.  That works for points in a point vector 
feature.

Using d.what.vect or v.what, however, I cannot control whether the 
function reports a boundary or centroid.  This makes it hard to
replicate the behavior of v.distance for a few points on the fly.
Is there some way to access the existing point-in-polygon routine from 
an existing GRASS function to simulate what I'm after?

Looking at the progammer's manual, writing something like v.pip (= to 
d.what.vect reporting always area cat) appears to be a bit more than I 
want to bite off at present.

Perhaps one approach would be

d.where > file
(edit file as necessary for separator)
v.in.ascii in=file out=points fs=whatever
v.distance from=points to=area dmax=0 upload=cat column=areacat

That works but seems like a lot of work for what I'm after.

Thanks much, Dave


Hamish wrote:
> Dave wrote:
>>     I think it would indeed be handy if a switch
>> was added to v.what and
>> d.what.vect in GRASS 7.  In the meantime it might be
>> handy to write a v.pip command for GRASS 6.  I have
>> written a point-in-polygon routine for other software
>> (including R), but I would have to really study the file
>> structures of GRASS vectors to attempt that in
>> GRASS.   If any GRASS programmers are
>> interested let me know and perhaps we could work it out as
>> an add-on.
> 
> point in polygon is a standard function in the vector library for
> years.
> 
> 
> Hamish
> 
> 
>       
>