On 10/08/10 15:17, Moritz Lennert wrote:
On 10/08/10 13:49, Nikos Alexandris wrote:
Markus M:
If a point is inside an area (the polygon composed of the area's
boundaries), the distance is 0 (zero):
This sentence makes me think that it is a priori known (based on
something
else - related to topology?) when a point is inside an area. Why all
the need
to measure distances then in order to count how many points are inside?
As you can see in the code referenced by Markus, there is a
Vect_point_in_area(), so yes, it is possible to more directly check if
points are in areas. It all depends on which modules were written using
this function. At this stage all point-in-polygon attempts in GRASS are
scripts using workarounds...
As a follow-up:
The counting points in polygons algorithm I prefer at this stage is
(using municipal boundaries and hospitals in the NC data set with an
SQLite backend - DBF won't work):
g.copy hospitals,myhospitals
v.db.addcol myhospitals col="cat_municip int"
v.distance from=myhospit...@sqlite to=boundary_mun...@permanent
upload=cat column=cat_municip dmax=0.0
db.select sql="select cat_municip, count(*) from myhospitals group by
cat_municip"
If your hospital attribute table contains number of beds (nbeds), the
you could sum the number of beds as such:
db.select sql="select cat_municip, sum(nbeds) from myhospitals group by
cat_municip"
etc...
Using 6.5 to test a similar case to yours (I assume):
g.region vect=boundary_municp
v.random out=mypoints n=600000
v.db.addtable mypoints col="cat int, cat_municip int" (that's veeeeery
slow, probably because of 600000 update statements to the database in
the v.to.db call...)
time v.distance from=mypoi...@sqlite to=boundary_mun...@permanent
upload=cat column=cat_municip dmax=0.0
real 2m2.119s <= not so bad
db.select sql="select cat_municip, count(*) from mypoints group by
cat_municip"
So, using the combination of v.distance and db.select I cannot reproduce
your problem with 600,000 points, but maybe the number and nature of
polygons can also play a role...
Moritz
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