[GRASS-user] Point count with large vectors

GRASS GRASS user
1

Hi all,

I'm not sure if this has been solved before.

I have two large points files (>20,000 sites), and I want to find the number of
points in A falling within a given distance of the points in B. So the desired
output would be:

Cat Point count from B
1 10
2 3
3 12
...etc

I can use a buffer from Map A and run v.distance, but it takes an extremely long
time, so suggestions would be welcome!

Richard

2

On 09/05/09 00:51, Richard Chirgwin wrote:

Hi all,

I'm not sure if this has been solved before.

I have two large points files (>20,000 sites), and I want to find the number of
points in A falling within a given distance of the points in B. So the desired
output would be:

Cat Point count from B
1 10
2 3
3 12
...etc

I can use a buffer from Map A and run v.distance, but it takes an extremely long
time, so suggestions would be welcome!

Don't know if this would be much faster (what takes a long time in your approach ?), but you could use only v.distance to create a distance matrix (-a), and setting the threshold dmax to your distance.
Then a simple aggregate query on the reulting table should give you what you need.

Moritz

3

Moritz Lennert wrote:

On 09/05/09 00:51, Richard Chirgwin wrote:

Hi all,

I'm not sure if this has been solved before.

I have two large points files (>20,000 sites), and I want to find the
number of
points in A falling within a given distance of the points in B. So the
desired
output would be:

Cat Point count from B
1 10
2 3
3 12
...etc

I can use a buffer from Map A and run v.distance, but it takes an
extremely long
time, so suggestions would be welcome!

Don't know if this would be much faster (what takes a long time in your
approach ?), but you could use only v.distance to create a distance
matrix (-a), and setting the threshold dmax to your distance.
Then a simple aggregate query on the reulting table should give you what
you need.

As to "a long time", I left one loop running for several days and only completed
one state of Australia!

And now an embarrassing admission, in spite of using v.distance *lots*, I never
saw the -a flag before. Many thanks, Moritz!

Richard

Moritz