[GRASS5] How to get point counts?

[Thomas Adams]

Michael,

What about creating a circle vector (centered at the point of interest), 
convert it to a raster which you then use as a mask; then use R to do 
your statistics on the layer of interest?

Tom


Michael Barton wrote:

>I'm working on a little script that will do Local Density Analysis for
>archaeology, and have run into an odd problem for a high-end GIS like GRASS.
>
>I am having considerable difficulty in getting the count of points within a
>given distance of another point. That is, for each point, I'd like to get
>the number of points (in either the same or, better, another map layer)
>within 100m, 200m, 300m, etc.
>
>I first wrote my script using v.neighbors--reading the documentation rather
>than trying it out (I know, bad idea)--followed by r.sum. It seemed perfect,
>but it turns out that v.neighbors doesn't do what it seems it ought to do
>given the documentation. When have time to figure out exactly what it IS
>doing, I will see if I can clarify the docs.
>
>R.neighbors is too restrictive in its window size (only 3-25) and the window
>size is in grid units rather than in ground units (making my task
>additionally complicated).
>
>Creating a raster buffer with bands at the set of distances I need, followed
>by r.statistics seemed like a good idea. But again, the actual behavior of
>r.statistics does not seem to match quite what the docs imply--though the
>docs also need to be updated a lot. Again, if I can be sure of what
>r.statistics is doing I can try to update the docs.
>
>Perhaps v.distance might be a place to start. However, I was trying to test
>it on the Spearfish dataset to understand its behavior and stymied by the
>lack of a dbf attribute file for any of the point data AND GRASS's apparent
>inability to construct one from scratch (it seems like v.in.db will add
>fields to an existing table but won't make one if it doesn't exist).
>However, I'm still not sure what distances will end up being put into the
>selected distance field if I choose the flag to calculate all distances
>within a threshold. And I'm still left with a question about how to count
>points within a set distance unless GRASS supports count(*) in its SQL
>subset (it doesn't say that it does, but perhaps someone knows for sure).
>
>As far as I can see, there are no distance functions in r.mapcalc.
>
>Right now, I'm looking at doing a series of buffers, making each a mask and
>running r.sum for each mask. This seems klunky but doable.
>
>I am hoping that there is some kind of solution to this that I am missing.
>Any suggestions? Thanks in advance.
>
>Michael
>____________________
>C. Michael Barton, Professor of Anthropology
>School of Human Evolution and Social Change
>PO Box 872402
>Arizona State University
>Tempe, AZ  85287-2402
>USA
>
>Phone: 480-965-6262
>Fax: 480-965-7671
>www: <www.public.asu.edu/~cmbarton>
>
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>

-- 
Thomas E Adams
National Weather Service
Ohio River Forecast Center
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