[postgis-users] Calculate variance of a multipoint

[Ben Madin]

I'm not quite clear to me what you are trying to demonstrate - do you want to know the density of the points... relative to their total size (area / number?), or relative to some defined area?

cheers

Ben


On 28/05/2011, at 6:19 AM, Aren Cambre wrote:

> Did anyone have thoughts on this? :-)
> 
> Aren
> 
> On Wed, May 4, 2011 at 2:12 PM, Aren Cambre <aren at arencambre.com> wrote:
> The more I think about it, is this a job for R? I know I need to start using R at some point, just haven't begun yet.
> 
> Aren
> 
> 
> On Wed, May 4, 2011 at 1:42 PM, Aren Cambre <aren at arencambre.com> wrote:
> Suppose you have a geometry type with a multipoint. How would you calculate the variance of the points in that multipoint?
> 
> I looked through the PostGIS 1.5 function reference and am not coming up with any easy way.
> 
> A hard way seems to be using st_centroid(multipoint) to find the multipoint's  center. From there, I can calculate the distance of each point from its center, and use that towards calculating the variance (each distance is squared, all squared distances are added together, then divide by number of points).
> 
> I guess my ultimate need is to measure relative dispersion of multipoints. The multipoints that have the most dispersion are suspect, but I need a way of identifying which ones are like this.
> 
> Aren
> 
> 
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