Aren,
Your purposed approach sounds reasonable to me. You can do it all in one
query like:
select c.gid, sum(c.dist*c.dist)/count(*) as variance
from (
select b.gid, b.cent, st_distance(b.geom, b.cent) as dist
from (
select a.gid, (st_dump(a.the_geom)).geom as geom,
centroid(a.the_geom) as cent
from (
select 99 as gid, 'MULTIPOINT(1 2,2 3,3 4,4 5)'::geometry
as the_geom
union all
select 88 as gid, 'MULTIPOINT(1 2,2 3,3 4,4 5,3 5,9
9)'::geometry as the_geom
) as a
) as b
) as c
group by gid order by variance desc;
You should be able to replace the select...union all select ... with
your table of multipoints.
-Steve W
On 5/27/2011 6:19 PM, Aren Cambre wrote:
Did anyone have thoughts on this? :-)
Aren
On Wed, May 4, 2011 at 2:12 PM, Aren Cambre <a...@arencambre.com
<mailto:a...@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 <a...@arencambre.com
<mailto:a...@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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