[postgis-users] Calculate variance of a multipoint

[Aren Cambre]

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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