Cool, a PL/R user!
Brent, I'm curious about why you make a distinction between the centroid
and the average, because as far as I can tell they are the same thing.
[email protected] wrote:
Hi guys,
A bit more difficult, & way out in left field, but if you use PL/R to create a
median function for Postgres, you could build your point from the median(X) & (Y)
values instead of the average.
Where this would actually lie obviously depends on the distribution of the points.
The centroid is most affected by (actually defined by) outlying points, the avg
somewhat less & the median less still.
Of course once you have PL/R to play with, you have much more flexibility to
look at returning statistics from datasets than just the median.
Cheers,
Brent Wood
--- On Tue, 9/15/09, Kevin Neufeld <[email protected]> wrote:
From: Kevin Neufeld <[email protected]>
Subject: Re: [postgis-users] Center of Points Collection
To: "PostGIS Users Discussion" <[email protected]>
Date: Tuesday, September 15, 2009, 8:51 AM
Paul Ramsey wrote:
Faster than creating a multipoint is to recognize that
ST_Centroid()
is just going to return the center of the bbox of the
collection
anyways...
Unfortunately, Paul, ST_Centroid returns the center of
mass, not the center of the bbox.
SELECT astext(st_centroid(st_collect(column1))),
FROM (values ('POINT(0 0)'),
('POINT(0 1)'),
('POINT(0 2)'),
('POINT(1 0)')) as foo;
astext
------------------
POINT(0.25 0.75)
(1 row)
Your second post, taking the avg of the x,y does seem to be
the nice approach, and produces the same results as
ST_Centroid - the center of mass.
SELECT astext(st_makepoint(avg(st_x(column1)),
avg(st_y(column1))))
FROM (values ('POINT(0 0)'),
('POINT(0 1)'),
('POINT(0 2)'),
('POINT(1 0)')) as foo;
astext
------------------
POINT(0.25 0.75)
(1 row)
If Dustin is after the center of the collection, then
something along your first suggestion might be more
appropriate.
(taking the center of the extents)
Cheers,
Kevin
_______________________________________________
postgis-users mailing list
[email protected]
http://postgis.refractions.net/mailman/listinfo/postgis-users
_______________________________________________
postgis-users mailing list
[email protected]
http://postgis.refractions.net/mailman/listinfo/postgis-users
--
Regards, (please note new mobile number below)
Chris Hermansen mailto:[email protected]
tel+1.604.714.2878 · fax+1.604.733.0631 · mob+1.778.840.4625
Timberline Natural Resource Group · http://www.timberline.ca
401 · 958 West 8th Avenue · Vancouver BC · Canada · V5Z 1E5
_______________________________________________
postgis-users mailing list
[email protected]
http://postgis.refractions.net/mailman/listinfo/postgis-users
