[postgis-users] Center of Points Collection

[Chris Hermansen]

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.

pcreso at pcreso.com 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 <kneufeld at refractions.net> wrote:
>
>   
>> From: Kevin Neufeld <kneufeld at refractions.net>
>> Subject: Re: [postgis-users] Center of Points Collection
>> To: "PostGIS Users Discussion" <postgis-users at postgis.refractions.net>
>> 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
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>>
>>     
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Chris Hermansen         mailto:chris.hermansen at timberline.ca
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