[postgis-users] 'Clustering' records in space and time

Pierre Racine Pierre.Racine at sbf.ulaval.ca
Thu Jul 15 12:16:35 PDT 2010

What should happen when event A is at a distance n minus epsilon from B, B is at a distance n-epsilon from C but A is at a distance 2*n-epsilon from C? Should A and C be in the same cluster with B?


>-----Original Message-----
>From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-
>bounces at postgis.refractions.net] On Behalf Of Dylan Beaudette
>Sent: 15 juillet 2010 15:10
>To: will at thearete.co.uk; PostGIS Users Discussion
>Subject: Re: [postgis-users] 'Clustering' records in space and time
>Can you give us some hints about your data?
>1. how many records
>2. temporal domain (i.e. 1 year?)
>3. spatial domain (local, regional, continental?)
>If you don't have too much data, you may be able to standardize them, and
>apply an algorithm like PAM, or CLARA (see cluster package in R).
>On Thursday 15 July 2010, William Furnass wrote:
>> I have a PostGIS table of records describing events therefore the
>> table has a timestamp attribute.  I wish to replace 'clusters' of
>> events that occur within a m-hour window and a spatial radius of n
>> with single events which have the mean timestamp and central position
>> of the cluster.  I understand that I can quantize my data spatially
>> using the St_SnapToGrid function but using this function alone I lose
>> some of the distinct events that occurred at the same point in space
>> but at very different times (it's my understanding that St_SnapToGrid
>> only allows one point to be stored at each node in the grid).  Also, I
>> am unsure as to how I could use St_SnapToGrid in such a way so as not
>> to relocate points that are unique within the aforementioned spatial
>> and temporal window boundaries.
>> Has anyone any suggestions as to how this can be achieved
>> programmatically using SQL (rather than a graphical tool)?  Should I
>> perhaps be looking to use R to spatially and temporally cluster my
>> data?  Apologies if the description of my problem isn't particularly
>> clear; it's been a long day:)
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>Dylan Beaudette
>Soil Resource Laboratory
>University of California at Davis
>postgis-users mailing list
>postgis-users at postgis.refractions.net

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