I would suggest you ask your question to the r-sig-geo mailing list. You will get a R solution. You can then get your PostGIS table from R using the gdal/ogr package or use PL/R in PostgreSQL.
Pierre >-----Original Message----- >From: Dylan Beaudette [mailto:debeaude...@ucdavis.edu] >Sent: 15 juillet 2010 16:02 >To: Pierre Racine >Cc: PostGIS Users Discussion; w...@thearete.co.uk >Subject: Re: [postgis-users] 'Clustering' records in space and time > >On Thursday 15 July 2010, Pierre Racine wrote: >> 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? >> >> Pierre > >Interesting. The choice of clustering algorithm would need to be based on the >questions the OP was trying to answer. Without much thought (warning!) I >pictured a 3D space (x, y, time) partitioned around medoids (PAM algorithm) >of data. > >In this very simple case chunks of data in (x, y, time) space would be >collected based on their proximity. For this to work, space and time >coordinates would need to be standardized accordingly... For x and y, I think >that subtracting the mean and dividing by the standard deviation should do. I >am not sure about the standardization of time... maybe the same thing, but >applied to the number of seconds | minutes | hours | days elapsed since the >start of the experiment? > >Dylan > > >> >-----Original Message----- >> >From: postgis-users-boun...@postgis.refractions.net [mailto:postgis-users- >> >boun...@postgis.refractions.net] On Behalf Of Dylan Beaudette >> >Sent: 15 juillet 2010 15:10 >> >To: w...@thearete.co.uk; PostGIS Users Discussion >> >Subject: Re: [postgis-users] 'Clustering' records in space and time >> > >> >Hi, >> > >> >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). >> > >> >Cheers, >> >Dylan >> > >> >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:) >> >> _______________________________________________ >> >> postgis-users mailing list >> >> postgis-users@postgis.refractions.net >> >> http://postgis.refractions.net/mailman/listinfo/postgis-users >> > >> >-- >> >Dylan Beaudette >> >Soil Resource Laboratory >> >http://casoilresource.lawr.ucdavis.edu/ >> >University of California at Davis >> >530.754.7341 >> >_______________________________________________ >> >postgis-users mailing list >> >postgis-users@postgis.refractions.net >> >http://postgis.refractions.net/mailman/listinfo/postgis-users > > > >-- >Dylan Beaudette >Soil Resource Laboratory >http://casoilresource.lawr.ucdavis.edu/ >University of California at Davis >530.754.7341 _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
