[OpenLayers-Users] Cluster strategy but on the server

[Nils Weinander]

users-request at openlayers.org wrote:
Didrik:
> 
>> I don't know if this is interesting for you but this postgis thread was
>> related to the subject of your mail :
>>
>> http://postgis.refractions.net/pipermail/postgis-users/2005-July/008642.html

Interesting, but as one of the replies said, the clustering
algorithm is iterative in nature, which is hard to do in a
SQL query.

Yves:
 >
> That series of posts is about statistical data clustering as in k-means
> and the like.  Give a k-means like algorithm a number of seeds N, it
> will minimize some sum of squares of distances between those seed points
> and the data points and your N clusters will be produced.  
> 
> That's not how the clustering strategy in OL is implemented AFAIK.  Come
> to think of it a pregenerated set of clusters could be done on distance
> to seed points (e.g. know site locations ?), but that would need to be
> done for every zoom level and even every "tile" in a given zoom level.  

Yes, definitely. For my needs I have come up with this:

* A separate table for the cluster features, with columns
   for the geometry (a point), a feature count and the scale
   (i.e. I store the cluster points for all scales in one
   table)

* A view for each scale

   "create view cluster_1M as select * from clusters where scale = 1000000"

   etc.

* A stored procedure which runs the same clustering algorithm
   as OpenLayers.Strategy.Cluster on the original spatial table
   and adds the clusters in the cluster table.

* Then I'll set up the layers and scale limits so that the
   cluster layer for each scale is dispalyed for that scale
   only.

* I'll probably add a trigger on the original spatial table
   and add each single new point to the clusters rather than
   have to regenerate the whole thing.

The backend is Oracle Spatial (10.2).

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