[postgis-users] Determining clusters of points

[Sébastien Lorion]

Nevermind my last question, I actually don't need the last step of creating
a box/circle/hull around the merged buffers. Only the set of clusters with
their points is sufficient.

Thanks again,

Sébastien

On Tue, Dec 7, 2010 at 14:03, Sébastien Lorion <sl at thestrangefactory.com>wrote:

> I like your idea very much, especially since something I did not say is
> that the points have a range attribute which determines how far they can
> interact with other points. So the circle buffer you talk about would have a
> diameter equals to the range of each point.
>
> What would be fastest as the last step : bounding box, minimum bounding
> circle or minimum convex hull ? I am guessing the BB, but is the difference
> significant enough that one should be chosen over another ?
>
> Sébastien
>
> On Tue, Dec 7, 2010 at 13:42, Emilie Laffray <emilie.laffray at gmail.com>wrote:
>
>>
>>
>> On 7 December 2010 17:01, Sébastien Lorion <sl at thestrangefactory.com>wrote:
>>
>>> Hello,
>>>
>>> I am trying to find an efficient way to find clusters of points as shown
>>> in the attached image. The only clustering criteria is the distance between
>>> the points. The dataset can be very large (millions of points) and point
>>> distribution is mostly clustered with some sparse points in the gaps.
>>>
>>> I searched the net and this mailing list and found two promising solution
>>> paths:
>>>
>>> - use a statistical tools such as R with a density function (
>>> http://www.r-project.org)
>>> - use a clustering algorithm like those explained here
>>> http://www.med.nyu.edu/biostatistics/people/Ilana%20Belitskaya-Levy/Courses/MAS/Handouts/clustering.pdf (agnes
>>> seems the most promising for my purposes)
>>>
>>>
>>> <http://www.med.nyu.edu/biostatistics/people/Ilana%20Belitskaya-Levy/Courses/MAS/Handouts/clustering.pdf>I
>>> would like your advice to help me find which approach would be best suited
>>> with PostGIS (maybe there is even something already made that I can use?).
>>> Whatever solution I pick, it must be efficient and the workload must be able
>>> to be distributed on a cluster of commodity hardware.
>>>
>>> I am new to GIS and this mailing list, so please excuse me if I am not
>>> using the right vocabulary.
>>>
>>> Thank you very much!
>>>
>>>
>> Hello,
>>
>> Some time ago I have worked on something similar, except that I was using
>> circles instead of boxes which should not be a problem. I am just giving the
>> logic as I don't have access to the code right now.
>> You can start by creating a buffer around each of your points of the
>> distance you want.
>> The next step is to create an UNION of all the buffers that intersect.
>> You get the list of points included in each of the resulting polygons and
>> then you create either a bounding box around them or use a minimum bounding
>> circle (Postgis 1.5 and above).
>>
>> Emily Laffray
>>
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>>
>>
>
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