[postgis-users] Sample - Points into Polygons

[Vispo Leblanc]

Thanks for your support, I was able to make it work!

On Fri, May 25, 2018 at 4:01 PM Thijs van den Berg <thijs at sitmo.com> wrote:

> Or associate each point with a grid cell id using scaling and rounding of
> coordinates cell_id=int(x/cell_width)+rn_cols*(y/cell_width). If it were a
> 3x3 tic tac toe grid you'd have 0,1,2,..,8.
>
> Then select distinct points on that grid cell id?
>
> Op 25 mei 2018 23:42 schreef "Darafei "Komяpa" Praliaskouski" <
> me at komzpa.net>:
>
>> Hi,
>>
>> I see two ways to address your problem.
>>
>> First step, by area, calculate expected number of points in polygon.
>>
>> Then, depending on quality/speed tradeoff,
>>
>> fast: for each polygon you make ST_GeneratePoints, and for each point
>> take 1 closest point using KNN query, then apply DISTINCT to filter
>> duplicates.
>>
>> with some eyecandy guarantees: take KMeans of all the points in polygon
>> with K you calculated. In each cluster, take ST_GeometricMedian. Take point
>> from each cluster closest to cluster center.
>>
>>
>>
>> сб, 26 мая 2018, 00:26 карыстальнік Vispo Leblanc <vispoleblanc at gmail.com>
>> напісаў:
>>
>>> Hello PostGIS users,
>>>
>>>
>>>
>>> I have one table with thousand of polygons and another one with million
>>> of points. I would like to select all the points that are within the
>>> polygons, BUT I just want a sample of the data. Let me explain.
>>>
>>>
>>>
>>> Let’s say I have a polygon of 1,000 square meter with 60 points, ideally
>>> I would like to take only 1 points. But if the polygon is 10,000 square
>>> meter with 600 points then ideally I would take only 10 points. Also, in a
>>> perfect world, I would only select a sample of points that are spread out
>>> enough to be representative of the polygons. Let’s take the 10,000 square
>>> meters polygon for example, I would only select 10 points that are
>>> everywhere in the polygon and not 10 points that are within 10 meters of
>>> each other’s.
>>>
>>>
>>>
>>> I draw the following diagram to explain:
>>>
>>>
>>> [image: image.png]
>>>
>>> Please let me know if you have any ideas or functions I can use to solve
>>> my problem.
>>>
>>>
>>>
>>> Thank you :-)
>>>
>>> Vispo
>>>
>>> _______________________________________________
>>> postgis-users mailing list
>>> postgis-users at lists.osgeo.org
>>> https://lists.osgeo.org/mailman/listinfo/postgis-users
>>
>>
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