[Qgis-user] Not so Random Points in Polygon generation

[Chris Berens]

Thanks for the full answer Richard, and the brain baffling eye-candy.

Previously I have used the manual approach of generating a regular
grid at the required resolution (R) and adding a random x,y (scaled
max R) to each co-ord, which is tricky when you have a required sample
per polygon. Alternatively, generating a dense regular grid, adding a
random 0-9 column, sorting by ID&random and skimming the required
sample off the top.  This 'appears' more random.

Regards,
Chris
Chris Berens
www.mapland.co.za
+27 (0)82 567 9322



On 19 February 2015 at 21:32, Richard Duivenvoorde <rdmailings at duif.net> wrote:
> On 19-02-15 19:54, Chris Berens wrote:
>> Hi all,
>>
>> The random points I generate inside polygons with QGIS always seem to
>> follow lines.  This is also evident on the Linfiniti manual example of
>> same:
>> http://manual.linfiniti.com/en/vector_analysis/spatial_statistics.html
>> Is there a reason for this?
>
> Hi Chris,
>
> I do not see 'lines', but I do see some 'flocking'...
> It all depends on the algorithm used:
>
> https://github.com/qgis/QGIS/blob/master/python/plugins/fTools/tools/doRandPoints.py#L154-L170
>
> not sure if you can deduct what is the 'problem' there.
>
> For a plugin I created for a client: Feature Grid Plugin (to create a
> regular grid of points in a polygon for small work plots), I had a look
> into some algorithms to create a given number of points in a polygon
> which are random but also fill up the space at best. The best algorithm
> to use (and is very nice to look at) comes from
> http://bost.ocks.org/mike/algorithms/  <== look at those !!!
> and is called 'Poisson-disc:
> http://www.jasondavies.com/poisson-disc/
>
> If people are interested I think it is nice to either add this to ftools
> or to my own plugin.
>
> Regards,
>
> Richard
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