[GRASS-user] sampling points in a grid

[Johannes Radinger]

Maybe you can use a database selection approach for example in sqlite.
First update your points with your grid cell ID and then randomly select
7 rows from the selection where the grid cell ID == 42.

In sqlite there exists the function random(). Here you can find
an approach to select random rows from a sqlite table:
http://stackoverflow.com/questions/2279706/select-random-row-from-an-sqlite-table

I have not tested that approach, but maybe worth a try...

/Johannes

On Wed, Feb 18, 2015 at 11:55 AM, Margherita Di Leo <diregola at gmail.com>
wrote:

>
>
> On Tue, Feb 17, 2015 at 5:30 PM, Markus Neteler <neteler at osgeo.org> wrote:
>
>> On Tue, Feb 17, 2015 at 4:49 PM, Margherita Di Leo <diregola at gmail.com>
>> wrote:
>> > Hi,
>> >
>> > I have a vector of points. According to a regular grid, I need to
>> (randomly)
>> > sample a certain number n of these points in each cell of the grid. Such
>> > number n is given in a column of the attribute table of the grid. How
>> would
>> > you do this?
>>
>> With a small script this should be doable:
>> - generate the grid as polygons (v.mkgrid)
>> - loop over each polygon
>>      - fetch category number or v.extract
>>      - fetch corresponding number of points to be created from DB
>>      - v.random, using the current grid polygon for restricted creation
>> feature
>>
>> http://grass.osgeo.org/grass70/manuals/v.random.html#restriction-to-vector-areas
>
>
> Here lies the problem. I need to randomly pick points that are already
> existing. Example: for grid cell that has ID 42, I have 50 points, and have
> to randomly pick 7 out of these 50.
>
>>
>>      - save point map
>> - patch all point maps into single resulting map
>> - remove single point maps
>>
>> Something like this...
>>
>> Markus
>>
>
>
>
> --
> Best regards,
>
> Dr. Margherita DI LEO
> Scientific / technical project officer
>
> European Commission - DG JRC
> Institute for Environment and Sustainability (IES)
> Via Fermi, 2749
> I-21027 Ispra (VA) - Italy - TP 261
>
> Tel. +39 0332 78 3600
> margherita.di-leo at jrc.ec.europa.eu
>
> Disclaimer: The views expressed are purely those of the writer and may not
> in any circumstance be regarded as stating an official position of the
> European Commission.
>
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