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 <direg...@gmail.com> wrote: > > > On Tue, Feb 17, 2015 at 5:30 PM, Markus Neteler <nete...@osgeo.org> wrote: > >> On Tue, Feb 17, 2015 at 4:49 PM, Margherita Di Leo <direg...@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-...@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. > > _______________________________________________ > grass-user mailing list > grass-user@lists.osgeo.org > http://lists.osgeo.org/mailman/listinfo/grass-user >
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