Re: [GRASS-user] sampling points in a grid
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
Re: [GRASS-user] sampling points in a grid
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 ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] sampling points in a grid
Margherita, Actually, my approach with be little changed either way; without know of a better GRASS-only approach, the kind of thing I have done is to: (1) write-out to a text file for all grids x, y, value (2) index the lines, producing, id, x, y, value (3) in R randomly sample the index (4) from (3) you have id, x, y, value for the random sample (5) import (4) into grass using v.in.ascii This will take a little scripting… Best, Tom On Tue, Feb 17, 2015 at 8:49 AM, 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? I have half idea to play with the coordinates and Monte Carlo, but I have the feeling that I'm not thinking GIS and there must be a easier way. Thank you for any hint -- 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 -- Thomas E Adams, III 718 McBurney Drive Lebanon, OH 45036 1 (513) 739-9512 (cell) ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
Re: [GRASS-user] sampling points in a grid
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 - save point map - patch all point maps into single resulting map - remove single point maps Something like this... Markus ___ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user
