On Tue, May 20, 2014 at 5:41 PM, Remi Genevest <rgenev...@free.fr> wrote: > Hi, > My question is very trivial. I want to create a "blank" grid that contains > regular cells from 20meter*20meter in a WGS84 CRS, covering Europe. > My purpose is to used this grid as a "base layer" to then add some > additional layers (with different resolution and extent) and make some > spatial analyses with all these stuff. > > Which way should I follow to manage this ?
Step 1 is "buy a bigger computer". Europe is about 3,000km EW and NS (wikipedia gives the area as about 10,000,000 square km - that might be the land area? Anyway, orders of magnitude...), which is 150,000 of your 20metre cells by 150,000. Or twice as many if that was just the land area. That's 22 billion cells. And you haven't put any data in them yet. Step 2 is the tricky step. You can't create what's commonly thought of as a 'regular grid' on a sphere. You can make each cell centre 20m from its nearest neighbours (I think, at least until you run up against the pole), but it won't be 40m from its next-nearest neighbours when measured along the great circle distance. You can use a coordinate system other than WGS84, and make a 20m spaced-out set of cells, but the distances computed along the sphere won't be right. There are some standard map projections that european-wide data seems to use, mostly based on Lambert conformal conical projections. You might want to copy the grid used for CORINE data: http://www.eea.europa.eu/data-and-maps/data/corine-land-cover-2000-raster-3 - which goes to 100x100m resolution (or 250x250 if you don't want to have to buy a new PC to load the whole continent...). Barry _______________________________________________ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo