Reproject the points to the coordinate system of thr grid, do the overlay then copy the attributes back. No need to warp a raster just for over(lay).
On Saturday, December 8, 2012, Roger Bivand wrote: > On Fri, 7 Dec 2012, Sarah Goslee wrote: > > Hi Simon, >> >> I've copied this back to the list, as is encouraged. >> >> On Fri, Dec 7, 2012 at 9:41 AM, O'Hanlon, Simon J >> <simon.ohan...@imperial.ac.uk> wrote: >> >>> Hi Sarah, thank you. >>> >>> My data points are located in West Africa, so I think a good projection >>> would be Lambert Azimuthal Equal Area. >>> >>> This does raise one more question for me. I also have some raster data >>> that I wish to use as covariates which is also in geographic coordinates. >>> If I try to simply reproject a raster, I think it would screw up the >>> regular grid. Does it make sense if I convert the lat-long raster to a >>> SpatialPointsDataFrame and transform this to LAEA so I could then create a >>> grid in the desired planar coordinates from scratch and use over() to >>> assign values from the reprojected spatial points onto the raster? >>> >> >> A reprojected raster is still a raster, and thus a regular grid. Or am >> I missing something? >> > > Yes, a warped raster is a raster, but a reprojected raster will in general > be a set of irregular points. So resampling is invoved one way or the > other. Whether one queries the raster as-is with spatial points, and then > projects the output, or warps the raster doing spatial query with projected > points will depend a bit on the support(s) of the data sets. Maybe see: > ?projectRaster in raster for warping. > > Roger > > >> Sarah >> >> >> Thanks again for your help. >>> >>> Cheers, >>> >>> Simon >>> >>> -----Original Message----- >>> From: Sarah Goslee [mailto:sarah.gos...@gmail.com] >>> Sent: 07 December 2012 14:18 >>> To: O'Hanlon, Simon J >>> Cc: r-sig-geo@r-project.org >>> Subject: Re: [R-sig-Geo] Distance between two points >>> >>> Precisely. You should use great-circle distances with lat-lon >>> coordinates, rather than Euclidean distance, because the actual length >>> varies with position on the globe. >>> >>> Converting to UTM or something similar is one solution if your points >>> are not too far apart. >>> >>> There are many other R solutions: searching for "great circle distance" >>> at rseek.org will get you quite a list. >>> >>> Sarah >>> >>> On Fri, Dec 7, 2012 at 8:10 AM, O'Hanlon, Simon J < >>> simon.ohan...@imperial.ac.uk> wrote: >>> >>>> Dear list, >>>> I am using the package geoRglm to do some predictive mapping. There is >>>> a function that calculates the distance between observed data points and >>>> the prediction locations using a .C call to a function which eventually >>>> calculates the length of the hypotenuse between one location and the other >>>> given the vertical and horizontal separation distance of those points. >>>> >>>> My question is, is this method of distance-finding incompatible with >>>> long-lat style coordinates? Should I first transform my data and prediction >>>> locations into something where the unit of measurement is in metres rather >>>> than decimal degrees? >>>> >>>> Many thanks, >>>> >>>> Simon >>>> >>>> >> >> -- >> Sarah Goslee >> http://www.**functionaldiversity.org <http://www.functionaldiversity.org> >> >> ______________________________**_________________ >> R-sig-Geo mailing list >> R-sig-Geo@r-project.org >> https://stat.ethz.ch/mailman/**listinfo/r-sig-geo<https://stat.ethz.ch/mailman/listinfo/r-sig-geo> >> >> > -- > Roger Bivand > Department of Economics, NHH Norwegian School of Economics, > Helleveien 30, N-5045 Bergen, Norway. > voice: +47 55 95 93 55; fax +47 55 95 95 43 > e-mail: roger.biv...@nhh.no > > ______________________________**_________________ > R-sig-Geo mailing list > R-sig-Geo@r-project.org > https://stat.ethz.ch/mailman/**listinfo/r-sig-geo<https://stat.ethz.ch/mailman/listinfo/r-sig-geo> > -- Michael Sumner Hobart, Australia e-mail: mdsum...@gmail.com [[alternative HTML version deleted]] _______________________________________________ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo
