Thanks Miha, but it doesn't look like this function supports spatial data frames and great circle distances.
On Tue, Apr 7, 2009 at 10:54 PM, Miha Staut <mihast...@yahoo.co.uk> wrote: > > Dear Greg, > > A while back I did a similar task with the function interp.surface() from > the fileds package. It briefly does bilinear interpolation from a regular > grid to arbitrary point. It is computationally quite inexpensive since the > method of interpolation is quite simple. > > Hope I understood you correctly. > > All best > Miha > > > --- On Tue, 7/4/09, Greg King <gregs...@kooji.com> wrote: > > > From: Greg King <gregs...@kooji.com> > > Subject: Re: [R-sig-Geo] Spatial Interpolation of Regularly Gridded Data > > To: roger.biv...@nhh.no > > Cc: r-sig-geo@stat.math.ethz.ch > > Date: Tuesday, 7 April, 2009, 10:32 PM > > Thanks for your reply Roger. I > > little while back I got kriging working in > > the gstat package with my data, but found that for my > > purposes it was too > > computationally expensive (read slow). Therefore I am > > looking for something > > a little quicker. I now appreciate I cannot use > > interp due to it treating > > coordinates as planar. I found a good tutorial for > > kriging in gstat at > > http://casoilresource.lawr.ucdavis.edu/drupal/node/442. > > From a quick play > > around I can't immediately see how functions like Tps in > > the field package > > meet my needs. Could anyone provide directions to > > primers on the web?. > > Googling for R is quite tricky (even with rseek). > > > > Essentially I want a fast simple method for interpolating a > > single point > > value from its surounding points. It shouldn't be too > > difficult but I must > > admit I am struggling a little! > > > > Greg > > > > On Tue, Apr 7, 2009 at 8:26 AM, Roger Bivand <roger.biv...@nhh.no> > > wrote: > > > > > On Mon, 6 Apr 2009, Greg King wrote: > > > > > > Hi, > > >> > > >> I'm fairly new to R, but finding the experience a > > good one. However I am > > >> a > > >> little overwhelmed by the number of packages and > > not sure I am necessarily > > >> using the most appropriate ones. > > >> > > > > > > Have you read the Spatial Task View on your nearest > > CRAN? There you should > > > find some information to help. Firstly, akima treats > > all coordinates as > > > planar, so don't use interp(). Both fields and gstat > > can interpolate using > > > geographical coordinates, gstat with IDW and kriging, > > fields with thin plate > > > splines (I believe using rdist.earth, but have not > > checked) and kriging. You > > > are trying to interpolate, so why not use a > > geostatistical method? You could > > > see whether the new automap package (running gstat > > internally) can handle > > > geographical coordinates - check by comparing the > > output with runs with the > > > proj4string set and unset. > > > > > > Hope this helps, > > > > > > Roger > > > > > > > > > > > >> Here is the background to what I am trying to > > achieve: I have a CSV file > > >> which contains weather forecast data for latitude > > and longitude points > > >> (see > > >> attached out.csv, the data I understand is on > > WGS84 > > >> http://www.ready.noaa.gov/faq/geodatums.html). > > The sample points are at > > >> half degree intervals. My objective is to > > work out what the forecast data > > >> is at any specific given latitude/longitude by > > interpolating data from the > > >> 0.5x0.5 degree grid. I am doing this for a > > number of different time > > >> points > > >> using the following functions: > > >> > > >> library(akima) > > >> library(rgdal) > > >> > > >> gks <- function(inLat,inLon,dframe,variab) { > > >> wind.grid <- > > expand.grid(lat=inLat, lon=inLon) > > >> coordinates(wind.grid) <- ~ > > lat+lon > > >> proj4string(wind.grid) <- > > CRS("+init=epsg:4326") > > >> pnt<-interpp(coordinates(dframe)[,1], > > coordinates(dframe)[,2], > > >> z=as.data.frame(dframe)[,1], > > >> > > coordinates(wind.grid)[,1],coordinates(wind.grid)[,2],linear=TRUE) > > >> return(pnt$z) > > >> } > > >> > > >> interp_gk <- function(lat, lon) { > > >> wind<-read.csv(file="/Users/greg/Out.csv",head=TRUE,sep=",", > > >> > > colClasses=c("POSIXct","numeric","numeric","numeric","numeric")) > > >> coordinates(wind)=~lat+lon > > >> proj4string(wind) <- > > CRS("+init=epsg:4326") > > >> > > >> times<-unique(wind$vt) > > >> columns<-names(wind)[2:length(names(wind))] > > >> > > >> dOut<-data.frame(dateTime=times[1]) > > >> > > >> for (i in 1:length(times)) { > > >> dOut[i,"dateTime"]<-times[i] > > >> for (j in > > 1:length(columns)) { > > >> > > sset<-subset(wind, wind$vt==times[i], > > select=c(columns[j])) > > >> > > dOut[i,columns[j]]<-gks(lat,lon,sset,columns[j]) > > >> } > > >> } > > >> dOut<-cbind(dOut, > > mag=sqrt(dOut$ugrd_0^2+dOut$vgrd_0^2)) > > >> return(dOut) > > >> } > > >> > > >> However, I have the following concerns: > > >> > > >> - Should I really be using akima? The > > documentation states it is not for > > >> use on regularly spaced grids - what are my > > alternatives? > > >> - The interp funcrtion will not work for > > cubic spline "linear=FALSE" > > >> interpolation (is it because my data is > > regularly gridded?). How can I > > >> achieve cubic spline interpolation? > > >> - Is my function really using the Cordinate > > reference system specified? > > >> When I comment out the CRS lines, my > > functions return the same values? > > >> > > >> Lots of questions I appreciate, but I am > > curious! It seems R can achieve > > >> what I am trying to do... but I may just be > > missing some vital bits of > > >> information. > > >> > > >> Thanks, > > >> > > >> Greg > > >> > > >> > > > -- > > > Roger Bivand > > > Economic Geography Section, Department of Economics, > > Norwegian School of > > > Economics and Business Administration, Helleveien 30, > > N-5045 Bergen, > > > Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43 > > > e-mail: roger.biv...@nhh.no > > > > > > > > > > [[alternative HTML version deleted]] > > > > _______________________________________________ > > R-sig-Geo mailing list > > R-sig-Geo@stat.math.ethz.ch > > https://stat.ethz.ch/mailman/listinfo/r-sig-geo > > > > > > > [[alternative HTML version deleted]] _______________________________________________ R-sig-Geo mailing list R-sig-Geo@stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo
