[R-sig-Geo] SpatialPoint to SpatialGrid with unequal distributed points

[Edzer Pebesma]

Stefan, your data are definitely not on a grid, so either you transform
them back to the grid on which they were, or alternatively you could
interpolate them to a new grid, e.g. by

library(sp)
link <- "http://z107.de/uni/example16092010.RData"
load(url(link))
coordinates(data) <- c("lon","lat")
diff(sort(unique(coordinates(data)[,2])))
diff(sort(unique(coordinates(data)[,2])))
bb = bbox(data)
gt = GridTopology(c(bb[1,1],bb[2,1]), c(.18,.18), round(c(diff(bb[1,]),
        diff(bb[2,]))/.18) + 1)
plot(data)
plot(SpatialGrid(gt),col='red',add=T)
sp = SpatialGrid(gt)
fullgrid(sp)=F
spx = SpatialPixelsDataFrame(sp, data.frame(x = rep(NA, 44*16)))
library(gstat)
#proj4string(data)="+proj=longlat"
#proj4string(spx)="+proj=longlat"
sp2 = idw(Therm.Eignung~1,data,spx)
image(sp2,axes=T)
spplot(sp2[1], col.regions=bpy.colors())


On 09/16/2010 08:31 PM, Stefan Muthers wrote:
> Hello,
> 
> this is probably a beginners question, but I found no solution on the
> web.
> 
> If am using data from a climate model with the cell positions given in
> geographic coordinates. Unfortunatly the distance between the cells is
> uneven (due to runding errors and maybe the projection) and the
> "gridded" command fails to determine the correct cell size.
> 
> 
> Consider the following example:
> 
> 
> library(fields)
> library(sp)   
> 
> link <- "http://z107.de/uni/example16092010.RData"
> load(url(link))
> 
> coordinates(data) <- c("lon","lat")
> gridded(data) <- TRUE  
> # Warnmeldungen:
> #1: In points2grid(points, tolerance, round, fuzz.tol) :  
> #  grid has empty column/rows in dimension 1
> #2: In points2grid(points, tolerance, round, fuzz.tol) :
> #  grid has empty column/rows in dimension 2
> 
> data_sp <- as(data, "SpatialGridDataFrame")
> g_plot <- as.image.SpatialGridDataFrame(data_sp["Therm.Eignung"])
> image.plot(g_plot)
> 
> 
> 
> The resulting map contains only small stripes and much empty space.
> summary(data) reveals, that the cellsize if much to small (cell should
> be quadratic with a cellsize of ~ .16) and to many cells were found in
> the y-direction. 
> 
> When I replace the lat and lon values with simple x and y coordinates,
> everything works fine, but then it is not possible to overlay a shape
> file, that displays the country borders.
> 
> Any ideas, why the cellsize-estimation is so different from the real
> value and what I can do, to create a raster map with this data?
> 
> 
> thank you and best regards,
> 
>  stefan
> 
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo at stat.math.ethz.ch
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo

-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de