[R-sig-Geo] sp package/ spatial points operation works in R <= 2.8, fails for R >= 2.9 - resend with file download link

[Edzer Pebesma]

Hi Rick, thanks for the clean and reproducable example.

instead of

  SampPointsOneDeg <- as(SampGridOneDeg, "SpatialPoints")

you now need to do a

   SampPointsOneDeg <- as(as(SampGridOneDeg,
"SpatialPixels"),"SpatialPoints")

to make your example work.

As these coercion functions were written a long time ago I believe that
the reason it stopped working are changes in the methods package rather
than in sp. I'd agree that the first one should work as well, and I'll
have a look at modifying sp such that it will, again, in the future.
--
Edzer

On 05/21/2010 02:10 AM, rick reeves wrote:
> Hello List:
> 
> The following script creates a point grid at user-supplied spatial
> resolution;
> this grid (SpatialPoints) is then used as an input to sp/overlay() to
> extract
> points at regular intervals from a SpatialPolygonsDataFrame object.
> 
> Downloat the code and data for this example at:
> http://www.nceas.ucsb.edu/files/scicomp/Dloads/CreatePointGridSP.zip
> 
> CreatePointGrid <- function()
> {
>    library(rgdal) # loads sp package
> 
> # Read ESRI Shape File into SpatialPolygonsDataFrame
> 
>    EcoRegion <-
> readOGR("BaileysEcoRegionUsaGP.shp","BaileysEcoRegionUsaGP")
>    theProjection <- proj4string(EcoRegion)
> 
>    vals <- EcoRegion at bbox
>    deltaLong <- as.integer((vals[1,2] - vals[1,1]) + 1.5)
>    deltaLat <- as.integer((vals[2,2] - vals[2,1]) + 1.5)
> 
> # grid resolution units: decimal degress.
> 
>    gridRes <- 1.0
>    gridSizeX <- deltaLong / gridRes
>    gridSizeY <- deltaLat / gridRes
> 
> # GridTopology object is basis for sampling grid
> 
>    GridTopoOneDeg <- GridTopology(vals[,1],
>                                   c(gridRes,gridRes),
>                                   c(gridSizeX,gridSizeY))
> 
> # Create the SpatialGrid object....
> 
>    SampGridOneDeg <- SpatialGrid(GridTopoOneDeg,proj4string =
> CRS(theProjection))
>    SampPointsOneDeg <- as(SampGridOneDeg,"SpatialPoints")
>    message("hit key to see first 20 points of One Degree Grid Points...")
>    browser()
>    print(SampPointsOneDeg[1:20,])
>    message("done")
> }
> The issue: the line:
> 
>   SampPointsOneDeg <- as(SampGridOneDeg,"SpatialPoints")
> 
>   ...generates the required list of points when the script is run under
> R version 2.8 and earlier.
>   However, if the script is run under R 2.9 or greater, the point list
> contains only 2 coordinate pairs.
> 
>   Question: What has changed in R 2.9 and later? Is this technique
> deprecated, and is there a better
>                   way of creating such a point list?
> 
> Thanks,
> Rick R
> 
> < Attached zip file contains the script and sample data set>
> 

-- 
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