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 <- ecoreg...@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>
--
Rick Reeves
Scientific Programmer/Analyst and Data Manager
National Center for Ecological Analysis and Synthesis
UC Santa Barbara
www.nceas.ucsb.edu
805 892 2533
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