Dear R geographers:
I'm able to replace the coordinates of any given polygon, but not
construct apply() family commands or for() loops to accomplish the same
task. How can I construct a simple loop or apply() command that will
sequentially replace all of the coordinates in a SpatialPolygons object?
As far as I understand, the coordinates of a polygon are kept in a slot
referred as follows:
m...@polygons[[x]]@polygons[[...@coords
Using which, I can create mapply(foo, indexp, indexP) commands to
extract the original coordinates and transform them.
# Define a pair of vectors indexp, indexP that together index every
Polyon in the SpatialPolygons object map.
# Then extract coordinates as follows:
coords <- function(x,y) {...@polygons[[indexp]]@polygons[[index...@coords}
# Or operate a function foo on the old coordinates
coords.alt <- mapply(foo,
m...@polygons[[indexp]]@polygons[[index...@coords[,1],
m...@polygons[[indexp]]@polygons[[index...@coords[,2])
So I can create lists and matrices of coordinates, and transform the
coordinates.
It is possible to write a command that replaces coordinates using
integers; but these commands fail when I attempt to use mapply() to
replace existing coordinates. For example,
p...@polygons[[1]]@polygons[[...@coords <- foo(1,1)
will work fine, but the following will not.
newcoords <- function(indp, indP) {
p...@polygons[[indp]]@polygons[[ind...@coords <- foo(indp, indP)
}
mapply(newcoords, indexp, indexP)
Have I missed something in the manual on coordinates that would enable
me to transform the coordinates of all the Polygons in a
SpatialPolygonsDataFrame according to some regular function of the old
coordinates (x,y)? Is the only alternative to build a new
SpatialPolygons data frame from scratch, beginning with the new coordinates?
If it would be useful, I can provide sample code that works on wrld_simpl.
Please advise. Thank you!
--
Ben Mazzotta, PhD Candidate
The Fletcher School, Tufts University
benjamin.mazzo...@tufts.edu
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