working with a rotated grid of SpatialPolygons
How about just using high-level maptools? Following on directly from your code, the function elide can rotate the grid polygons: library(maptools) plot(elide(grdp, rotate = -45), add = TRUE) On Thu, Nov 14, 2013 at 3:00 PM, Waichler, Scott R
<Scott.Waichler at pnnl.gov> wrote:
I have an environmental process model whose domain is a regular grid that is not oriented in the cardinal directions. I want to show an outline of the active part of the domain, as well as other subgroupings of cells within the domain, on a map figure. These will be "blocky" polygons oriented at an angle to E-W, N-S. I assume I should do the rotation on (x,y,z) with rotate3d() of rgl. If there is a way to rotate sp objects directly, do tell. library(rgl) library(sp) x <- y <- seq(0.5, 9.5, by=1) # cell centers of a toy grid xy <- expand.grid(x, y) xyz <- matrix(data = 0, nrow=nrow(xy), ncol=3) xyz[,1] <- xy[,1] xyz[,2] <- xy[,2] # rotate the grid 45 degrees counterclockwise xyz.r <- rotate3d(obj=xyz, angle=-pi/4, x=0, y=0, z=1) # plot the cell centers (not vertices) of the original and rotated grids plot(xyz[,1], xyz[,2], col="blue", type="p", asp=1, xlim=c(-6, 10), ylim=c(0, 15), main="grid cell centers") points(xyz.r[,1], xyz.r[,2], col="red", type="p") # I have learned this for defining polygons based on the original grid: grd <- GridTopology(cellcentre.offset=c(0.5,0.5), cellsize=c(1,1), cells.dim=c(10,10)) grdp <- as.SpatialPolygons.GridTopology(grd) How can I efficiently extract the vertices of all the grdp polygons (each one a square) so that I can rotate them? I think str() and slot() are needed but I don't understand them. Once I have the rotated vertex coordinates, how do I use them to define the new SpatialPolygons? Once I have SpatialPolygons, I want to use unionSpatialPolygons(), so I also need to understand how to carry through an attribute for classifying grid cells. I could easily rotate vertices instead of cell centers, but I wouldn't know how to make grid cell polygons from the matrix of coordinates. Scott Waichler Pacific Northwest National Laboratory, Washington, USA scott.waichler at pnnl.gov
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Michael Sumner Hobart, Australia e-mail: mdsumner at gmail.com