overlay polygons (from shp) and raster (from geotif)
On Tue, 23 Oct 2007, Agustin Lobo wrote:
Hi there!
I'm trying to do the overlay of polygons on raster.
I do:
pols <- readOGR("../AllTransectPolygons02", layer="AllTransectPolygons02")
a <- readGDAL("t1s2comb/t1s2combfim95.tif") #geotif file
str(a) shows the correct proj info:
..@ proj4string:Formal class 'CRS' [package "sp"] with 1 slots
.. .. ..@ projargs: chr " +proj=utm +zone=18 +south +ellps=WGS72
+units=m +no_defs"
My goal is to get, for each polygon in pols,
the surface covered by each value of a.
Then:
delme <- overlay(pols, a)
delme
AREA PERIMETER Site
NA NA NA <NA>
NA.1 NA NA <NA>
which is not too useful and
delme2 <- overlay(a, pols)
takes for ever and what I finally get is:
str(delme2)
num [1:207024] NA NA NA NA NA NA NA NA NA NA ...
delme2[!is.na(delme2)]
[1] 86 86 86 86 87 87 87 87 88 88 88 88 89 89 89 89 90 90 [19] 90 90 90 91 91 92 92 93 93 93 93 94 94 94 95 95 95 96 [37] 96 96 96 98 101 101 101 101 102 102 102 102 104 104 104 104 104 104 [55] 104 104 105 105 105 105 105 105 106 106 106 106 that is, just the ids of the polygons on the cells of a.
This is what you would need, using this column to make a SpatialPixelsDataFrame out of the input data, then doing a tapply or similar to tabulate the categories. But with 200K cells, large cells, and small polygons, the point-in-polygon approach is converting the cells to their cell centre points, and missing most of the cell-polygon overlaps, that is those where the cell centre falls outside the polygon. Large cells and small polygons is not really the usual case. In addition, it might be helpful to subset pols first to avoid looping over polygons that cannot belong to a. I'm not sure how starspan does polygon/cell overlay - if it measures the overlap area, you might find that more helpful. If not, reduce pols to the a area, and resample a to a finer resolution so that multiple cells fit inside each polygon (change g.region in GRASS, for example). It will take time to do, because it does a C point-in-polygon operation for each polygon (in nested lapply() calls). Hope this helps, Roger
Note that the extent of pols is much larger than the one of a (i.e., may polygons are outside the raster and I'm not interested on those). Also, cells are much larger then the width of the polygons (I can send a jpg with an example). So it's clear I'm doing something wrong... any advice? Thanks
Roger Bivand Economic Geography Section, Department of Economics, Norwegian School of Economics and Business Administration, Helleveien 30, N-5045 Bergen, Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43 e-mail: Roger.Bivand at nhh.no