On Sat, 14 Feb 2009, Jonathan Greenberg wrote:
Thanks -- another few questions along these lines:
Perhaps you could simplify by providing code examples with a built-in data
set and so on? Verbatim copies do let others see what is going on. So I'll
start by trying to reconstruct what you seem to be asking about:
library(rgdal)
dsn <- system.file("vectors", package = "rgdal")[1]
layer <- "scot_BNG"
scot_BNG <- readOGR(dsn, layer)
library(maptools)
SG <- Sobj_SpatialGrid(scot_BNG)$SG
o <- overlay(scot_BNG, SG)
SGDF <- SpatialGridDataFrame(slot(SG, "grid"),
proj4string=CRS(proj4string(SG)), data=o)
spplot(SGDF, "SMR", col.regions=bpy.colors(20))
The Sobj_SpatialGrid() function in maptools takes a maxDim= argument,
which indirectly controls the (square) cell resolution. It is used in
creating PNG devices for displaying arbitrary objects in geographical
coordinates on Google Earth, so is related to vector to raster. An
alternative is to create a GridTopology object from scratch, here with
10km by 10km cells:
bb <- bbox(scot_BNG)
grd <- GridTopology(cellcentre.offset=floor(bb[,1]), cellsize=c(10000,
10000), cells.dim=ceiling(diff(t(bb))/10000))
o <- overlay(scot_BNG, SpatialGrid(grd))
SGDF <- SpatialGridDataFrame(grd, proj4string=CRS(proj4string(scot_BNG)),
data=o)
spplot(SGDF, "SMR", col.regions=bpy.colors(20))
1) Is there any way to determine the type of vector file in advance of
defining a layer name? How can I list layer names affiliated with a given
vector (it seems to be part of ogrinfo from the main gdal release, but when i
do a ogrinfo(filename) I get yelled at to feed it a layer name first).
ogrInfo(dsn, layer)
system(paste("ogrinfo", dsn))
# only works if ogrinfo is in your PATH
list.files(dsn)
suggests that your description isn't adequate - that is my external
ogrinfo program appears only to choose one driver, based on the fact that
it has been passed a directory. If the dsn was suited to other drivers, it
would have found them, but so would, for example:
list.files(dsn, pattern="shp$")
2) Is there a way to tell the feature type (point, polygon, line, etc...) of
a readOGR object?
l <- list.files(dsn, pattern="shp$")
sapply(l, function(fn) getinfo.shape(paste(dsn, fn, sep="/")))
for shapefiles and using a function in maptools, type is as in the ESRI
documentation. I'll try to add a similar facility to rgdal for available
drivers - in fact, the type is declared for each feature, but is assumed
uniform on return after accessing all features.
3) How do I get a polygon/line's node coordinates? Using
coordinate(readOGR(...)) I only get a single coordinates associated with each
polygon/line in the vector.
What do you mean by "node coordinates" - no topology is being done here,
so no arc/node analysis is available. If you mean the coordinates from
single Polygon or Line objects, see the thread at:
https://stat.ethz.ch/pipermail/r-sig-geo/2009-February/005037.html
and my reply yesterday on stepping through Polygons and Polygon lists
using lapply, and where to look for sample code. Basically:
all_the_coords <- lapply(slot(scot_BNG, "polygons"), function(Plns)
lapply(slot(Plns, "Polygons"), function(Pln) slot(Pln, "coords")))
is a list of lists of coordinate matrices.
Hope this helps,
Roger
Thanks!
--j
Hengl, T. wrote:
If you work with large shapes/grids, try also SAGA:
> rsaga.get.usage(lib="grid_gridding", 3)
SAGA CMD 2.0.3
library path: C:/Progra~1/saga_vc/modules
library name: grid_gridding
module name : Shapes to Grid
...
But before SAGA, you need to reproject the polygons first (if necessary).
see also some examples from:
http://spatial-analyst.net/wiki/index.php?title=Export_maps_to_GE#Polygon_maps
HTH
Tom Hengl
-----Original Message-----
From: [email protected] on behalf of Paul Hiemstra
Sent: Sat 2/14/2009 10:07 PM
To: Jonathan Greenberg
Cc: [email protected]
Subject: Re: [R-sig-Geo] Vector to raster?
Jonathan Greenberg schreef:
> How do I take a polygon in some OGR supported vector layer (say, a
> shapefile), and rasterize it given a pixel size and projection/datum?
>
> --j
>
Take a look at the spsample() function.
Paul
--
Drs. Paul Hiemstra
Department of Physical Geography
Faculty of Geosciences
University of Utrecht
Heidelberglaan 2
P.O. Box 80.115
3508 TC Utrecht
Phone: +31302535773
Fax: +31302531145
http://intamap.geo.uu.nl/~paul <http://intamap.geo.uu.nl/%7Epaul>
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