On Tue, 26 May 2009, John Callahan wrote:
I have a DEM (2 meter spacing, IMG file format) and I'd like to create
contour vector data output, shapefiles would be great. I just downloaded R
2.9.0. Looking for more info, I keep finding references to the contour
function in the grahics package, but that seems like it only produces plots
instead of output vectors.
I know the sp and spatstat packages are good for various types of geospatial
analysis. Is one of these 'the next generation' of the other? Would you
recommend one or the other specifically for contour generation from a DEM?
(My platform is Windows.) Thanks.
Just roughly:
download.file("http://geomorphometry.org/data/DEM25m.zip";,
destfile="DEM25m.zip")
fname <- zip.file.extract(file="DEM25m.asc", zipname="DEM25m.zip")
file.copy(fname, "DEM25m.asc")
# to give some data to play with
library(rgdal)
dem <- readGDAL("DEM25m.asc")
# GDAL has many formats, IMG isn't very descriptive, try and see which
# driver suits for reading to a SpatialGridDataFrame - I'm assuming one
# band only, using the first and only band next
im <- as.image.SpatialGridDataFrame(dem)
cl <- contourLines(im)
# contourLines() takes the same interval arguments as contour()
library(maptools)
SLDF <- ContourLines2SLDF(cl)
# convert to a SpatialLinesDataFrame with the contour labels as
# attributes and export
writeOGR(SLDF, ".", "my_contours", driver="ESRI Shapefile")
mc <- readOGR(".", "my_contours")
summary(dem)
summary(mc)
image(dem, col=gray.colors(20))
plot(mc, col=terrain.colors(8), add=TRUE)
The colours in the last line are rather sleight of hand, but for
demonstration they work this time.
Hope this helps,
Roger
- John
PS - At first pass, I'm going to give gdal_contour
(http://www.gdal.org/gdal_contour.html) a try. I'd really like to see how
this compares with some method within R.
I'm also looking at SAGA for the R interface options. I just installed the
most recent QGIS with the ManageR plugin to see what tools that brings in.
And GRASS is always an option (either through QGIS or standalone) but I don't
quite understand how mapsets work yet.
**************************************************
John Callahan
Geospatial Application Developer
Delaware Geological Survey, University of Delaware
227 Academy St, Newark DE 19716-7501
Tel: (302) 831-3584 Email: john.calla...@udel.edu
http://www.dgs.udel.edu
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--
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.biv...@nhh.no
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