[GRASS-user] Retaining holes in buffers
Roger.Bivand at nhh.no
Mon Jan 25 10:20:00 EST 2010
This problem arose in comparing vector and raster buffering on a recent
course. Using 6.4.0RC5 v.to.rast for a line, then buffering with r.buffer
gave a plausible outcome, but we wished to look at the vector equivalent.
In addition, we wanted to cookie-cut a raster with the vector buffer, to
match using r.mapcalc on the raster workflow, in R taking the cell centres
as representative points.
The problem is that while the internal vector representation of the buffer
has a hole, the hole is filled with an island on export (using either
shapefile or GML OGR drivers). The setup in R for a test dataset is:
L1 <- Line(cbind(c(1, 2, 3, 5), c(1, 3, 5, 10)))
L2 <- Line(cbind(c(2, 1.5, 3), c(3, 4, 5)))
LA <- Lines(list(L1, L2), ID="A")
LB <- Lines(list(Line(cbind(c(4, 6), c(3, 1)))), ID="B")
SL <- SpatialLines(list(LA, LB))
SLDF <- SpatialLinesDataFrame(SL, data=data.frame(ID=c("A", "B"),
writeOGR(SLDF, ".", "buff_island", driver="ESRI Shapefile")
(also on: http://spatial.nhh.no/misc/buff_island.zip).
then in GRASS (using wx) v.in.ogr to a new location, v.buffer with
distance 0.25, display (OK, 2 centroids), export as area as shapefile or
GML (visual inspection of the GML shows that the hole gets filled on
export), import back, and the hole is filled (three centroids). I'm sure
I'm missing something, but my cookie-cutter was cutting the island too,
and should have avoided the hole.
I'd be very grateful for leads.
PS. Where is v.to.rast on the wx menu system?
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
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