merging data with a dataframe based on a common identifier

On Sun, 12 Jul 2009, stephen sefick wrote:

I have used v.strahler in GRASS to calculate the stream order. ?This
module writes its results to a text file. ?The line column of the
output data is the common name between the two data sets. ?I would
like to attach the stream order to the streams vector. ?I have looked
at the data once in R with

pseudocode:
streams <- readVECT6("streams")
str(streams)

Well, v.strahler is a very fresh GRASS addon, not a regular part of GRASS,
and does require a clean topology. Running it on streams in spearfish, with
the elevation.dem DEM, gives an output vector map, say st_o0:

v.strahler --verbose --overwrite input=streams output=st_o0 \
?dem=elevation.dem txout=st_o0.txt sloppy=0 layer=1


st_o0 <- readVECT6("st_o0", type="line,boundary", with_c=TRUE,
?remove.duplicates=FALSE, plugin=FALSE)

gets 116 lines. They agree with the first 116 of 119 lines in st_o0.txt:

tbl <- read.table("st_o0.txt", skip=1, header=TRUE)

by plotting. For example:

plot(st_o0, col=rainbow(27)[tbl[1:116,]$Basin.])

looks plausible. However, where the streams are not fully connected, basins
get split, with obvious consequences for the order. The cat in the vector
layer seems to be the order too, as stated on the help page - I think that
this is where the graphics on the help page are coming from. I think that
this agrees with advice that your got on the grass-users list. It may be
that you need to check the cleanness of the topology of the stream lines
carefully - this is repeated on the help page. You may also need to read the
v.strahler code to check what it is assigning, as:

plot(st_o0, col=tbl[1:116,]$Order.)

or equivalently:

plot(st_o0, col=st_o0$cat)

don't match my view of drainage in spearfish.

Hope this helps

Roger

and it is a huge list. ?I am not sure the best way to tackle this problem.
thanks for any help,

--
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