[GRASS-user] Tracing dotted lines from raster (and convert to vector)

Nikos Alexandris nikos.alexandris at felis.uni-freiburg.de
Fri May 23 11:46:40 EDT 2008

Dear list,

I am curious to find out if it is possible to successfully convert the
"boundaries" of a raster map (contains some administrative boundaries)
a vector map.

The problem with this raster map is that the boundaries
are represented by non-continuous lines (dots).
Here is the link to the bad-looking gif files which contain
administrative boundaries (prefectures in Greece):

Is there a way to get the biggest portion out of it as vector
lines/boundaries and correct manually as less as possible?

Thank you,

P.S. My "quick" attempts so far


I 've tried using r.zc (edge detection) with no satisfactory results.


1. Save gif from http://www.ypes.gr/kapodistrias/images/i_trik.gif

2. grass -text # and create an xy location

3. r.in.gdal input="/grass_db/kapodistrias/data/i_trik.gif"

4. d.rast trikala cat=3 # cat=3 is the blue color, that is the
boundaries of interest.

5. r.mapcalc trikala_3="if(trikala == 3, 3, null())"

6. r.thin input="trikala_3" output="trikala_3.thinned" iterations=200

7. r.to.vect input="trikala_3.thinned" output="trikala_3_thinned"

8. v.clean input="trikala_3_thinned" output="trikala_3_thinned_clean"
type="line" tool="snap,rmdangle,rmbridge,rmdupl" thresh=16

Probably only snap help here but anyway I tested several options. snap
threshold above 20 and below 14 gives few/bad lines.

If there is a way to connect the "blue dots" in the raster map then I
suppose it would be more or less easy to grab lines from the raster

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