With QGIS 2.10, the following should work (probably also OK
for earlier versions).
Check the band values of your colour using the info button,
and use the values in the expression below (assuming the
name of your raster layer is yourraster) instead of 116, 20
and 31:
("yourraster@1" = 116 AND "yourraster@2" = 20 AND "yourraster@3" = 31) * 1
This should give you a binary raster with 1 in the pixels
that have your colour and 0 in the pixels with other colours.
In the style dialogue you can use the following to see the
resulting binary image:
Colour gradient: "white to black"
Min: 0
Max: 1
You should be able to use r.thin on that image, but I am not
able to test this now, as I am on Ubuntu, and have not been
able to get GRASS to work through Processing on that platform
for QGIS 2.10.
Håvard
On 29. juli 2015 16:40, Nick Papadonis wrote:
Hi Håvard,
I found the raster calculator and read the online manual for it, however am
still a bit lost. I was also playing with the Style properties and color bands
last night to try to accomplish this. I was unable to get a single binary
image with the single color (red in this case). I think the understand the
r.thin suggestion and that it will thin the single color thick lines for
vectorization. Appreciate if you can expand on obtaining a binary skeleton of
the one color of the trails.
Thanks again,
Nick
On Jul 29, 2015, at 7:18 AM, Havard Tveite <[email protected]> wrote:
If you are going to use GRASS r.to.vect (for instance through QGIS
Processing), you will need a binary skeleton image.
You could get one by using the raster calculator to extract
your single colour as a binary image, and then GRASS r.thin
(available in QGIS Processing) to get the skeleton.
Håvard
On 29/7/2015 8:36:AM, Nick Papadonis wrote:
Hi Folks,
I have a map with numerous trails in a single color and would like to vectorize
these. Is there a mode Qgis can be placed in such that a color can be selected
for inclusion in the vectorization process? I.e. to make converting these
routes to vector format easier?
Thanks again
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