[Qgis-user] How to convert raster routes into vector format?

[Nick Papadonis]

Hi Havard,

Unfortunately, the expression below results in a complete black TIFF.  I am referencing the correct layer.  Any ideas?

Thanks
Nick

> On Jul 29, 2015, at 4:34 PM, Håvard Tveite <havard.tveite at nmbu.no> wrote:
> 
> 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 at 1" = 116  AND "yourraster at 2" =  20 AND "yourraster at 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 <havard.tveite at nmbu.no> 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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>>>> 
>>