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I used image classification technique in GRASS GIS v7 and generated the image mask below in which white area has NULL value and red area contains 1.

enter image description here

I am going to clip those red color areas in a LiDAR file so I need to convert those to a polygon shapefile. I know that I need to use a combination of image analysis techniques to achieve this but don't know whether GRASS has similar functionality to generate polygon shapefile from a mask.

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Did you check the r.to.vect module? You find the documentation here: http://grass.osgeo.org/grass70/manuals/r.to.vect.html

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  • This isn't useful as I need only polygon shapes not points or lines. my sesong question is: let's say I have an ASCII file including rows of X Y Z coordinates. Is there any command in GRASS that I could use to add another column attribute to the ASCII file (let's call it "H") and get its values from the mask (as shown above in raster format). Thanks – user2148425 Jun 4 '14 at 5:52
  • Ok, I see. What about a GDAL alternative? gdal.org/gdal_polygonize.html – webrian Jun 4 '14 at 6:37
  • Werbrian,I used that command and that worked. However, I ended up with large number of polygons so I decided to solve the problem in another way, which is what I posted yesterday. – user2148425 Jun 7 '14 at 0:43
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Regarding webrian's answer, the "feat=area" option to r.to.vect will create only polygons.

As for adding a column to your ASCII file, you'll need to import it into grass as a vector, then using v.what.rast you will upload raster values to each point. Something like:

v.db.addcolumn <your_vector_layer> column="h double precision"
v.what.rast <your_vector_points> raster=<your_raster_layer> column="h"

Now, if you need the data as an ASCII file, export back out. (v.out.ascii)

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  • Is the "feat=area" option the same as the "type=area"? I'm just asking since I can't find the "feat" option in the documentation. – webrian Jun 4 '14 at 7:31
  • Yes, in v7.0 the syntax is now "type=area". (In the previous v6.4 it was "feature=area") – Micha Jun 4 '14 at 7:40
  • Hi Micha,I tried to run v.db.addcolumn but I got the message "ERROR: There is no table connected to this map. Run v.db.connect or v.db.addtable first." I thought when I import an ASCII with X Y Z structure, GRASS generates a table and attached it to the resulting vector map. Then in the v.db.addtable, I have to define "columns". My question is after defining columns (let's say X Y Z) in v.db.addtable, how do I assign the actual x, y and z values to the table? – user2148425 Jun 6 '14 at 6:40
  • The v.in.ascii import creates a table only when it is "necessary", that is, only when there are additional data columns, other than x-y. If you have defined the x-y-z columns then do v.to.db with the parameters: option=coor columns="x,y,z". – Micha Jun 6 '14 at 10:01
  • Hi, I am confused. As I said in my previous posts, I want to import a text file, add another column (to get the structure of x y z status) and populate the “status” attribute from a raster map. The data type of “status” will be int. I followed the following steps. First I ran: v.in.ascii -n input= pts.txt output=pts_vecorMap separator=’space’. File is read Ok. Then I run the following command: v.to.db map=pts_vecorMap@PERMANENT type=point option=coor columns='x,y,z' but I got ERROR: Unable to open database <D:\Astron\Grass\TextFileTest\PERMANENT\sqlite\sqlite.db> by driver <dbf> – user2148425 Jun 6 '14 at 14:45

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