On 12/20/19 5:40 PM, Luis Moreira wrote:
Hello everyone,
I'd like to obtain the area between two drainage networks. The difference
between them is due to different calculation methods.
<http://osgeo-org.1560.x6.nabble.com/file/t385794/photo5181593200849168458.jpg>
The image above illustrates the methodology of mesuring the "error" between
two drainage networks.
Each one of the drainage networks is in a shapefile, and I need the sum of
all the areas between the segments of the drainage networks, so that I can
mesure the difference between them.
How could I do that with GRASS?
It seems that you want to get the area between two criss-crossing line
vectors. To do this in GRASS, you can try the procedure below. (Note
that there will always be some dangling line segments at the end that
are not closed, unless the two line vectors end exactly at the same point)
Assuming I have two line vectors 'streams1' and 'streams2'...
# First patch the two lines together
micha@RMS tmp $ v.patch input=streams1,streams2 output=streams_patch --o
WARNING: Vector map <streams_patch> already exists and will be overwritten
Patching vector map <streams1>...
Patching vector map <streams2>...
Building topology for vector map <streams_patch@micha>...
Registering primitives...
Building topology for vector map <streams_patch@micha>...
Intersections at borders will have to be snapped
Lines common between files will have to be edited
The header information also may have to be edited
v.patch complete. 2 vector maps patched
# Use v.clean to break lines at each intersection
micha@RMS tmp $ v.clean streams_patch output=streams_clean
tool=break,rmdangle --o
--------------------------------------------------
Tool: Threshold
Break: 0
Remove dangles: 0
--------------------------------------------------
WARNING: Vector map <streams_clean> already exists and will be overwritten
Copying features...
100%
Rebuilding parts of topology...
Building topology for vector map <streams_clean@micha>...
Registering primitives...
--------------------------------------------------
Tool: Break lines at intersections
100%
--------------------------------------------------
Building topology for vector map <streams_clean@micha>...
--------------------------------------------------
Tool: Remove dangles
100%
--------------------------------------------------
Rebuilding topology for output vector map...
Building topology for vector map <streams_clean@micha>...
Registering primitives...
# Now change the line to a boundary. This creates enclosed polygon areas
micha@RMS tmp $ v.type input=streams_clean output=streams_boundary
from=line to=boundary --o
WARNING: Vector map <streams_boundary> already exists and will be
overwritten
Building topology for vector map <streams_boundary@micha>...
Registering primitives...
Building areas...
100%
Attaching centroids...
100%
WARNING: Number of incorrect boundaries: 24
# Run v.centroids to get individual areas each with a centroids
micha@RMS tmp $ v.centroids input=streams_boundary output=streams_areas --o
WARNING: Vector map <streams_areas> already exists and will be overwritten
Processing features...
10 new centroids placed in output map
Copying attribute table(s)...
Building topology for vector map <streams_areas@micha>...
Registering primitives...
Building areas...
100%
Attaching centroids...
100%
WARNING: Number of incorrect boundaries: 24
v.category complete. 10 features modified.
# Add an attribute table to hold the area of each polygon
micha@RMS tmp $ v.db.addtable streams_areas column="area_sqm REAL"
WARNING: Values in column <cat> will be overwritten
Reading features...
100%
Updating database...
100%
10 categories read from vector map (layer 1)
10 categories read from vector map don't exist in selection from table
10 records updated/inserted (layer 1)
micha@RMS tmp $ v.to.db streams_areas option=area unit=meters
column=area_sqm
Reading areas...
100%
Updating database...
100%
10 categories read from vector map (layer 1)
10 records selected from table (layer 1)
10 categories read from vector map exist in selection from table
10 records updated/inserted (layer 1)
# Check results:
micha@RMS tmp $ v.db.select streams_areas
cat|area_sqm
1|483620.195889
2|320097.009754
3|440533.925655
4|330833.778807
5|472005.399364
6|80935.144153
7|130299.225048
8|275763.239
9|281143.425416
10|446053.906035
HTH, Micha
Thanks in advance.
--
Sent from: http://osgeo-org.1560.x6.nabble.com/Grass-Users-f3884509.html
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--
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918
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