Dear list,
I hope you can help me to understand the behavior of GRASS for the
following: I need calculate the network-distances for intersections in
this network. What seemed easy, starts to get very tricky....
My approach is shown below, but has following problems:
1.) As intersections need to be dedicated with a category, I extraced
the intersection-points with v.net so that I can connect them to their
network. (is this really needed?) The following execution of the command
v.net.allpairs generated errors: I think this might be due to missing
categories for the new points? But adding a table, connecting it to the
layer and filling in categories resulted in errors concerning already
existing attribute tables. Can GRASS handle several layers for one map
in PostgreSQL at all?
2.) Exporting and reimporting is a bit hacky, but solves issue 1. After
connecting this new pointlayer to the network, I get a map consisting of
points and lines in the same layer, as expected by v.net.allpairs.
However, I have the feeling that the categories are not distinct, so I
have a point and a line with the same cat-values. I expect this to have
an effect on v.net.allpairs? The command v.net.allpairs does run (with
the code below), but the output is not at all clear: I get the expected
number of objects in the table (square of number of points), but the
categories start with "1", which is not even existing in my original
network. So what will cat "1" stand for than and how can I get
v.net.allpairs to use the category of the points in the output?
My code runs on GRASS 6.4.3 from Ubuntu-Repos with PostgreSQL 9.1. as
database.
#___network cleaning and extracting intersections connecting to new network
v.build.polylines in=net_clean out=net_segm cats=first --overwrite
v.net in=net_segm out=intersections op=nodes --o
v.out.ogr in=network dsn="network.shp" type=point #re-import to
create pt_layer with category
v.in.ogr dsn="network.shp" out=network_pt type=point
v.net gr_streets_clean points=network_pt op=connect thresh=200
out=network nlayer=1 --o
#___ create o-d-table with costs
v.net.allpairs in=network out=streets_shortpath_od af=cost_speed
ab=cost_speed -a --overwrite
I'd appreciate any hint!
Patrick
Patrick Schirmer,
Institute for Transport Planning and Systems
ETH Zürich
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