On Fri, Jun 9, 2017 at 3:36 PM, Mira Kattwinkel < [email protected]> wrote: > > Dear list members, > > has nobody any idea what's going on (see below)? > > When using forward and backward costs or only backward costs in v.net.iso the backward end (i.e. upstream) always gets too short / too few segments.
I have an idea what could cause these differences, but I need some time for testing. More soon. Markus M > > Can anybody please point me into the right direction. > > All the best, Mira > > > Subject: wrong result in v.net.iso for backward direction > Date: Thu, 8 Jun 2017 10:46:42 +0200 > From: Mira Kattwinkel <[email protected]> > To: grass-user <[email protected]> > > Dear list members > > I am using v.net.iso to split a stream network at a certain distance > from sampling points. > > First, I create a network from vector lines (streams) and vector points > (sampling sites) using v.net. The lines feature have a 'cat' column, > 'length', 'backward_cost' and 'forward_cost'. I would use -1 for forward > costs because I am only interested in the upstream part, and length for > backward costs in v.net.iso: > > v.net.iso input=test_edges arc_layer=2 node_layer=3 > output=test_edges_bw_2000 center_cats=55 arc_column=forw_cost > arc_backward_column=backw_cost costs=2000 > > However, the backward part of the resulting lines with cat 1 is always > too short. Likewise, if I give just 1 for the backward costs and set the > costs to 5, it gets 4 segments with cat 1. Working in both directions at > the same time gives correct values for the forward end, but too short > for the backward end. I then realised that the numbers would be correct > if the first part of the forward end was added to the backward part (see > attached example). The forward part all the costs (lengths) sum up > correctly to 2000 (1443.19 + 556.81). For the backward part it would be > 45.72 + 511.09 = 556.81. However, if the first segment of the forward > part is added, it gives the correct cost sum (45.72 + 511.09 + 1443.19 = > 2000). > > Do I use the function in the wrong way or is this a bug? > > Thanks a lot, > Mira > > URL: < http://lists.osgeo.org/pipermail/grass-user/attachments/20170608/97469a58/attachment.png > > > URL: < http://lists.osgeo.org/pipermail/grass-user/attachments/20170608/97469a58/attachment-0001.png > > > PS > > I just realized that it works correctly for length in both direction if > the parameters arc_column and arc_backward_column are not given. > However, for me this is inefficient because I only need the backward end. > > > > _______________________________________________ > grass-user mailing list > [email protected] > https://lists.osgeo.org/mailman/listinfo/grass-user
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