[GRASS-user] wrong result in v.net.iso for backward direction

[Markus Metz]

On Fri, Jun 9, 2017 at 3:36 PM, Mira Kattwinkel <
kattwinkel-mira at uni-landau.de> 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 <kattwinkel-mira at uni-landau.de>
> To: grass-user <grass-user at lists.osgeo.org>
>
> 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.
>
>
>
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