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

[Markus Metz]

On Thu, Jun 8, 2017 at 10:46 AM, Mira Kattwinkel <
kattwinkel-mira at uni-landau.de> wrote:
>
> 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).

For the record in the ml, the problem was that several lines had the same
category, thus uploaded lengths for each category were the sum of all lines
with that category. The problem was solved by creating a new layer and
adding unique categories for lines to that layer. This new layer was then
used as arc_layer.

Markus M

>
> Do I use the function in the wrong way or is this a bug?
>
> Thanks a lot,
> Mira
>
> --
> Dr. Mira Kattwinkel
> Quantitative Landscape Ecology
> Institute for Environmental Sciences
> University of Koblenz-Landau
> Fortstraße 7
> 76829 Landau
> Germany
> Phone: + 49 6341 280-31553
> Office: Building I, Room 2.02
>
>
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