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

[Mira Kattwinkel]

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

-- 
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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