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

[Mira Kattwinkel]

Hello

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.

Thanks for any suggestions,

Mira



-------- Forwarded Message --------
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

-- 
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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