[GRASS-dev] [GRASS GIS] #2669: v.net.iso: different results depending on direction

GRASS GIS GRASS GIS developer
1

#2669: v.net.iso: different results depending on direction
----------------------------------------------+-------------------------
Reporter: mlennert | Owner: grass-dev@…
     Type: defect | Status: new
Priority: normal | Milestone: 7.0.1
Component: Vector | Version: svn-trunk
Keywords: network v.net.iso direction cost | CPU: Unspecified
Platform: Unspecified |
----------------------------------------------+-------------------------
Testing v.net.* modules to see whether the number of vertices in lines
makes a difference for results (for info: it doesn't), I came upon a weird
bug.

I digitized three +/- parallel lines (with different number of vertices)
and start points at more or less similar positions on these lines.

Then:

{{{

v.net test_lines points=test_points op=connect thresh=500 out=network
v.db.update network col=cost qcol="length/(speed*1000/60)"
v.net.iso input=network@isochrones output=isos center_cats=1-3
costs=5,10,15,20,25,30,35,40 arc_column=cost

}}}

So cost is the same in both directions. However, the result I get is very
different depending on the direction from the starting point (cf attached
image - numbers are the categories from v.net.iso, blue circles are the
start points).

--
Ticket URL: <https://trac.osgeo.org/grass/ticket/2669&gt;
GRASS GIS <http://grass.osgeo.org>

2

#2669: v.net.iso: different results depending on direction
--------------------------+----------------------------------------------
  Reporter: mlennert | Owner: grass-dev@…
      Type: defect | Status: new
  Priority: normal | Milestone: 7.0.1
Component: Vector | Version: svn-trunk
Resolution: | Keywords: network v.net.iso direction cost
       CPU: Unspecified | Platform: Unspecified
--------------------------+----------------------------------------------
Changes (by mlennert):

* Attachment "v_net_iso_direction_bug.png" added.

image showing different result depending on direction

--
Ticket URL: <https://trac.osgeo.org/grass/ticket/2669&gt;
GRASS GIS <http://grass.osgeo.org>

3

#2669: v.net.iso: different results depending on direction
--------------------------+----------------------------------------------
  Reporter: mlennert | Owner: grass-dev@…
      Type: defect | Status: new
  Priority: normal | Milestone: 7.0.1
Component: Vector | Version: svn-trunk
Resolution: | Keywords: network v.net.iso direction cost
       CPU: Unspecified | Platform: Unspecified
--------------------------+----------------------------------------------

Comment (by mlennert):

Apparently the problem arises when lines are diagonal (in comparison to NS
or EW direction). Here's a reproducible testcase:

* horizontal

{{{
v.in.lines in=- out=testline sep=, --o << EOF
610000,225000
680000,225000
EOF
echo "645000|225000" | v.in.ascii in=- out=testpoint --o
v.net testline point=testpoint op=connect thresh=500 out=testnet --o
v.db.addtable testnet col="length double precision, cost double precision"
v.to.db testnet op=length col=length
v.db.update testnet col=cost qcol="length/(100*1000/60)"
v.net.iso testnet out=testiso center_cats=1-999 costs=10,20,30,40
arc_column=cost --o
}}}

* diagonal

{{{
v.in.lines in=- out=testline sep=, --o << EOF
610000,200000
680000,300000
EOF
echo "645000|225000" | v.in.ascii in=- out=testpoint --o
v.net -s testline point=testpoint op=connect thresh=50000 out=testnet --o
v.db.addtable testnet col="length double precision, cost double precision"
v.to.db testnet op=length col=length
v.db.update testnet col=cost qcol="length/(100*1000/60)"
v.net.iso testnet out=testiso center_cats=1-999 costs=10,20,30,40,50,60,70
arc_column=cost --o
}}}

* vectical

{{{
v.in.lines in=- out=testline sep=, --o << EOF
645000,200000
645000,300000
EOF
echo "645000|250000" | v.in.ascii in=- out=testpoint --o
v.net -s testline point=testpoint op=connect thresh=50000 out=testnet --o
v.db.addtable testnet col="length double precision, cost double precision"
v.to.db testnet op=length col=length
v.db.update testnet col=cost qcol="length/(100*1000/60)"
v.net.iso testnet out=testiso center_cats=1-999 costs=10,20,30,40,50,60,70
arc_column=cost --o
}}}

I'll attach the respective images. In the diagonal case, the isochrones
are much closer towards the South-West (~37km - using the wxgui measuring
tool - to reach the beginning of cat=8) than towards the North-East (~78km
to reach the beginning of cat=8). As cost is defined via only one column,
it should be identical in both directions.

--
Ticket URL: <https://trac.osgeo.org/grass/ticket/2669#comment:1&gt;
GRASS GIS <http://grass.osgeo.org>

4

#2669: v.net.iso: different results depending on direction
--------------------------+----------------------------------------------
  Reporter: mlennert | Owner: grass-dev@…
      Type: defect | Status: new
  Priority: normal | Milestone: 7.0.1
Component: Vector | Version: svn-trunk
Resolution: | Keywords: network v.net.iso direction cost
       CPU: Unspecified | Platform: Unspecified
--------------------------+----------------------------------------------
Changes (by mlennert):

* Attachment "horizontal_line.png" added.

--
Ticket URL: <https://trac.osgeo.org/grass/ticket/2669&gt;
GRASS GIS <http://grass.osgeo.org>

5

#2669: v.net.iso: different results depending on direction
--------------------------+----------------------------------------------
  Reporter: mlennert | Owner: grass-dev@…
      Type: defect | Status: new
  Priority: normal | Milestone: 7.0.1
Component: Vector | Version: svn-trunk
Resolution: | Keywords: network v.net.iso direction cost
       CPU: Unspecified | Platform: Unspecified
--------------------------+----------------------------------------------
Changes (by mlennert):

* Attachment "diagonal_line.png" added.

--
Ticket URL: <https://trac.osgeo.org/grass/ticket/2669&gt;
GRASS GIS <http://grass.osgeo.org>

6

#2669: v.net.iso: different results depending on direction
--------------------------+----------------------------------------------
  Reporter: mlennert | Owner: grass-dev@…
      Type: defect | Status: new
  Priority: normal | Milestone: 7.0.1
Component: Vector | Version: svn-trunk
Resolution: | Keywords: network v.net.iso direction cost
       CPU: Unspecified | Platform: Unspecified
--------------------------+----------------------------------------------
Changes (by mlennert):

* Attachment "vertical_line.png" added.

--
Ticket URL: <https://trac.osgeo.org/grass/ticket/2669&gt;
GRASS GIS <http://grass.osgeo.org>