Hi there,
I am new to Grass, and am utilizing it within the QGIS GUI (i am not
familiar with the coding typically associated with Grass). I need to create
a surface reflecting drivetime isochrones from a point on a network. I
understand that i need to use v.net.iso in order to create a isolines from
which i can create a surgface. I am looking for a very BASIC walkthrough on
how to do this, as i am running into issues where the v.net.iso GUI will not
recognize any of my point files (i.e. they are not showing up). I think this
has to do with the fact that i have to use the v.net function in order to
'connect' my point to the network? but i have no idea what that means! any
help will be apprecaited, hopefully keeping any code inputs to a minimum! 
like i said, i have a network shapefile (roads) and a separate grass
shapefile for my centre point.... from there i am stuck :S
Cheers,
R
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**UPDATE**
Ok, so i have figured out one part of the problem. Perhaps somone can
provide some further guideance?
I have used the v.net function on my arc shapefile, using the nodes
operation to create another shapefile with just the nodes of the network. I
can then input into the v.net.iso tool both of these files (the arc and
nodes layers respectivlely). up to this point i am stuck, becuae i can't
figure out how to specifiy the centre points for the isoline calculations...
i am thinkning i need to modify the attribute table of my nodes layer, but
not sure how this should be done... does anyone have any experience in using
v.net.iso in GUI format, that could help me?
Any input is much appreciated.
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On 16/06/11 19:54, RKiTect wrote:
**UPDATE**
Ok, so i have figured out one part of the problem. Perhaps somone can
provide some further guideance?
I have used the v.net function on my arc shapefile, using the nodes
operation to create another shapefile with just the nodes of the network. I
can then input into the v.net.iso tool both of these files (the arc and
nodes layers respectivlely). up to this point i am stuck, becuae i can't
figure out how to specifiy the centre points for the isoline calculations...
i am thinkning i need to modify the attribute table of my nodes layer, but
not sure how this should be done... does anyone have any experience in using
v.net.iso in GUI format, that could help me?
Any input is much appreciated.
1) Connect centre points to roads:
v.net input=roads points=centre_point output=roads_with_point operation=connect threshold=DistanceBetweenPointAndNearestLineInNetwork
2) Create isoline vector
v.net.iso input=roads_with_point output=isolines ccats=1-999 costs=TheDistanceThresholds
Please be aware that this module does not create a surface, but affects each line of the network to one of the classes defined by your cost thresholds. You could possible use interpolation afterwards to create a surface from these lines.
Moritz
Thanks for your help...
unfortunately i am using the GUI and i must have an input for the 'nodes'
section. my centre layer doesn't show up for inputting into this... so i
think my point file requires some sort of pre-processing, but not sure...
if you try running the gui tool you will see what i mean... inputs required
for arcs and nodes, but a standard grass pointfile does not show up in nodes
dropdown box...
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[You seem to have gotten beyond some of your troubles, but just for the record, some answers.]
On 20/06/11 05:50, RKiTect wrote:
Thanks for your help...
unfortunately i am using the GUI and i must have an input for the 'nodes'
section.
Which module are you speaking about ? Neither v.net nor v.net.iso have a 'nodes' parameter.
my centre layer doesn't show up for inputting into this... so i
think my point file requires some sort of pre-processing, but not sure...
You have to connect your points to the streets using v.net. By default this will add the points as nodes to the street network and these nodes will have category values in layer 2. v.net.iso will by default look for these category values in layer 2, so you normally don't have to worry about the layers.
if you try running the gui tool you will see what i mean... inputs required
for arcs and nodes, but a standard grass pointfile does not show up in nodes
dropdown box...
What do you mean by "standard grass pointfile" ? Your points have to be a vector layer. So, if you have them as an ascii file in the classic grass point format, you have to import it with v.in.ascii first.
Moritz
i think i have figured out where my problem lies. It is in the centre node
costs... from the manual is says right at the top that centre node must be
opened (costs >= 0). however i have not defined costs for the centre node at
any point... could this be the issue? how do i go about defining costs for
the centre node?
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On Sat, Jun 25, 2011 at 8:02 PM, RKiTect <evan.apostol@gmail.com> wrote:
i think i have figured out where my problem lies. It is in the centre node
costs... from the manual is says right at the top that centre node must be
opened (costs >= 0). however i have not defined costs for the centre node at
any point... could this be the issue? how do i go about defining costs for
the centre node?
Defining costs or a cost column for nodes is not needed. If no cost
column is given, a cost value of 0 (zero) is used for nodes, which is
most of the time ok.
It would help a lot if you could post the commands and the output,
then it is much easier to figure out what goes wrong. You can cut and
paste the output from the GUI output window. The command can be copied
by clicking on the copy button.
Markus M