[GRASS-user] v.net.alloc having difficulty with specific dataset

[Markus Metz]

On Wed, Oct 5, 2016 at 3:05 AM, Sam Snellings <ssnellings at gmail.com> wrote:
> Markus,
>
> Thank you for the suggestions. I ran the following commands nine times on
> the data, with thresholds .00000000000000000001, 0.0000000001, 0.0000001,
> 0.00001, 0.01, 0.1, 1, 10, and 100. Let me know if I'm not going small
> enough.
>
> v.clean --overwrite input=roads output=roads_snap -c tool=snap thresh=[see
> above]
> v.net --overwrite input=roads_snap points=schools output=network
> operation=connect thresh=1000
> v.net.alloc --overwrite input=network output=network_allocation ccats=1-2
>
> I did not get a complete allocation of road segments from any of them, and
> as far as I can tell it got stuck at the same vertices on each run.
>
> I will email you a link to the data off-list, thanks for taking a look.

thanks for providing a link to the data! The problem is that the roads
are 3D with z coordinates not matching at the end of a road segment
and the beginning of another road segment. The solution is to import
roads as a 2D vector with v.in.ogr -2. 2D snapping with v.clean
tool=snap thresh =1 might be helpful.

Markus M

>
> -Sam
>
> On Tue, Oct 4, 2016 at 3:53 PM, Markus Metz <markus.metz.giswork at gmail.com>
> wrote:
>>
>> On Tue, Oct 4, 2016 at 5:09 PM, Sam Snellings <ssnellings at gmail.com>
>> wrote:
>> > I've been running some trials through v.net.alloc using town road
>> > networks
>> > as the arc layer and schools as the node layer. I have one set of
>> > roads/schools that breezes through v.net.alloc without issue (Town 1),
>> > while
>> > another one only allocates the first few arcs connected to each school
>> > and
>> > leaves the rest of the network unallocated (Town 2). There is probably a
>> > problem with the data from Town 2, but I can't figure out what it is.
>> > Does
>> > this issue sound familiar to anyone?
>> >
>> > I am using the GUI. I load each pair of shapefiles into GRASS and then
>> > run
>> > the following statements (I only import two school points to simplify).
>> > The
>> > two towns are pulled right from the state-wide GIS office, but the towns
>> > are
>> > in different states (ie - different agencies provide the data).
>> >
>> You do not need this for v.net.alloc:
>> > v.net input=roads output=road_nodes operation=nodes
>> it will create a point for every node, the first point will have
>> cat=1, the second cat=2 which collides with the points you want to
>> attach
>>
>> > v.net input=road_nodes points=schools output=network operation=connect
>> > thresh=1000
>>
>> rather use
>> v.net input=roads points=schools output=network operation=connect
>> thresh=1000
>>
>> > v.net.alloc input=network output=network_allocation ccats=1-2
>>
>> the real problem is probably a data problem: roads are not properly
>> connected, i.e. the end of one road does not match the beginning of
>> another road. Try v.clean input=roads output=roads_snap -c tool=snap
>> with a very small threshold. This kind of error is quite common with
>> shapefiles (not a fault of the shapefile format but of the software
>> used to create the shapefiles).
>>
>> >
>> > Thanks in advance for any advice, I can also email images/data if that
>> > would
>> > be helpful.
>>
>> If v.clean -c tool=snap does not help, (links to) original data of the
>> schools and road network for Town 2 would be helpful (data please only
>> off-list).
>>
>> Markus M
>
>
>
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