[GRASS-user] Topological Trouble

James Keener jim at jimkeener.com
Mon Sep 1 16:26:03 PDT 2014

Thanks for replying Markus,

> Note that their projection is in meters, while...
>>>> I am using the following shapefiles in SRID 2272 (NAD83 / Pennsylvania
>>>> South (ftUS)):
> ... your's is in US feet (http://epsg.io/2272).
> This affect the threshold you use below:

I know.  For testing purposes it seemed to be reasonable, though
(increments of 1000 ft. For my final map I'll probably do 1/4mi, 1/2mi,
3/4mi, and 1mi increments).

>>>>>  v.net input=roads points=stops output=network operation=connect
>>>>> thresh=500
> Note that the threshold is specified in map units (so, here feet in your case).

I spot checked a lot of the bus stops, and they appeared to be connected
to the road graph. FWIW.  The TIGER road maps do appear to make
intersections overlap, but don't put a point that all the adjoining
roads connect to. (This brings up something I was going to worry about
later: bridges vs intersections?)

> Sidenote: in GRASS GIS 7 the module is a bit more advanced:
> http://grass.osgeo.org/grass70/manuals/v.net.html
> Perhaps (guessing, I didn't have time to test your data), the initial
> threshold makes the difference?

I'll have to give GRASS 7 a shot:) (GRASS 6 is what was in the repo I
was using).

I'm completely new at this so the biggest problem for me is not know
exactly where to look when something isn't working.

Thanks again for your input!


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