[GRASS-user] Topological Trouble

[James Keener]

I'm trying to create a map with isolines for the road-distance places
are from bus routes.  I was trying to roughly follow
http://jcastellssala.wordpress.com/2012/05/07/basic-network-analysis-with-grass/
but I don't quite get similar results (I feel like the network isn't
completely connected, but this is the first time I'm doing this so it's
more likely I'm just doing it wrong.

I am using the following shapefiles in SRID 2272 (NAD83 / Pennsylvania
South (ftUS)):

http://jimkeener.com/grass-topo-question/roads.tar.bz2 which is the
TIGER road graph for Allegheny County, PA, USA (state fips = 42, county
fips = 003), but reprojected to 2272 and a "length" attribute added.

http://jimkeener.com/grass-topo-question/stops.tar.bz2 which is derived
from the Port Authority of Allegheny County GTFS file found at
http://www.portauthority.org/paac/CompanyInfoProjects/DeveloperResources.aspx
and reprojected to 2272.


So, the first thing I did is import these files:

> v.in.ogr dsn=roads.shp layer=roads output=roads
> v.in.ogr dsn=stops.shp layer=stops output=stops

(http://jimkeener.com/grass-topo-question/ac.png)

Then I connect the stops to the network

> v.net input=roads points=stops output=network operation=connect thresh=500

Connect the require database layer

> v.db.connect map=network table=schools layer=2

Then allocate the network

> v.net.alloc --overwrite --verbose input=network output=network_alloc ccats=1-6965

(http://jimkeener.com/grass-topo-question/ac-alloc-zoom-thumb.png when
displayed with d.vect -c) (the little Xs are bus stops

And create isolines at 1-, 2-, and 3- thousand feet

> v.net.iso --overwrite input=network output=network_isolines ccats=1-6965 costs=1000,2000,3000 afcolumn=LENGTH abcolumn=LENGTH

(http://jimkeener.com/grass-topo-question/ac-isolines-zoom.png also with
d.vect -c)

All but a few of the roadways near the open space to the right (a park
and very hilly (even by Pittsburgh standards:-p) area) are within 3000
feet of a bus stop.  Therefor I was expecting almost all of the roads to
be highlighted in different colors representing their distance from a stop.

Thank you for any help!
Jim

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