# [GRASS-user] Creating isolines of walking distance

Josh Doe josh at joshdoe.com
Fri Aug 19 22:47:54 EDT 2011

```Hello all,
I'm a newbie to GIS and especially GRASS, but I'm trying to create
walking distance isolines (isospace?) from OpenStreetMap data, and so
far I've been successful using PostGIS, pgRouting (driving_distance()
function), QGIS, and the QGIS contour plugin. However I'd like to be
able to do this via the command line for a web service, and would like
to try and do this in GRASS, however I have had some difficulties.

Here is what I've created in QGIS and want to create in GRASS (the blue areas):
http://wiki.openstreetmap.org/wiki/File:SRTS_walkability_before_new_sidewalk.jpg

This is the Shapefile of points resulting from pgRouting's
driving_distance() function, with the cost column containing the
distance in km from the entrance to a school:
http://dl.dropbox.com/u/23634456/terra_centre_1mile_points.zip

I'm not sure what the best (and more importantly, fastest) method is
to produce this, but here's what I have so far. I only care about
creating filled areas for 1/4, 1/2, 3/4 and 1 mile (0.4, 0.8, 1.2, 1.6
km), so don't really need or want full interpolation (e.g. a
gradient), especially if it takes more processing time.

# read in shapefile to costs layer in terra_centre mapset
v.in.ogr dsn=/home/user/points.shp output=costs at terra_centre
# set bounds to match costs layer, and set resolution (for rasters)
g.region vect=costs at terra_centre rows=256 cols=256
# idw interpolation doesn't seem to work well
#v.surf.idw --overwrite input=costs at terra_centre output=interp_idw column=cost
# perform interpolation of costs layer using cost column, and output
to interp_rst layer
v.surf.rst input=costs at terra_centre elev=interp_rst zcolumn=cost
# set colors to be four solid shades of blue
r.colors map=interp_rst at terra_centre color=rules
0 0:0:255
0.4 0:0:255
0.401 50:50:255
0.8 50:50:255
0.801 100:100:255
1.2  100:100:255
1.201 150:150:255
1.6 150:150:255
1.601 white
10 white
end
# create contour lines at 1/4, 1/2, 3/4, and 1 mile
r.contour --overwrite input=interp_rst at terra_centre
output=contour at terra_centre levels=0.4,0.8,1.2,1.6

My questions are:
1) Is there a better way to achieve what the contour plugin of QGIS does?
2) How could I create a polygon layer (for the four bands) from the
contour layer?
3) How could I create a similar result using v.net.iso instead of pgRouting?

Thanks for any help,
-Josh
```