[GRASS-user] v.net.iso usage - including speed as factor

GRASS GRASS user
1

Hi all,

I'm trying to calculate drive time 'zones' using v.net.iso. I've
worked through the examples using spearfish data, and seem to get
suitable results using only the line 'distance' as the factor.
However, I'd like to include the speed limit as another factor in the
calculation, and can't seem to find any examples of how to do this.

I've looked through the examples here:
http://grass.itc.it/grass64/manuals/html64_user/v.net.iso.html
and here:
http://www.dinama.gub.uy/grass/grass57/tutorial/network.html (which
mentions "This calculates the reachability map based on the vector
length for 1km, 2.5km, 5km and 10km. Another option were to use vector
line attributes from a DBMS table which may contain velocity, street
types etc." - so it appears to be possible!

Can anyone point out what I'm missing, and/or the appropriate command or switch?

Cheers,

Darren Cope
http://dmcope.freeshell.org
http://bluesignweekly.blogspot.com/

2

Darren Cope wrote:

I'm trying to calculate drive time 'zones' using v.net.iso. I've
worked through the examples using spearfish data, and seem to get
suitable results using only the line 'distance' as the factor.
However, I'd like to include the speed limit as another factor in the
calculation, and can't seem to find any examples of how to do this.

look in the help page for v.net.path for a speed limit example.
(cost is inverse)

good luck,
Hamish

3

Hi Hamish/List,

Thanks. I presume the example you meant on the v.net.path page was the
one where they entered two fields to calculate speed (e.g. The correct
fastest path can then be found by specifying afcol=length/max_speed).
However, trying to enter fields in this format in v.net.iso causes it
to crash (using command below--note I also tried surrounding
speed/length with single and double quotes, to no avail):

v.net.iso input=myroads_net output=outputname ccats=1-999999
afcolumn=speed/length costs=0.01666,0.1666,0.5,1,2,3

I then tried calculating a new field (t) manually with the 'time'
value, and ran v.net.iso using that field as 'afcolumn'. Results look
close, although I'm noticing some anomalies... I ran:

v.net.iso input=myroads_net output=outputname ccats=1-999999 afcolumn=t
costs=0.01666,0.1666,0.5,1,2,3

The attached image shows the results. Most of the categories are
'close' to what I would expect, but still seem a bit off. The real
odd result is the "1" category (themed red), which shows up
(correctly) in the middle of the image, but also (incorrectly) along
the whole N/W corner of the image. Why is this occurring? Any
insights into what I'm doing wrong?

Cheers,

Darren Cope
http://dmcope.freeshell.org
http://bluesignweekly.blogspot.com/

On Fri, Feb 13, 2009 at 2:09 AM, Hamish <hamish_b@yahoo.com> wrote:

Darren Cope wrote:

I'm trying to calculate drive time 'zones' using v.net.iso. I've
worked through the examples using spearfish data, and seem to get
suitable results using only the line 'distance' as the factor.
However, I'd like to include the speed limit as another factor in the
calculation, and can't seem to find any examples of how to do this.

look in the help page for v.net.path for a speed limit example.
(cost is inverse)

good luck,
Hamish

(attachments)

vnetiso.png