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

[Darren Cope]

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 at 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
>
>
>
>
>
>
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