[GRASS-user] slope along defined section (e.g. road, stream,)

[Jachym Cepicky]

Hi,

Tim Michelsen píše v St 06. 02. 2008 v 17:22 +0000:
> Hello,
> I would like to get the slope of a defined section like a part of a river or
> a road.
> 
> The only post dealing with such a idea was left unanswered:
> [GRASSLIST:7280] Hazard Assessment
> -
> http://thread.gmane.org/gmane.comp.gis.grass.user/8702
> 
> My idea is the following:
> 1) define a starting vector point and a end vector point of a stream or road
> section. 
> 2) then run a module to calculate the slope along that section
> 3) add the slope to the attribute table of the stream/road vector
> 
> While point 1 and 2 are quite clear to me I would ask you for hints on how to
> calulate the slope as lined out in point 2.
> What I am looking into would be a slope version of d.measure...
> 

This is imho not so easy, how it looks. While vector lines are kind of
discrete objects (from node to node is one line), slope changes
continuously. The attribute of the slope will always have to be some
average along the line. You can cut the line e.g. every 200m, to fit it
better.

Anyway, with v.drape you can fit your vector network to digital
elevation model. With v.to.db you can store the length of the line and
it's nodes coordinates to the database and slope is
length/(end_node_z-start_node_z), which can be performed with db.execute

What more: I just find out, that v.to.db has an option "slope" as well -
no need to calculate it manually :-)

  Hope, it helps

Jachym
 
> To have an idea on what one could like to do with the results:
> * Get a first estimate (based on DEM resolution) on hydraulic gradients in
> rivers
> * assess the risk of certain roads in mountainious regions
> 
> Thanks in advance for your help.
> 
> Kind regards,
> Timmie
> 
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-- 
Jachym Cepicky
e-mail: jachym.cepicky at gmail.com
URL: http://les-ejk.cz
GPG: http://www.les-ejk.cz/pgp/jachym_cepicky-gpg.pub

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