[GRASS-user] movement direction surface in r.drain

[Markus Metz]

On Tue, Jan 8, 2013 at 3:41 PM, Margherita Di Leo <diregola at gmail.com> wrote:
> Thanks for super quick answer,
>
> On Tue, Jan 8, 2013 at 3:25 PM, Markus Metz <markus.metz.giswork at gmail.com>
> wrote:
>>
>>
>> Check if the friction map is not all zero or NULL for the second
>> polygon,
>
>
> the friction map within the ending polygon has everywhere value 1.

And what are the values of the friction map outside the ending
polygon? Should also be 1 I guess.

>>
>> that the second
>> polygon is actually reachable from the first polygon in the output of
>> r.walk,
>
>
> Not sure how to verify that, but i digitized a random point within the
> ending polygon (i placed it somewhere in the center) and i used it as
> starting point for r.drain, and obtained a nice least cost path line.

Interesting. Are the costs anywhere larger than zero?

>
>>
>> Also
>> have a look at the direction output of r.walk.
>
>
> Looks OK (it has values) but i'm still not sure what should i check in it.

That the directions make sense. In GRASS 7 as of r54582 they are
degrees CCW from East, zero is undefined, 360 is due East.


A little example in the NC dataset:

# walking from Macon county to Dare county

g.region -p rast=elev_state_500m at PERMANENT

v.to.rast in=boundary_county at PERMANENT use=val val=1 where="NAME =
'DARE'" out=dare

v.to.rast in=boundary_county at PERMANENT use=val val=1 where="NAME =
'MACON'" out=macon

r.mapcalc "friction = 0.0001"

# use Knight's move
r.walk elevation=elev_state_500m at PERMANENT friction=friction
output=dare_cost outdir=dare_dir start_rast=dare -k

r.mask raster=macon

r.out.xyz in=dare_cost out=- | sort -t "|" -k 3 | head -n3

# gives
# 234250|147750|490937.498669579
# 234250|147250|491132.5043559544
# 234250|146750|491258.3433474401

# Coordinates with lowest cost in Macon county are 234250,147750, and
the lowest cost is > zero.

r.mask -r

# use directions input because the input is a cost surface
# and vector output because of Knight's move in r.walk

r.drain -d input=dare_cost indir=dare_dir output=macon2dare
vector_output=macon2dare start_coordinates=234250,147750

Markus M

>
> Thanks,
> madi
>>
>>
>> HTH,
>>
>> Markus M
>>
>> >
>> > Thanks in advance.
>> >
>> > Best,
>> > madi
>> >
>> > --
>> > Margherita DI LEO
>> > Postdoctoral Researcher
>> >
>> > European Commission - DG JRC
>> > Institute for Environment and Sustainability (IES). Unit H03 – FRC
>> > Via Fermi, 2749
>> > I-21027 Ispra (VA) - Italy - TP 261
>> >
>> > Tel. +39 0332 78 3600
>> > margherita.di-leo at jrc.ec.europa.eu
>> >
>> > Disclaimer: The views expressed are purely those of the writer and may
>> > not
>> > in any circumstance be regarded as stating an official position of the
>> > European Commission.
>
>
>
>
> --
> Margherita DI LEO
> Postdoctoral Researcher
>
> European Commission - DG JRC
> Institute for Environment and Sustainability (IES). Unit H03 – FRC
> Via Fermi, 2749
> I-21027 Ispra (VA) - Italy - TP 261
>
> Tel. +39 0332 78 3600
> margherita.di-leo at jrc.ec.europa.eu
>
> Disclaimer: The views expressed are purely those of the writer and may not
> in any circumstance be regarded as stating an official position of the
> European Commission.