[GRASS-user] affecting cat of closest feature as pixel value

[Moritz Lennert]

On 10/07/09 14:53, Markus Neteler wrote:
> On Fri, Jul 10, 2009 at 9:37 AM, Moritz
> Lennert<mlennert at club.worldonline.be> wrote:
>> Using r.cost with multiple starting points with my students, the question
>> came up whether it is possible to use the resulting cumulative cost surface
>> to assign each pixel the cat (or other info) of its closest starting point,
>> so something like r.grow.distance, but taking into account the cost, not
>> necessarily the distance.
>>
>> Any ideas ?
> 
> (Un)related wish: while you are at it, could you also compare the results
> of r.terracost in the Addons repository [1]? If as expected, I would suggest
> to replace r.cost with the code of r.terracost in GRASS 7.

Using two different examples I use in current teaching (dataset nc_spm_06):

g.region rast=elev_state_500m
v.to.rast nc_state use=val val=500.0 out=cost
v.to.rast urbanarea use=val val=1 out=urbanarea
r.cost -k in=cost start_rast=urbanarea out=dist_urban
r.terracost in=cost start_rast=urbanarea out=dist_urban_terracost
r.mapcalc diff=dist_urban-dist_urban_terracost

results in

http://geog-pc40.ulb.ac.be/grass/misc/comparison_rcost_rterracost_1.png
with
dist_urban: maximum: 83450.8, mean: 11829.7
dist_urban_terracost: maximum: 85324.1, mean: 11932.2


g.region rast=landclass96
r.reclass landclass96 out=cost_landcover << EOF
1 = 200
2 = 20
3 = 5
4 = 50
5 = 30
6 = 1000
7 = 5
EOF
r.cost -k in=cost_landcover out=dist_rcost coordinate=629980,228527.25
echo "629980|228527.25" | v.in.ascii out=point
#would be nice if r.terracost also accepted a vector map or coordinates 
as starting points
v.to.rast point use=val val=1 out=point
r.terracost in=cost_landcover out=dist_terracost start_rast=point
r.mapcalc diff2=dist_rcost-dist_terracost

results in

http://geog-pc40.ulb.ac.be/grass/misc/comparison_rcost_rterracost_2.png
with
dist_rcost: maximum: 30320, mean: 10641.8
dist_rterracost: maximum: 30755.4, mean: 10868

So, differences are non-negligeable, but no idea how to interpret them 
at this stage, i.e. don't know which one is "correct".

Moritz