[GRASS-user] how to measure distances in a constrained space

[Moritz Lennert]

> Moritz Lennert escribió:
>> Hello,
>>
>> I would like to measure distances between points in a constrained 
>> space, such as distances between cities, but only taking into account 
>> distances by land, and so excluding any water bodies. Concretely, this 
>> means finding those cities in a first set of cities which are at least 
>> 150km from the nearest city in the a second set of cities.

On 12/11/08 19:46, Facundo Muñoz wrote:
 > I've implemented exactly the same idea as yours in this script:
 > http://www.geeitema.org/doc/guenmap//docs/v.costdist.mat.zip
 >
 > ...with the particularity that it stores the computed distances in the
 > attribute table of the second set of cities, which was needed for my
 > application.
 > You can try it, and see if it works for you.

Thank you ! It is actually a bit overkill for me as I don't need the 
distance from every city in set 1 to every city in set 2, but only the 
distance for every city in set 1 to the closest city in set 2. As I have 
over 1000 cities in one and 556 cities in the other set, it is a bit 
heavy for your script.

I actually now get a correct result with the following steps (my 
original cost map was wrong, that's why my results where weird):

- create a cost map where all land parts are equal to the resolution and 
all water masses are null
- run r.cost with start_points=set2 (actually no need for null_cost) tio 
create the distance map
- run v.what.rast vector=set1 raster=distance column=dist_column

And results seem perfectly correct.

Now just for curiosity, any possible solution in vector space ? I 
thought about v.net.visibility, but it would need to be altered to allow 
a visibility graph from vector1 to vector2, instead of only within 
vector1. Then I guess one could use something like v.net.iso or loop 
through v.net.path...


Moritz