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

[Richard Chirgwin]

>
>
>
> ------------------------------
>
> Message: 3
> Date: Wed, 12 Nov 2008 19:14:11 -0800 (PST)
> From: Hamish <hamish_b at yahoo.com>
> Subject: Re: [GRASS-user] how to measure distances in a constrained
> 	space
> To: Facundo Mu?oz <famuvie at alumni.uv.es>,	Moritz Lennert
> 	<mlennert at club.worldonline.be>
> Cc: grass-user at lists.osgeo.org
> Message-ID: <209101.3681.qm at web110013.mail.gq1.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
>
> Moritz Lennert wrote:
>   
>>>> 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.
>>>>         
>
> 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.
>>>       
>
>   
>> 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...
>>     
>
>
> just some extra notes:
> - r.cost in GRASS 6.4 should be 60x faster than in older versions
> - have a look at the new r.grow.distance in GRASS 7, even faster
> - FWIW when I've done this I've made the cost map 1,NULL and multiplied
>   the cost distance by map res. knights move is essential.
> - See the v.surf.icw script in wiki addons. it is like IDW interpolation
>   but replaces euclidean distance with true (cost) distance and goes
>   around corners/coastlines.
> - Somewhere I had a script for generating distance matrix tables for
>   distance between sites taking obstacles into account using r.cost.
>   I'd have to hunt for a backup copy.
>
>
> It would definitely be interesting to hear if you figure something out
> with v.net.visibility. you are looking for as-the-bird-flies distance,
> not road distance, right?
>
> see ongoing geodesic distance thread on the PROJ4 list for more on the
> dangers of measuring long distances on a grid map projection.
>
>
> Hamish
>   
As one burned by measuring distance on grid projections, a minor
observation ...

Keep in mind "fit for purpose". If I'm asking "how many people live
within 1km of a railway station?", other errors (such as assumptions of
the distribution of people within an area) will probably be bigger than
the measurement of 1km. For these "rough" measurements, I keep a
database table of degrees per km at tenth-degree increments. That way,
the attribute table of point data can include a roughly-accurate number
for buffering based on latitude. Rough, but re-usable!

RC