[GRASS-user] Length of a political boundary

[Hamish]

ambijat wrote:
> No, Hamish,
> This is not the case as I see it.

ok, sorry if I misunderstood. how do you get a political boundary
from SRTM data? by watershed? is knowing the "length" actually
important? could whatever you are looking at be answered instead
by a less problematic metric like area or proportion?

> My fundamental method is as follows:
> 1. in a 3x3 matrix of neighbours analysis I want to fit in
> one worthy formula.
> 2. For every such matrix there are going to be at least 3
> non-zero values and 6 zero values.

so far sounds like a job for r.mfilter or r.mapcalc's neighbor
modifier,

> 3. as we know that c^2 = a^2 + b^2, i would be counting on
> a = elevation difference between the two adjacent cells, and
> b = 90 meters, which is the resolution of the cell.

ok. is the center cell always non-zero/null? (ie skip over those)

> 4. So, c values for each such set of two adjacent cells can
> be calculated, and a sum of all such values which are actually
> hypotenuse values would give me more accurate value of the total
> length of the boundary.
> 
> This is a kind of algorithm I have in mind. Anybody who can
> translate into formula?

probably it would be r.mapcalc (incredibly versitile), but I
guess you could get a head start by multiplying a r.slope.aspect
slope map into the mix.


good luck,
Hamish