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Which way do I calculate bottomlines of valleys or even more preceise: which way do I "find" valleys (for example suggested tectonic faults).

Sample below shows a height profile (profile from line) based on Aster data and it's that I at most want to follow the larger valleys (long arrowed lines).

I could go for hydrological methods (even though havent been working with them), but using hydro plugins/tools a small breakthrough to lower areas would cause a runoff, while I want to follow the big valley.

Any ideas?

--- (edited) To underline my problem, I've just taken a circa bottom line profile of the valley pointed to by the most left arrow in pic-1 (arrow'ed). The intersection is at about 8km of 1st diagramm (with arrows) and at km 35 of second profile.

---- (edited) Just to spread some ideas: Rotating the 1st image arount the center between point 1 an 2 (1st and 2nd arrow, Height Profile) also shows "valleys" with different idth/height ratios, but still significant.


QGIS 1.8, Ubuntu, Aster Elev Data

Profile-1

First and second "sharp" valley from left (large arrows) are of most interesting to get the idea. It might help to focus on focusing on strong beeing above/below average. Just an idea :-/

Profile-2

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I don't understand the "small breakthrough" issue. Hydrological methods make a (crude) estimate of total runoff; you then identify the cells where that estimate exceeds a (rather large) threshold. This eliminates all valleys whose total catchment is less than the threshold, leaving a network of apparent stream centerlines to delineate the largest valleys. –  whuber Dec 17 '12 at 19:19
    
@whuber: Ref to the small breakthrough.... suggest simple some kind of terrace (your place to sit in the evening if owning a hillside home). If there would be a - in compare to whole surface - small hole within right in the middle at a slightly deeper location, the water would run off.... but netherless the terrace continues. Sorry - I'm not a native english speaker, so sometimes I have to give very self-plaining visible and simple examples to explain my thoughts/ideas and intensions. –  Chris Pallasch Dec 17 '12 at 20:48
    
I do not follow your example. Do you, or do you not, want such a "small terrace" to be considered a "valley"? If you do, set the flow accumulation threshold low; if not, set the threshold higher. –  whuber Dec 17 '12 at 20:52
    
@whuber - (skippinh hydrology) your ideas/thoughts are pretty difficult for me to translate; but if I did not get it totally wrong, you do pretty much understand my problem - any why I cant use hydrologic plugins/tools - and where I'm heading for! –  Chris Pallasch Dec 17 '12 at 20:53
    
@whuber - I'm searching the big terraces. I do not want to follow the run off. My english is awful, sadly, I do know :-/ Say there would be a hole in the middle of Golden Gate Bridge covering 2-4 lanes (6 total if I recall my holiday some 20+ years ago) ---- I still want to go straigt on an not fell into the hole, following the bridge! –  Chris Pallasch Dec 17 '12 at 21:01

1 Answer 1

I assume that your graphs came from a R-script and that you are capable of using R. Here is a solution in R, which finds local maxima and minima along a data sequence

x <- rnorm(50,mean=1500,sd=800) # Example-Data
r <- rle(x) # Generate run sequence object

min <- which(rep(x = diff(sign(diff(c(-Inf, r$values, -Inf)))) == 2, 
                 times = r$lengths))
max <- which(rep(x = diff(sign(diff(c(-Inf, r$values, -Inf)))) == -2, 
                 times = r$lengths))
plot(x,type="l")
points(x[min]~min,pch=19,col="red") # Plots the minima points
points(x[max]~max,pch=19,col="blue") # Plots the maxima points

enter image description here

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That identifies local minima of a single profile, but I believe the question asks to identify valleys within a two dimensional region: the profile was offered only as an illustration. –  whuber Dec 17 '12 at 19:17
    
@Curlew: Sadly these are no R-Results, due to I learned /read knowledge of R would help me a lot :-/ Could you please point me to a suitable R newbie Documentation plus maybe an documented example for local min/max calculations? -> quick entry for local min/max calc. –  Chris Pallasch Dec 17 '12 at 19:28
    
@whuber: yes you are right. Profiles are circa 1st NE-SW, 2nd W-E The "to calculate" line doesnt need to focus on 3D (height in the end is unnecessary), but simly show the 2D path of the valley. –  Chris Pallasch Dec 17 '12 at 19:34
    
Imagine a huge rotovator driving up and down on a random path through a mountain area..... I want to follow it's path! –  Chris Pallasch Dec 17 '12 at 19:42
    
nah, sorry. Obviously i misunderstand your question (and i still do). Maybe instead of a straight line you could look for the local minima/maxima in a polygon (2D), where the z-axis equals the elevation. Sth. like this (tinyurl.com/cpxevju) ? –  Curlew Dec 17 '12 at 22:11

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