Calc bottomline of valleys

Question

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

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 :-/

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