# Problem Solving: Finding slope breaks in DEM

kfroese
Geotechnical EngineerPosts:

**165**Trusted User
I am working on a project considering development boundaries on a piece land next to a high river bank. I'm interested in finding the major breaks in slope along this property. I typically export my DEM as slope, bring it back into GM, and then contour on 7degrees. This 7d line has typically given me the top-of-bank line fairly well. However, it is not working at this site to find the bottom of the slope as the plateaus adjacent to the river are sitting anywhere from 6d to 20d.

What I have tried to figure out is how to contour were there is a significant change in slope. Remembering back to my university calculus, it seems that what I'm looking for is essentially the 2nd derivative (slope angle being the 1st). So, I exported my DEM of slopes as slopes (which gives the slope of the slopes, or rate of change of slope). It seemed like a good idea but I had too much noise in the data to cleanly extract a contour line (unlike the top-of-bank 7d line).

Any thoughts from the community?

What I have tried to figure out is how to contour were there is a significant change in slope. Remembering back to my university calculus, it seems that what I'm looking for is essentially the 2nd derivative (slope angle being the 1st). So, I exported my DEM of slopes as slopes (which gives the slope of the slopes, or rate of change of slope). It seemed like a good idea but I had too much noise in the data to cleanly extract a contour line (unlike the top-of-bank 7d line).

Any thoughts from the community?

## Comments

17,238Let me know if I can be of further assistance.

Thanks,

Mike

Global Mapper Guru

gmsupport@bluemarblegeo.com

http://www.globalmapper.com

165Trusted UserThanks.

454Trusted UserE.g.

AvgValue(Slope(Slope(LowPass1([DEM]), 3), 3), 1)

This would apply a moderate low pass (smoothing) filter to the source DEM, calculate the slope over a window of 7 x 7 cells, then the same again (slope of slope), and finally apply averaging to the results over a 3 x 3 window. Because it happens so quickly, it's essentially costless to experiment to get the result you need.

Horses for courses. Happy to send you back a processed sample if you can supply source data.

165Trusted Usertjhb: can you send me a private message with your email address so I can send you some sample data? I'm curious to see what Manifold can do with this as it is a program I have considered looking into.

454Trusted UserI can't see how to PM you here--nor how you could PM me. I thought we had email addresses in our profiles but apparently not. I'd certainly like to help though.

Try my user name at clear dot net dot nz.