Manifold Community Site: Slopetendency

RE: Slopetendency

volker — Thu, 03 Jul 2008 00:57:51 GMT

is it possible to measure the slopetendency in a surface ?

or over the entire surface with a given tendency (like in my attached example (in german)) ?

Attachments:
hangneigung.jpg

RE: Slopetendency

Nick Verge — Thu, 03 Jul 2008 01:44:11 GMT

Not directly ( i think), but can you clarify what you mean by slope tendency.

RE: Slopetendency

volker — Thu, 03 Jul 2008 01:54:09 GMT

Hi Nick,

i try to clarify (i don`t know if "slope tendency" the right word, it`s a free translation of me).

Like in my attached example you have a surface and i want to know all locations that have a

tendency of say equal or more than 15 degrees (or in percent). Make an Area Drawing of this

locations that is the target.

Volker

RE: Slopetendency

ColinD — Thu, 03 Jul 2008 02:44:13 GMT

Volker, this sounds like you need Edit Save Mask/Channel Slope from your surface and then from the resulting slope surface create contours Areas Between at the intervals you would like.

RE: Slopetendency

Nick Verge — Thu, 03 Jul 2008 02:59:29 GMT

This just requires performing a boolean-based selection on the elevation gradient surface object either in % or degrees. Then, tracing the outline of the selected cells. There are several ways of doing this procedure.

All i would caution you about is being careful in how you calculate gradient. Depending on whether the DEM elevation values refer to points or finite areas (cells) and the gradient algorithm used, you will get different (and sometimes signifcantly different) results.

RE: Slopetendency

Graeme — Thu, 03 Jul 2008 02:58:30 GMT

Colin has one option which wouldn't require the surface tools extension. If you do have the surface tools extension, do the save mask channel first (assume you call it "slope") then in the surface transform dialogue enter:

IIf ([Slope]<15,Null,[Slope]). Check save result as new component and resulting Slope2 will be all pixels <15°

RE: Slopetendency

volker — Thu, 03 Jul 2008 05:29:46 GMT

Thank you all.

So i am a proud ownerof the surface tools i try the method

of graeme. It seems to me it work perfect, but if i do it

with the formula "IIf ([Slope]<15,Null,[Slope])" it generate

a surface with the tendency >15° (good for me), but why ?

Look at the attached Map image.

I want to attach the Map-file, but it`s too big.

Attachments:
Map Tendency Image.jpg

RE: Slopetendency

Graeme — Thu, 03 Jul 2008 05:58:50 GMT

Understand your confusion Volker.

What the transform is asking is to set those pixels with a slope of less than 15° to Null. So what you're left with in the result [Slope2] is those pixels where the slope is greater than 15. If you were seeking only areas where slope is more than 15°, you'd simply reverse the operator and use IIf([Slope]<15,Null,[Slope]).

Cheers, Graeme.

RE: Slopetendency

volker — Thu, 03 Jul 2008 06:14:28 GMT

oh thank you, now i understand

and it works perfectly

RE: Slopetendency

Graeme — Thu, 03 Jul 2008 06:21:42 GMT

If you haven't discovered the "linked surfaces" functions, you can create linked surfaces from queries (see the help topic "linked surfaces"). In your case create a query containing:

SELECT * from [Slope] WHERE [Height (I)] > 15;

Then file/link/surface/this project/<name of your query>

You'll need to assign a projection and format the display, but depending on your requirements, it may be an option worth considering.

RE: Slopetendency

volker — Thu, 03 Jul 2008 06:43:13 GMT

yeah it`s a fast alternative.

Are you a Surface-specialist ?

RE: Slopetendency

Graeme — Fri, 04 Jul 2008 02:20:52 GMT

Did a lot of surface work when natural resources formed a large part of our task mix, but that sector has diminished for us. Probably a little rusty!