AVERAGE SLOPE OF A PROFILE

It might depend on the flavor of cad.?ÿ Seems the best approach would be a least squares regression calc, but I doubt there are any using that.?ÿ Civil 3D gives a mean slope for a TIN which apparently is a weighted average of the triangles over the entire surface.?ÿ I know that's different than an average slope of a profile, but I'd presume with Civil 3D it would be calculated the using the same algorithm.?ÿ (Does C3D even give an average slope of a profile?)

C3D doesn't calc an average slope in profile it calcs the slope at the edge and across each 3dface.

Surface slope tool calcs the slope of each 3dface with the single point or using 2-point it calcs the difference at the selected elevations divide the length no real magic

• @mlschumann I'm going to argue that your Rocket Science diagram is showing nothing more than the slope of a line from point A to point B. Not average. Just plain old slope.?ÿ

I have always used a formula involving the length of the contours and the contour interval, but for the life of me I can't find the formula.

Here is my process using Carlson w/oem acad.

Create profile from surface file or screen entities.?ÿ Create 3d line from profile. Inquiry> polyline info. The result will generate an average slope. Which is different from a simple rise over run.

My thoughts are that each vertical slope between vertices is weighted by its length to calculate the average slope of the entire profile.

I just couldn't find any white paper on the subject.?ÿ

Are we trying to find the slope (dashed line) between two points as illustrated above, which in one sense could be called the average slope ...

Or are we trying to find the average magnitude (always taken as positive) over the land between the points (shorter solid lines), as a measure of roughness? The latter can be extended to an average over an area.

However, the latter has a fractal problem in that the closer together your measurement points the larger the answer as every clump or grain of soil adds more.

• @dave-lindell Was this what you were thinking of Dave?
  

  

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In my experience this can be done with TopoDot in Microstation.?ÿ Which is a point cloud utility.?ÿ The user will need to define parameters such as minimum length and width, along with Deflections.?ÿ There is also a routine within OpenRoads and InRoads. If you need consultation with this task feel free to DM me.

@mag-eye : That looks something like it!

The question by MAG-EYE is:
Does anyone [k]now how cad calculates the average slope of a profile over irregular terrain.

The solution I presented is the average slope of a profile, NOT the average slope of a part of surface as provided by Dave Lindell. While Lindell's solution may be correct for the surface contour data presented, it is not the average slope of a profile as asked in the question. A profile, by convention, is a series of line segments representing a vertical terrain or surface cross section. I have not yet seen a profile to be defined as a part of a surface.

A follow-up diagram presents how the solution for the average slope of a profile is determined. Of particular note is that the diagram demonstrates that the length of a slope is fundamentally the weight or influence of that slope in the solution.

And, there is good probability that a cad program determines an average surface slope using the digital terrain model (DTM) triangles instead of contours.

C3D doesn't calc an average slope in profile it calcs the slope at the edge and across each 3dface.

Surface slope tool calcs the slope of each 3dface with the single point or using 2-point it calcs the difference at the selected elevations divide the length no real magic

I've read that C3D calcs a weighted average slope for the TIN, presumably triangle slope/triangle area as the unit.?ÿ It doesn't, that I know of, calc an "average slope in profile" which, if it did, I would presume it uses the same algorithm (but I repeat myself).?ÿ What do you mean by "at the edge and across each face"? Wouldn't that be the data used to compute a weighted average??ÿ And where do you see this output?

I'm not trying to be contentious but I've made up my own spreadsheet to do this from C3D profile data for use in drainage calcs and if it's already doing it for me somewhere I'd love to know.

If by "surface slope tool" you're talking about the surface slope annotation that allows you to pick a single point or use a two point average, then your response is irrelevant to the OPs question.

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How does cad calculate the average slope of a 3d polyline?

Below is the report of a 3d polyline that cad generates.

Low Z: 503.00 At: 4991.32,5069.74
High Z: 542.00 At: 5100.21,4976.61
Polyline length: 412.52 Slope distance: 422.70
Avg elev: 528.95 Avg slope: 14.33%, 6.98:1 Open

The line includes 37 vertices and the two end point elevations are 503.00' & 526.73'

How does cad calculate the average slope of a 3d polyline?

Below is the report of a 3d polyline that cad generates.

Low Z: 503.00 At: 4991.32,5069.74
High Z: 542.00 At: 5100.21,4976.61
Polyline length: 412.52 Slope distance: 422.70
Avg elev: 528.95 Avg slope: 14.33%, 6.98:1 Open

The line includes 37 vertices and the two end point elevations are 503.00' & 526.73'

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What "cad" are you using to get this?

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