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How does Civil3D Calculate Average Slope?

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Message 1 of 4
dankoss
4695 Views, 3 Replies

How does Civil3D Calculate Average Slope?

Does anyone know what formula Civil3D uses to calculate the average slope of a surface?

 

The formula I am using is S = I*L/A

 

S = Average Slope

I = Contour Invertval

L = Length of Contours (ft)

A = Area of parcel (sq. ft)

 

I just had to do four parcel calcs and the percentage I find using the formula is always 1-3% lower than the percentage Civil3D spits out. I can't find anything about how Civil3D arrives at that number, it just appears. I assume that Civil3D is more accurate but I can't give that number to a client without knowing how the program arrived there. Any clues?

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Message 2 of 4
sboon
in reply to: dankoss

The software apparently uses a weighted average formula.  Check this thread for more info.

Steve
Expert Elite Alumnus
Message 3 of 4
JamesMaeding
in reply to: sboon

I know this thread is from 2013, but I had the same question, here in 2019, and the "solution" here points to a thread on average elevation, not slope.

This is totally wrong. It was not answered and is not the solution. I have yet to find the answer for average slope but it has to be the average of area x slope value.

The question is if the area is the 2d or 3d.


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Message 4 of 4

currently researching this topic.  In doing so, the 2D area is used for the slope value.

 

To validate this. Create a rectangle that is 1000 Ft by 2000 ft.  Draw a 3D poly over the rectangle.  Set the elevation of the top left corner at 0. And the bottom right corner at elevation 10,000.  Adjust the corners in between.  Create a surface with the four points and create contours that are based on 100 intervals.

 

Look at the surface properties, and make note of the 2D surface area, 3D surface area, and the slope.

 

now extract the contours, measure the lines and use the contour interval approximation method to calculate slope.  Calculation  1: use the 3D area in the formula

calculation 2: use the 2D area in the formuka

 on the surface use the surface label properties to label the surface slope.

compare the answers.   You should find that the 2D area gives almost the exact same answer as C3D does.  This makes sense as slope is run over rise (for example 3H:1V), it is not based on the hypotenuse of the triangle.

 

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