Average level within an area

[Anonymous]

I have been able to undertake this with other software and have desperatly tried to find this same function in Civil 3D but am really struggling and therefore would appreciate some help.

Is there a command that returns the avaerage level of any particular surface within a specified area, for instance if we have a generated a large surface and then draw a square in  the middle of the surface, can it calcualte the average level within the square only?

Thanks in advance for any help that anyone can offer.

[ToddRogers-WPM]

You can create a Cropped Surface.  Once the cropped surface is created you can go to the Surface Properties, Statistics tab, and it will give you the Mean Elevation.

[mathewkol]

[...and it will give you the Mean Elevation....

Which of course is not average, but close enough?  Only the OP can know for sure.  I know of no way to get the average elevation in a surface without a 3rd party application.

[sboon]

I found a definition of Mean in Wikipedia.  Matt, are you saying that the value reported by the software is not the arithmetic mean?  If not then what is it?

[ToddRogers-WPM]

mean (mn)

n.

1. Something having a position, quality, or condition midway between extremes; a medium.

2. A number that typifies a set of numbers, such as a geometric mean or an arithmetic mean.

3. The average value of a set of numbers.

[mathewkol]

There are different ways to calculate mean.  Civil 3D doesn't state whether it's arithmetic mean or not, but that is moot.  I found this in the help file.  In this case mean is average.  in my original post I was thinking median.

Mean Elevation

Displays the mean or average elevation of all the points on the surface.

[sboon]

If I understand this correctly then for a Grid surface the reported value would be correct, but not for a TIN - unless the surface points were evenly distributed across the area.

Thinking about this some more - if you could generate a grid of surface elevation labels, then extract all of those values then the mean of those would be what the OP is looking for.

[mathewkol]

I have a surface with 4 points, 20,20,20,2.  Add them and divide by 4 you get 15.5.  My Civil 3D surface Mean is 14.  I assume the calculation takes surface area into account??  I don't know what math is used for the surface mean elevation.

At any rate, the OP is going to need to determine the nature of Civil 3D's calculation and see if that meets his needs.

[ToddRogers-WPM]

That's good information to know Matt.

[sboon]

The ideal calculation for a TIN surface would probably be a weighted average, based on the elevation at the centroid of each of the triangles and their surface area.  Matt, what happens with your example if you calculate it that way?

[mathewkol]

Thanks, hence why I brought it up.  Puzzled me when I first discovered it.  I have never been asked for the value so I never really pursued it further.

[mathewkol]

I'm not sure Steve.  I'm not sure about how to calculate that.

[sboon]

If you draw lines from the three vertices of a triangle to the midpoints of the opposite edges the intersection of those lines will be the centroid.  To get the weighted average you calcualate the sum of:

Elevation at the centroid of each triangle * (Surface area of the triangle / Total area of the Surface)

14*(50/100) + 20*(50/100) = 17

The result can be affected by the orientation of the TIN.  If I flip faces then I get a different answer.

[Hidden_Brain]

i have a surface with 415 points, Civil 3D reports the mean surface elevation as 373.172'. when i extract the points, i get 373.220' as the arithmatic mean. close, but not spot on. i remember seeing here on the forums someone mention how Civil 3D calculates it, but cannot seem to find it at the moment.

*EDIT: realized after inserting the images that they may be barely readable. the images are attached.

[Hidden_Brain]

also, in surf3.png, it may appear that all the data rows under "Position Z" column were not selected when calculating the mean, which is not the case. the mean reported is for all the Z values (415 values), which can be seen in surf3.png under the "Count", beside the "Average".

[Hidden_Brain]

i seemed to have found it, Christopher in his blog post mentions the mean grade/slope, and how it is calculated as a weighted average (i assume weighted average over triangle area). not sure if the mean elevation is the same way:

http://blog.civil3dreminders.com/2009/11/average-surface-slope.html

[7eeeshan]

Thank you 😘