Have you ever created a surface in Civil 3D and wondered how in the world can I calculate the area at a particular elevation? Well, if the elevation falls on the contour interval, that task is rather easy. Extract the contour line from the surface as a polyline and list it. What if the elevation does not fall on the contour interval? The process is similar, you just have to create a user contour.
Creating a User Contour is fairly simple. If you can do a slope or elevation analysis in your surface, you can create User Contours. The first thing to do is create the surface.
This surface could represent a pond or ground feature. Let’s assume that you needed to know the area at a 52.5 elevation. Open up the Surface Properties and go to the Analysis Tab. On the Surface Analysis tab, change the analysis type to User-Defined Contours, set the number of ranges, or user-defined elevations that you want to create and run the analysis. When the range is created you may have to manually adjust the elevation for the user-defined contour.
After running the analysis and changing the elevation of the contour, go back to the Information tab. On the information tab we need to edit the surface style so that the user-defined contour is displayed in the style.
After turning on the visibility of the user contours, we can click OK and close the dialog box. This should regenerate the surface and show the user-defined contour based on the applied surface style.
You can see in the image above that the user contour is yellow-orange and of a dashed linetype. Now, in my opinion, the easiest way to get the area of that contour is to extract a polyline from the surface. Click on the Modify Tab of the ribbon and then click on Surface. This will open up the contextual surface ribbon. On the Surface Tools panel you will see the Extract Objects command. Click on the Icon to initiate the command.
In this case we only want the user contours, so we will deselect the other check boxes. This will create a polyline from the user contour that we can then list to get the area. Creating user contours can also be beneficial if you are using the stage/storage tool as it will add the elevation into the table when computing the volumes.
Getting to the Dirt II 
August 4, 2010 at 1:14 pm
Another nice corollary of this method is the ability to create a lake water surface “Surface” using the extracted polyline. Use the polyline as a “breakline” in the definition and as a “boundary”. Then, run a volume between this surface and the lake bed surface to get a water volume.
October 22, 2010 at 4:27 pm
Nice application Jeff!
August 30, 2012 at 9:04 pm
This is a great thread. Very useful tool for contour data extraction! Very great job.
Is there a way to create a points from a polyline (not alignment). I would like to create points along a polyline using elevation as the basis? The purpose of this is to create assumed river bed elevation data base on the some sample shots along the river. This would be needed for flood analysis. Along the sections of the river, we can only get a few survey since the current is strong, that why I would like to simulate some points along the riverbed so that upon interpolation, there would be points along the river.
December 16, 2010 at 9:42 am
It’s not the true area if there is no data at the desired elevation, just an interpolated contour.
December 16, 2010 at 1:05 pm
You are correct that this is not the “true area,” but if you want to get technical, is it even a “true surface?” The surface that you use for design purposes is modeled based on the interpolation of elevations between at least 2 known points on a straight line grade between the 2 points. Is thus a “true” representation of the surface. I say it is not. As an industry we have come to accept certain criteria for collecting data in the field to create a topographic representation of the earth at the location we are interested in. It would be impossible and impractical to collect enough point information to create a “true” surface. So, from the context of the post, it is implied that we are accepting the survey criteria and in so accepting the interpolation as the exact surface.
August 30, 2012 at 9:04 pm
Thanks very great thread!