With the advent of Civil 3D, the geometry we see in modelspace is no longer a flat representation of our design.  Today, our geometry is “lifted” from the 2D plane and exists in 3D space.  By default, all Civil 3D objects, (point shots, survey figures, surfaces, corridors, pipe networks, etc.) are created at their real world elevations.
To verify this, simply click any of the hot spots on the ViewCube and you’ll see your project represented as a virtual 3D model.

Having our modelspace “earth” changing from a flat plane to 3D can create some challenges when taking measurements.  More often than not, we require 2D horizontal distances between objects, while most traditional AutoCAD commands (by default) display a true 3D measurement.

Fortunately Civil 3D has some specialized tools that allow us to take horizontal measurements in a 3D world.  Two of these tools can be found on the Analyze Tab within the Inquiry panel.

The first is called “List Slope” and (among other things) it will report the horizontal distance between any two selected object snaps in your drawing.

Here’s how it works…
Take a look at the two circles shown below.  (Notice there is a significant difference in their elevations.)  Let’s say I’d like to find the horizontal distance between the two center points.

Step 1: Launch the List Slope command.
Step 2: At the “Select object or [Points]:” prompt, press P and then hit Enter.
Step 3: Select the center of circle A, followed by the center of circle B.
Step 4: Check the command line to find the horizontal distance.

After seeing this, you may be wondering if this tool can be used to find a perpendicular measurement between entities.
(The distance from a sanitary line to a property boundary, for example.)

For a measurement like this, you’ll want to use the “Minimum Distance Between Entities” command.  This tool is also located within the Inquiry Panel.  Simply expand the panel to find it.

Minimum Distance Between Entities allows you to find the shortest horizontal distance between two objects drawn in 3D space.

Here’s how it works…
Take a look at the drawing below.  Let’s say the black linework represents a property boundary and the blue line represents an existing sanitary utility.
(Note that I have placed elevation labels to highlight the 3D placement of the geometry.) 

If I wanted to find the horizontal distance from the sanitary line to the property boundary I would use the following workflow.

Step 1: Launch the Minimum Distance Between Entities command.
Step 2: Select the property boundary, followed by the sanitary line.
Step 3: Check the command line, (or press F2) to see the minimum distance.

Step 4: At the “Draw shortest distance line? [Yes/No] <Yes>:” prompt, press Enter to create a horizontal line showing where the minimum distance was measured.  (If this line isn’t desired, simply choose NO, or press ESC to cancel the command.)

As you can see, even though our CAD geometry exists in three dimensional space today, finding horizontal measurements is just as easy as it was “back in the old days” when the earth was still flat.