Senior Engineering Designer Perry Medina shares with us a method of calculating the elevation of a point between two known points.
"If you want an approximate ground elevation at a point with no contours, draw a line between the two nearest 3D contours. Then use the ID command with the nearest osnap to get a 3D elevation at the selected point. It's a simple interpolation, a la AutoCAD. You can see an object's elevation via the properties palette or the humble List command."
Notes from Cadalyst Tip Patrol: This is why we use CAD — to lighten the load. For this to work, the contours must have a Z value, or an elevation. If they don't, that's easy enough to change. Select the contour lines, and in the properties manager, change their Z value to the elevation where they're located. Then proceed with the tip as described above. Or you can calculate the elevation using a slope intersect formula, but why bother with that when you have CAD!