Summary: OSNAP seems to be limited when it comes to Civil3D objects, and the difference between 2D and 3D OSNAP is still very primitive. Upgrade the system to work as intended for all Civil3D objects.
Description: For one, OSNAP in 2D and 3D mode still don't have as much functional difference as intended. For instance, a 2D OSNAP perpendicular to a line with different elevations on either ends defaults to a 3D perpendicular to the true line. 2D OSNAP is supposed to ignore the third plane and take a perpendicular to the line as seen (or in referenced planes).
And second, there are many cases where OSNAP does not work with Civil3D objects, such as profiles. I can't count the number of times where finding a new inflection point would have worked with an EXTEND command or finding an intersect with the grid for a station-elevation relation.
Product and Feature: AutoCAD Civil 3D - User Interface
Submitted By: Sebastien Leonard on 06/25/2012