There are times when it might be difficult to get a clean tracking point with the After Effects 3D Camera Tracker, and there may be times when you would want to isolate a point in the tracking data, and convert that to a 2D Tracking point. Here, Dylan Reeve provides a demonstration for doing just that.
A quick tutorial for After Effect CS6 and later about generating a 2D track from 3D Camera Tracker data
Dylan uses a good example to explain why you would want to do a conversion like this between a 3D Tracked point to 2D space. Using a simple toComp After Effects expression to grab the anchor point from the tracked point to the target 2D null, is a simple solution for doing a conversion from 3D space to 2D space in a shot. Check out the tutorial for Generating 2D Tracks from After Effects 3D Camera Tracker here.