Use a conventional Corner Pin track/ mocha or do it by hand. Sorry, but you are trying to be "scientific" for the sake of "doing it right" when more conventional techniques, including some quick&dirty manual keyframes, would work much better and faster. And of course next time plan your shoot better and place markers.
That shot is going to be tough to 3D camera track with AE's camera tracker. You need to eliminate the car from the track if you are going to try Camera Tracker. A screenshot showing the tracker points will give me a better idea but I suspect that you're getting very few tracking points on the flat green ground.
Syntheyes would probably track this shot but it would still take a bit of hand work.
I think the easiest way to do this is manually. Just add your replacement layer, add corner pin, set the corners, set the first keyframe then go to the last frame of the shot and adjust the corners, then split the difference and adjust the mask again, then again, then again.
Then use Rotobrush or manual masking to remove separate the car from the backgrouind on another layer and do your composite.
Hi thanks for the reply. Yeah have had problems with the very flat ground of this shot so have gone for the manual method you mentioned and am cracking on with it now, i will share how i get on to see if im doing it righ t and to the best quality i can.
I may need a bit of help with the rotobrushing part, i know how to do that but just wondered how do i separate the car to a new layer?
There is a technique in mocha called "unlinking" that is very useful for tracking shots like this.
In mocha AE, create a shape at the bottom of the road select perspective in the track tab.
Before you track, select the layer and "unlink" the search area from the track.
When you track the search area will stay static and automatically offset the track. Hopefully you will find this useful as it can really reduce the ammount of time on long pans or moves in which the majority of the pixels are on the same plane.
I think this video should be useful as well: http://www.imagineersystems.com/videos/fundamentals-ex.11-mask-shapes-roto-pt.-2