Reddit reviews Matchmoving: The Invisible Art of Camera Tracking

Research matchmoving. https://www.amazon.com/Matchmoving-Invisible-Art-Camera-Tracking/dp/111835205X

Basically...

Plan, lots of planning and storyboarding or some form of previz.

Then, shoot background, track camera movement.

Then replicate camera movement to track foreground subject shot over green screen. (Under consistent lighting.)

Combine. Color. Edit. Sell shit earphones.

You might want to see if your local library has a copy of Dobbert's Matchmoving: "The Invisible Art of Camera Tracking" (also Amazon, Book Depository).

So, there is tracking - identifying features and following them through out the frame - solving - taking tracked features and mathematically calculating the relationship between the features and/or the camera in 2D or 3D. Stable tracking on well selected features allows for more accurate solves.

Chances are that the solving process will provide you with the path of the camera, and locators for the features. Ideally, those locators will show you where those features are in space. All stuff you already know.

You might be able to assume that the level of the tracked ground is consistent and just build it out and see if it holds. It sounds like that is what you are doing already.

There's a few ways you could approach it:

• track some additional features around where you're interested in putting things
  
• look to other sources like lidar (which you won't have) or Digital Elevation Models (DEMs) (which you might be able to purchase for a relatively modest price) to provide additional guidance as to the shape of the terrain.
  
• eyeball it, see if you can find a level that doesn't slip. Unreliable, but you might find yourself needing to do this anyway
  
  Regardless, what I would suggest is looking at some matchmove showreels - you'll find some really talented artists proving just how good their solves are - often on insane shots - and apply the lessons you take from those reels to your own workflow. eg. /u/semmlerino posted a reel on vimeo.