There's a lot you can do with just a single camera and imu. PTAM is just one example there are other formulations but it tends to work well without using a ton of CPU/GPU resources
Combine this with GPS information and you can roughly "place it in the world" and make sure it doesn't clip the ground plane. After that, it is just pulling it up from your sparse 2D DB and dumping into the scene for other cameras. If they really care, there'll be some data included from the placement camera to help localize it.