In this article, 9-year veteran of augmented reality goes deep into the history of AR, and offers a point of view on why ARKit is a game changer. It uses a Visual Interial Odometry (VIO) system with Initial Measurement Unit (IMU) readings. “Visual / Optical measurements are made at the camera frame rate, so usually 30fps, and are based on distance (changes of the scene in between frames). Optical systems usually accumulate errors over distance (and time to a lesser extent), so the further you travel, the larger the error.” Working on their own, these two systems have their weaknesses, but when paired together they are much more successful. Check out the link for an extremely detailed explanation of the ARKit components.