The most basic necessity of a mobile robot is the capacity to measure its own motion. For otherwise, how could it determine which way to go or whether it ever got there? In the plane, the pose of the robot is defined as the position and orientation coordinates (x, y, ψ) of some distinguished reference frame on the body. In some cases, velocities or higher derivatives or perhaps articulations are desired and this more general problem is referred to as state estimation. Determining the exact pose of the robot is not usually a practical goal. The best that can be done is to estimate pose so it is common to refer to the problem of localization as pose estimation. Much of the theory of estimation is directly applicable.

For any element of the robot pose, an important distinction relates to the question of whether available measurements project onto the pose itself or onto any of its time derivatives. In the former case, algebraic equations relate the pose to the measurements and the technique is classified as triangulation. In the latter case, differential equations relate the pose to the measurements and the technique is classified as dead reckoning. The errors that occur in these two fundamental techniques behave in very different ways.