Kalman filter restricted by areal constraints
The simplest solution in terms of computational resources as much as simplicity, I guess, is the Unscented Kalman Filter (UKF). There, the clipping might easily be added to the calculation of the sigma-points, in a way that they remain in certain boundary. I recommend that you take a look at this article, for example (working in 2016.04.22, Constrained State Estimation Using the Unscented Kalman Filter, Rambabu Kandepu, Lars Imsland and Bjarne A. Foss)