# Noise in motion and measurement models

When using an EKF for SLAM, I often see the motion and measurement models being described as having some noise term.

This makes sense to me if you're doing a simulation, where you need to add noise to a simulated measurement to make it stochastic. But what about when using real robot data? Is the noise already in the measurement and thus does not need to be added, or does the noise matrix mean something else?

For example, in Probabilistic Robotics (on page 319), there is a measurement model: $z_t^i = h(y,j) + Q_t$, where $Q_t$ is a noise covariance. Does $Q_t$ need to be calculated when working with real data?