The paper discusses estimation of the potential accuracy of navigation systems using various parameters of the Earth's magnetic field with the help of the Cramer-Rao bound. For a simple measurement error model, a formula that relates the navigation accuracy and the spatial derivatives of the magnetic field is derived. A spectral method for the continuation of the magnetic field to a higher altitude is described. Using the data of geophysical surveys, the potential accuracy of navigation is estimated at various altitudes and for both field-based and gradient-based navigation systems.