Reconciling the navigational grid with the inertial navigation system

It is remarked that the concept of a `nonchanging north´ on the navigation grid overprinted on certain small-scale navigation and planning charts published by the Defense Mapping Agency is remarkably like that encountered in an inertial navigation system with the local-level platform untorqued (alpha-wander mode). However, the directional angles computed by the inertial navigation system rapidly diverge from those measured from the charts. The authors explore the reason for this divergence and propose a solution. Specifically, a formula for the alpha angle is presented