A high-accuracy inertial-navigation system (INS) is to be used to navigate a land-mobile Vehicle travelling at about 30 knots, Over time periods of 2-4 h or less. Important sources of navigation error are the errors in modeling the anomalous gravity forces acting on the vehicle. To obtain acceptable performance, the INS must be accurately compensated in real time for the anomalous gravity. A significant reduction in the growth of navigation errors can be obtained by stopping the vehicle periodically and processing zero-velocity updates in an on-board navigation filter. The land-navigation system design considerations that affect the growth of navigation error due to the gravity-model errors are examined here. These include the grid-spacing of the data base used to derive the real-time gravity compensation, the use of odometer and zero-sideslip measurement data for in-transit INS updating, and the frequency and accuracy of the at-rest zero-velocity updates which are the key to high-accuracy land navigation.