1. In the method of borehole mapping or surveying using a single angular rate sensor and a single acceleration sensor, both with input axes of sensitivity nominally normal to the borehole axis, the sensors being effectively rotated about the borehole axis; the sensors having outputs, the step that includes: (a) employing the acceleration sensor output together with the inertial angular rate sensor output to derive from the rate sensor output two components respectively in a horizontal plane normal to the plane containing the borehole axis and the gravity vector, and in a vertical plane, and deriving borehold azimuth from said component in a horizontal plane and from a known component in said horizontal plane of the earth's angular velocity vector.