Abstract: This invention relates to a device for providing data for determining a non-horizontal change from one direction to another of a reference axis fixedly associated with a body.The apparatus uses gravity sensor devices, such as accelerometers, to provide data for determining change in both azimuth and slant angle. In some embodiments that data may be derived from the signal of a single accelerometer.Some embodiments of the invention are probes for surveying in geological boreholes. In some of such embodiments a single gravity sensor is fixed to a mass which is mounted for free rotation.Drive elements are provided whereby the mass may be rotated and then allowed to rotate undriven.Thus the direction in which the sensor is responsive to the gravity vector is, due to inertia of the mass, independent from random axial rotation of the probe.