In the use of a backhoe digger an indication of the precise depth of the bucket is required. This may be done by measuring the angles and extensions of the elements of the backhoe and calculating the result. This is commonly done by means of angle resolvers and linear encoders. Retro-fitting and calibration of such equipment is very difficult, and according to the invention the same data may be obtained by means of an inclinometer and length measuring devices based on pulsed ultrasound.