US Patent 6854535 Bore location system and method of calibration

A method for calibrating a pitch measurement of a horizontal underground bore location system includes providing a probe having a pitch sensor that measures deviation between a predetermined orientation and a measurement axis. A receiver has an antenna that detects a signal radiated by the probe and has receiver circuitry that detects the deviation from the received signal. A bore head is aligned with the predetermined orientation. The probe is placed in the bore head. The receiver detects the deviation when the bore head is aligned with the predetermined orientation. The receiver offsets deviation measurements thereafter detected from the probe by the measured deviation.