The declinometer/inclinometer is a fluxgate magnetometer mounted on a non-magnetic theodolite to perform absolute measurements of declination and inclination. It is used to calibrate compasses or to periodically calibrate continuously recording magnetic variometers in magnetic observatories.

The angle at which its electronic fluxgate magnetometer reads a minimum value, is compared to a sighting through its optical theodolite. True north is determined by sighting a true north reference target mounted some distance away, or is derived from celestial navigation calculations on a sighting of the sun or another star.

MinGeo produces MinGeo 010 and 020 D/I instruments based on converted Zeiss THEO 010 and 020 optical theodolites to non-magnetic instrument. After very detailed optical and magnetic tests we mount a linear core fluxgate magnetometer Model G (from Danish Meteorological Institute) on the telescope and adjust its magnetic axis parallell to the optical axis. Magnetic adjustment and calibration is made in Tihany Geophysical Observatory and an independent geodetical laboratory. The instrument delivered with battery, charger and calibration documents. They are suitable for observatory and field use.