Theodolites combined the plain compass with a wye level or two pairs of open sights to create an extremely accurate instrument that could take bearings with greater exactness (due to the telescope with a spirit level as opposed to vane sights). The theodolite could also be used to take certain celestial observations. It is unknown if the Geographer‘s Department used theodolites, but they were in active use by advanced surveyors and cartographers during the period.