A portable navigational instrument has edge portions which are adjusted so that the instrument's orientation with respect to a vessel can be maintained constant in different positions within the vessel's wheelhouse. Bearings of objects are determined by a sighting arrangement which co-operates with a scale which is set in accordance with the vessel's steering compass bearing, corrected for deviation, whereby the need for a compass on the instrument is avoided. The sighting arrangement can comprise a single visible sight, and/or three such sights enabling horizontal angles between objects to be determined from a further scale, and/or a radio direction finding aerial.
