(marking the channel entrance) through the channel towards Trinity Inlet, with a lantern on shore serving as a leading light at night. Hydrographic surveying involves mapping the seabed and coastal features to enable safe navigation. It developed as a highly specialized branch of surveying in 19th century Britain. A triangulation network was established over the area to be surveyed, from the mainland to the sea or adjacent islands, with control points on coastal hills or headlands. "Coast liner" parties then conducted a land survey of coastal features; recorded high and low water lines; and took soundings to determine the depth and composition of the seabed. While the "mother" survey ship would take soundings in deep water, "chicks" (smaller boats, initially rowed and later steam-powered) would run lines of soundings from the shore to deep water, using a lead line. Sounding positions were fixed by reading sextant angles between the three nearest control points, and were then plotted on a field chart. Soundings were taken at all stages of the tide, and were later corrected for the height of the tide to reduce them to a common datum (a quantity or set of quantities that serve as a reference or basis for calculation of other quantities). On completion of a survey, the tides and datum were related to a permanent bench mark for use in future surveys. A bench mark is a point, at a known elevation above or below an adopted datum, marked on a durable object. Bench marks such as an upward-pointing broad arrow symbol (used to denote a government survey from 1852) could be chiselled on a rock - as was the case at Bessie Point. Alternatively, metal disks could be attached to a rock, or to a concrete block. Often a horizontal line was cut, balanced on the point of the arrow, although only an arrow was cut at Bessie Point. Other examples of broad arrow bench marks cut into rocks exist at Bowen (1887), and at Princess Charlotte Bay in the Flinders group of islands (1899, by ). In the case of the Bessie Point 1878 bench mark, which is cut into the largest boulder on the shoreline, the datum was low water ordinary spring tides, calculated as being below the broad arrow. All water depths (in feet) on the map of Cairns Harbour made in 1878 thus represented the depth available at low water ordinary