The skill of the hydrographic surveyor has its basis in the principles of land surveying. Where the topographic surveyor measures a position and height, the hydrographer measures a position and the depth of water. Indeed, part of his work consists of land surveying, for the topography and coastline adjacent to the water area have to be depicted accurately and, of course, the control for the water-work has to be established by normal triangulation methods. For these reasons a hydrographic surveyor is part seaman and navigator, and part land surveyor, a mixture that demands, for success, a particularly patient and painstaking personality. In recent years the instruments available to a surveyor have been greatly improved. The advent of the echo sounder is perhaps the largest single factor in increasing his output, and this, added to the use of lightweight alloys in instrument manufacture and the development of plastics as stable media for plotting his work on, have all helped to enhance the accuracy of his final product. They have also made his existence less reliant upon brute strength, for the lead no longer has to be heaved. Until a few years ago horizontal sextant angles, observed simultaneously by two observers, was the main method of fixing the position of the soundings. Today electronic means are being introduced to do this, and Lachlan was fitted in 1958 with a system widely used in other countries; a remarkable device designed to fix the ship to an accuracy of 5 metres at a distance from the controlling points of 150 miles. Radar is also extremely useful for surveying on a small scale, particularly in poor visibility. It is limited in use for large scale work by the accuracy with which distances can be measured. Basic seamanship and surveying knowledge remain the same as they always did and, as far as the skill of the individual surveyor is concerned, the only difference the new methods mean to him is an increasing requirement to be an electronic expert.

A typical day for the surveying ship begins with the crew being awakened by the quartermaster on watch, calling them over the internal ship's broadcast system at six o'clock in the morning. If the survey of a port or anchorage is in progress, then the ship will be at anchor in the survey ground and the first of the day's tasks will be to lower the sounding boats. Three boats fitted with echo-sounding equipment are normally carried. They spend the daylight hours working among the rocks and shoals off shore, also in the shallow water running up to the beaches. Each one traverses the water area allocated to it on parallel lines, rather like a ploughman turning the furrows up and down a paddock. The echo sounder produces on a wide paper tape a continuous record of the depth of water, and at the end of the day this tape can be compared with the positions the boat has passed through, which are also plotted on a field board. The related depths can then be inked on the board, and contour lines joining the positions of equal depth can be drawn.

Meanwhile the ship will have been traversing back and forth in the area of deeper water, a similar system being used to record the positions and depths plotted during her day's work. Towards dusk she will anchor, and the smaller craft will return to be hoisted inboard, replenished with fuel, food, and water, and prepared for the next day when they will again be away surveying. If the work on hand happens to be one of the small-scale coastal sheets, then the electronic fixing device will be used to control the ship. As visibility does not affect this equipment, the hours of darkness, as well as daylight, can be used for sounding. On some of the remoter parts of the coast this is particularly useful, as anchorages for the ship are few in number and usually a long way from the scene of operations. Thus the ship can survey continuously for twenty-four hours a day. This is a great asset although more time has to be spent in harbour maintaining the hull and machinery.

Today there are two units working in New Zealand. The major one is still HMNZS Lachlan; her internal layout now greatly altered and modernised. The second unit consists of two converted ex-harbour defence motor launches, HMNZ ships Takapu and Tarapunga. Both craft are 72 ft long with twin engines and, being highly manoeuvrable, are ideal for surveying ports and their approaches. Recently they were formed into an independent unit under the command of an officer qualified to take charge of surveys. In 1961 they carried out a survey of the Manukau Harbour, including the bar and approaches. This was their first major assignment. The previous survey of the port was done by HMS Pandora in 1853.

Chart compilation and drawing are in the hands of a team of civilian hydrographic officers and draughtsmen who work in the Hydrographic Branch of the Navy Department in Wellington. Charts are printed by the Government Printer, whose final product reflects the painstaking effort put into the work by all concerned. New Zealand's charts are of a high international standard and much experimental work has been done by the branch in the use of colour plates for depicting land and water detail.

The Hydrographic Surveying Service of the Royal New Zealand Navy is now firmly established as an asset to the country. In international circles it has become known as one of the smallest but most efficient organisations of its kind, and is an active member of the International Hydrographic Bureau. Moreover, the Service has invariably maintained harmonious relations with all the authorities, commercial and governmental, who have a stake in New Zealand's maritime affairs.

by Lt.-Cdr. Geoffrey Lewis Haskins, A.R.I.C.S., GRAD. N.Z.INST.SURV., Naval Hydrographic Surveyor in Charge of Survey, Royal New Zealand Navy.

• Manual of Hydrographic Surveying, G. Brit., Admiralty (1948).