| Summary: | In establishing the limits of the continental shelf, there are two aspects to the use of depth data. Shallow water depths are used to determine the low-water or drying line and hence the baseline from where the measurement of the various limits begin. Deepwater depths are used to determine the foot of continental slope and the 2500-m isobath, which in turn will help to decide where these limits should end. While the principle focus of this book is the delimitation of the outer limit of the continental shelf, some aspects of depth measurement are applicable to both shallow and deepwater measurements, and therefore, in order to provide a complete picture, all aspects will be considered in this chapter. For as long as people have ventured out to sea in boats, they have been interested in obtaining a knowledge of the depth of water and the position of underwater obstructions in order to avoid damaging and possibly losing their vessels. Information was valuable, and having been obtained by ships' captains, either by purchasing it from others or by carrying out their own surveys, it was not readily divulged to other people. It was the formation of national hydrographic offices in the 18th century that started the coordinated collection and wide dissemination of hydrographic data. The earliest methods of measuring depth involved the lowering of a weighted line over the side of the vessel until it hit the seabed or in the case of very shallow water, the use of a graduated pole. Measurements were restricted to shallow water until the latter part of the 18th century. Captain Phipps in HMS Racehorse recorded a depth of 683 fathoms in the Norwegian Sea in 1773. Measuring such depths was a very slow, weatherdependent process, but with the growing interest in the oceans, especially the desire to lay underwater telegraph cables in the second half of the 19th century, the techniques were improved. By 1855, Matthew Fountaine Maury of the U.S. Naval Observatory had accumulated sufficient depths to publish a first attempt at a ...
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