Nautical charts deliberately hide the deep water, and that is the point
A sea chart is not a faithful map of the seabed. It is biased toward the shallowest depths, because its job is keeping ships off rocks, not showing the true shape of the bottom. A deep hollow ringed by shoals may simply be left off, since no vessel could safely reach it.
Hydrography, from the Greek for water and writing, measures and describes oceans, coasts, lakes and rivers, chiefly so ships can navigate safely. Early charts were private property, often jealously guarded secrets that gave traders or warships an edge. As ocean trade and exploration grew, governments realized that collecting and sharing this knowledge was a strategic asset, and navies founded dedicated hydrographic offices.
Britain's story shows the shift. Royal Navy captains once supplied their own charts, so ships often sailed poorly informed and new surveys rarely reached everyone who needed them. In 1795 the Admiralty appointed Alexander Dalrymple as Hydrographer; within a year two centuries of existing charts were collated into a first catalogue, and the office's first chart, of Quiberon Bay in Brittany, appeared in 1800. Francis Beaufort, Hydrographer from 1829, gave his name to the wind scale and introduced official tide tables in 1833 and Notices to Mariners in 1834. By 1855 the catalogue covered the whole globe in 1,981 charts, and more than 130,000 copies were printed each year. In the United States, the Office of Coast Survey has charted waters since 1807.
A hydrographic survey records tides, currents, waves and every hazard on the bottom, from reefs and shoals to the type of seabed for anchoring. Unlike oceanography, it also maps features on land, such as hills, lights and towers, that help a navigator fix position. Shaded shallow-water bands on a chart are drawn on the safe side of the true shallowest point, so they are not equivalent to land contours. In remote waters the only depths may come from lead lines dropped from rowing or sailing boats, with nothing known between soundings, so charts carry caution notes about the quality of their data.
Since about 2003 to 2005 many offices have kept databases of depths as actually observed, producing cautious navigation charts from them afterward, so the same survey can also serve scientific seabed mapping. Uncrewed surface vessels carrying multibeam echosounders and side-scan sonar now do much of the work, and the International Hydrographic Organization coordinates efforts between nations.
Source: Hydrography
