Hydrography is the measurement of physical characteristics of waters and marginal land. Hydrography pertains to any waters.
Hydrography on a large scale generally applies to the oceans and national or international efforts to describe them for navigational purposes. The science of oceanography is, in large part, a direct outgrowth of classical hydrography. In many respects the data are interchangeable, but marine hydrographic data will be particularly directed toward marine navigation and safety of that navigation.
Hydrographic measurements will include the tidal, current and wave information of physical oceanography. They will include bottom measurements, but with particular emphasis on those marine geological features that pose a hazard to navigation such as rocks, shoals, reefs and other features that obstruct ship passage. Unlike oceanography, hydrography will include shore features, natural and manmade, that aid in navigation. A Hydrographic survey will therefore include accurate positions and representations of hills, mountains and even lights and towers that will aid in fixing a ship's position as well as the aspects of the sea and seabed.
An interesting historical relationship is that of James Whistler to hydrography. His artistic talents were applied to the sometimes beautiful shore profiles that appeared on charts during his work as a cartographer with both the civilian and naval U. S. hydrographic organizations. Those profiles on early charts were etchings designed to aid mariners in identifying their landfall and harbor approaches.
Hydrography's origin lies in the making of chart like drawings and notations made by individual mariners. These were usually the private property, even closely held secrets, of individuals who used them for commercial or military advantage. Eventually organizations, particularly navies, realized the collection of this individualized knowledge and distribution to their members gave an organizational advantage. The next step was to organize members to actively collect information. Thus were born dedicated hydrographic organizations for the collection, organization, publication and distribution of hydrography incorporated into charts and sailing directions.
Oceanic hydrography may be considered a specialized subset of oceanography that is particularly dedicated to marine navigation and its safety. Hydrography, partly for reasons of safety, tends to be more traditional in outlook and has conventions that are not entirely "scientific" in some views. For example, hydrographic charts will usually tend to over represent least depths and ignore the actual submarine topography that will be portrayed on bathymetric charts. The former are the mariner's tools to avoid accident. The later are best representations of the actual seabed, as in a topographic map, for scientific and other purposes.
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Hydrographic services in most countries are carried out by specialised Hydrographic offices. The international coordination of hydrographic efforts lies with the International Hydrographic Organization.