Hydrography

Table of geography, hydrography, and navigation, from the 1728 Cyclopaedia.

Hydrography is the mapping (charting) of water topographic features through the measurement of the depths, the tides and currents of a body of water and establishment of the sea, river or lake bed topography and morphology. Normally and historically the purpose of charting a body of water is for the safety of shipping navigation. Such charting includes the positioning and identification of things such as wrecks, reefs, structures (platforms etc.), navigational lights, marks and buoys and coastline characteristics. Hydrography does not include water quality or composition which are part of the broader field of Hydrology.

Overview

Large-scale hydrography is usually undertaken by national or international organizations which sponsor data collection through precise surveys and publish charts and descriptive material for navigational purposes. The science of oceanography is, in part, an 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. Marine resource exploration and exploitation is a significant application of hydrography, principally focused on the search for hydrocarbons.

Hydrographical measurements include the tidal, current and wave information of physical oceanography. They include bottom measurements, with particular emphasis on those marine geographical features that pose a hazard to navigation such as rocks, shoals, reefs and other features that obstruct ship passage. Bottom measurements also include collection of the nature of the bottom as it pertains to effective anchoring. Unlike oceanography, hydrography will include shore features, natural and manmade, that aid in navigation. Therefore, a hydrographic survey may include the 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 physical aspects of the sea and seabed.

Hydrography, mostly for reasons of safety, adopted a number of conventions that have affected its portrayal of the data on nautical charts. For example, hydrographic charts are designed to portray what is safe for navigation, and therefore will usually tend to maintain least depths and occasionally de-emphasize the actual submarine topography that would be portrayed on bathymetric charts. The former are the mariner's tools to avoid accident. The latter are best representations of the actual seabed, as in a topographic map, for scientific and other purposes. Trends in hydrographic practice since ca. 2003-2005 have led to a narrowing of this difference, with many more hydrographic offices maintaining "best observed" databases, and then making navigationally "safe" products as required. This has been coupled with a preference for multi-use surveys, so that the same data collected for nautical charting purposes can also be used for bathymetric portrayal.

Even though, in places, hydrographic survey data may be collected in sufficient detail to portray bottom topography in some areas, hydrographic charts only show depth information relevant for safe navigation and should not be considered as a product that accurately portrays the actual shape of the bottom. The soundings selected from the raw source depth data for placement on the nautical chart are selected for safe navigation and are biased to show predominately the shallowest depths that relate to safe navigation. For instance, if there is a deep area that can not be reached because it is surrounded by shallow water, the deep area may not be shown. The color filled areas that show different ranges of shallow water are not the equivalent of contours on a topographic map since they are often drawn seaward of the actual shallowest depth portrayed. A bathymetric chart does show marine topology accurately. Details covering the above limitations can be found in Part 1 of Bowditch's American Practical Navigator. Another concept that affects safe navigation is the sparsity of detailed depth data from high resolution sonar systems. In more remote areas, the only available depth information has been collected with lead lines. This collection method drops a weighted line to the bottom at intervals and records the depth, often from a rowboat or sail boat. There is no data between soundings or between sounding lines to guarantee that there is not a hazard such as a wreck or a coral head waiting there to ruin a sailor's day. Often, the navigation of the collecting boat does not match today's GPS navigational accuracies. The hydrographic chart will use the best data available and will caveat it's nature in a caution note or in the legend of the chart.

A hydrographic survey is quite different from a bathymetric survey in some important respects, particularly in a bias toward least depths due to the safety requirements of the former and geomorphologic descriptive requirements of the latter. Historically, this could include echosoundings being conducted under settings biased toward least depths, but in modern practice hydrographic surveys typically attempt to best measure the depths observed, with the adjustments for navigational safety being applied after the fact.

Hydrography of streams will include information on the stream bed, flows, water quality and surrounding land. Basin or interior hydrography pays special attention to rivers and potable water although if collected data is not for ship navigational uses, and is intended for scientific usage it is more commonly called hydrology.

History

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.

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. (An interesting account of his brief career with hydrographic charting can be found at: http://www.lib.noaa.gov/noaainfo/heritage/coastsurveyvol1/BACHE7.html.)

Organisations

Hydrographic services in most countries are carried out by specialised hydrographic offices. The international coordination of hydrographic efforts lies with the International Hydrographic Organization.

The United Kingdom Hydrographic Office is one of the oldest and most respected hydrography organisations in the world, supplying the widest range of charts covering the globe to other countries, allied military organisations and the public. In the United States, the hydrographic charting function has been carried out since 1807 by the Office of Coast Survey of the National Oceanic and Atmospheric Administration.

See also

• Hydrographic survey
• Bathymetric chart
• Challenger expedition
• Cartography
• Coastal geography
• Drought
• Evapotranspiration
• Flood
• Hydrology
• Hydrologic cycle
• Hydrometeorology
• Hydrometry
• Trasvasement
• Virtual water
• Pierre Desceliers

External links

• The International Hydrographic Organization- is an intergovernmental consultative and technical organization that was established in 1921 to support safety of navigation and the protection of the marine environment.
• The Academy Of Positioning Marine And Bathymetry (APOMAB)- is an independent, non-profit, non-governmental organization. It provides a forum for all those involved in activities related to the disciplines of marine positioning and bathymetry members who live or work anywhere in the world.
• International Federation of Hydrographic Societies (formerly The Hydrographic Society)