Editing Hydrography (section)

==Overview==
[[File:A plan of Milford Haven in the county of Pembroke south Wales.gif|thumb|right|250px|Survey of the strategic port of [[Milford Haven]] produced by [[Lewis Morris (1701–1765)|Lewis Morris]] in 1748]]
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 [[hydrocarbon]]s.

Hydrographical measurements include the [[tides|tidal]], [[Ocean currents|current]] and [[wave]] information of physical oceanography. They include [[seafloor|bottom]] measurements, with particular emphasis on those marine geographical features that pose a hazard to navigation such as rocks, [[shoal]]s, [[reef]]s 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 [[hill]]s, [[mountain]]s and even lights and [[tower]]s 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 [[:wikt:depth|depth]]s and occasionally de-emphasize the actual submarine [[topography]] that would be portrayed on [[bathymetric chart]]s. The former are the [[sailor|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 c.&nbsp;2003–2005 have led to a narrowing of this difference, with many more [[Hydrographic Office|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.

[[File:HMS Waterwitch, formally Lancashire Witch.jpeg|thumb|right|250px|[[HMS Waterwitch (1892)|HMS ''Waterwitch'']], a hydrographic survey vessel]]
Even though, in places, hydrographic survey data may be collected in sufficient detail to portray bottom topography in some areas, [[hydrographic chart]]s 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 predominantly 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&nbsp;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 its 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 [[geomorphology|geomorphologic]] descriptive requirements of the latter.  Historically, this could include [[Echo sounding|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 [[stream]]s will include information on the stream bed, [[Environmental flow|flow]]s, [[water quality]] and surrounding land. [[Drainage basin|Basin]] or interior hydrography pays special attention to [[river]]s and [[potable water]] although if collected data is not for ship navigational uses, and is intended for scientific usage, it is more commonly called ''[[hydrometry]]'' or ''[[hydrology]]''.

Hydrography of rivers and streams is also an integral part of water management. Most reservoirs in the United States use dedicated stream gauging and rating tables to determine inflows into the reservoir and outflows to irrigation districts, water municipalities and other users of captured water. River/stream hydrographers use handheld and bank mounted devices, to capture a sectional flow rate of moving water through a section and or current.

=== Equipment ===
[[Unmanned surface vehicle|Uncrewed Surface Vessels]] (USVs) and are commonly used for hydrographic surveys - they are often equipped with some sort of sonar. Single-beam echosounders, [[multibeam echosounder]]s, and [[Side-scan sonar|side scan sonars]] are all frequently used in hydrographic applications. The knowledge gained from these surveys aid in disaster planning, port and harbor maintenance, and various other coastal planning activities.<ref>{{Cite web |last=US Department of Commerce |first=National Oceanic and Atmospheric Administration |title=Hydrographic Surveying |url=https://oceanservice.noaa.gov/navigation/hydro/#:~:text=A%20NOAA%20survey%20ship%20uses,the%20sea%20floor%20and%20back |access-date=2024-01-12 |website=oceanservice.noaa.gov |language=EN-US}}</ref><ref>{{Cite web |title=A Smooth Operator's Guide to Underwater Sonars and Acoustic Devices |url=https://bluerobotics.com/learn/a-smooth-operators-guide-to-underwater-sonars-and-acoustic-devices/ |access-date=2024-01-12 |website=Blue Robotics |language=en-US}}</ref>