Hydrography

Hydrography is the measurement and description of physical characteristics of bodies of water and surrounding coastal areas.   The main objective of hydrography is to provide for the safe navigation of vessels through navigable waters including oceans, rivers and lakes.  This is done through the publication of up-to-date nautical charts and other forms of publications suitable for use in navigation.  Other activities supported by hydrography data include port and harbor maintenance, coastal engineering, coastal zone management, and offshore resource development.  Hydrographic services are typically carried out by national hydrographic offices.  International coordination of these efforts lies within the International Hydrographic Organization.

Hydrography involves the use of geodesy, physical oceanography, marine geology, geophysics, photogrammetry, remote sensing, and marine cartography.   The primary parameter of concern for most types of hydrographic surveys is water depth, although time, geographic position, and bottom type are also considered important basic parameters.  They also include tidal, current and wave information.  Hydrographic surveys are simply the process of "looking" at what the sea floor looks like by measuring the topographic features of the seafloor to produce maps similar to those of land areas.  Hydrographic surveys are typically conducted through the use of sonar (Sound Navigation and Ranging) which uses sound waves to identify objects within water and to determine water depth.  Also used to gain information are single beam echo sounders, divers least depth gauges, lead lines and LIDAR (Light Detection and Ranging) which uses light rather than sound to determine the water depth and objects in the water.

Hydrography is not to be confused with Bathymetry which is the detailed measurement and characterization of the seafloor's depth and features.  A Bathymetric chart is the equivalent to a topographic map.  They present a highly accurate visual presentation of the floor of a body of water.   It differs from a hydrographic chart because a bathymetric chart is an accurate presentation of underwater features while a hydrographic chart requires safe navigation.  That is, a hydrographic chart obscures actual features to give a simplified version to mainly aid in the safe navigation of mariners away from underwater hazards.   The Hydrographical charts also tend to over represent least depths.

Marine navigation has been dramatically altered with the introduction of Global Positioning System (GPS).  GPS is a continuous world-wide satellite positioning system developed by the Department of National Defense (DOD).  There are two main error sources in hydrographical surveying which include poor spatial soundings and error in the measurement of the soundings.  The first issue occurs due to the need to represent a continuous sea floor; the second error is usually due to poor calibration of the sounder and inaccuracies in the corrections for tides and waves.  GPS allows for real time changes to known information which can be crucial when applied to activities such as marine navigation, and can effectively reduce these errors.  GPS positional accuracies exceed those of any other hydrographic survey system and GPS also does not require time-consuming calibrations. 

Before the use of GPS, the measurement of tidal and wave corrections were done independently which is imprecise because the effects of both tides and waves are combined to create the current water level at any one instant.  Measuring both separately and then combining later during post-processing introduces inaccuracies and is much more complicated than measuring a single Water Level Correction (WLC).  GPS can provide real time measurement of a WLC through an accurate technique.  The synchronization of soundings and GPS can be calibrated accurately with ease to create a high precision WLC measurement compared to traditional sources of hydrographic surveying.

The product of the GPS/GIS is called an ENC (Electronic Navigational Chart) and is a digital version of a paper navigational chart.  The ENC and GPS position of the ship, when input to ECDIS (Electronic Chart Display and Information System) which is a computer system, provides a real time graphical display for navigational purposes including the ship's position along with the hydrographical data contained in the ENC.

Both GIS and GPS technology have been used for more than seven years in the field by the Naval Hydrographic Department (NHD). They use several sensors onboard their survey ships and boats to stream data and log onto a common GPS position and time stamp. This hydrographical data is then collected and inputted into ENC which is essential to enable worldwide navigation through GPS. 

The U.S. Navy SEALS use the Mk 107 Mod 0 Hydrographic Mapping Unit (HMU), which is a standalone navigation aid for hydrographic reconnaissance, mine countermeasures and ship-attack missions.  This handheld device uses sensors to navigate underwater accurately, to map the ocean floor, and to spot objects underwater.  The Hydrographic Mapping Unit is laptop-sized and allows the diver to plan a route with GPS coordinates and to navigate accurately and covertly.  The HMU chart continuously displays the diver's position and records hydrographic data at all times throughout the mission, as well as the position and images of underwater objects.