INTERNATIONAL LOW WATER

A hydrographic datum originally suggested for international use at the International Hydrographic Conference in London in 1919 and later discussed at the Monaco Conference in 1926. Th

international_low_water datum hydrographic_datum

Related Terms

HALF-TIDE LEVEL

A tidal datum midway between mean high water and mean low water. Mean sea level may coincide with half-tide level, but seldom does; the variation is generally about 3 centimeters and rarely exceeds 6 centimeters. Also called MEAN TIDE LEVEL.

half-tide_level water level variation datum tide high high_water low mean mean_high_water mean_sea_level sea_level tidal

GRID NORTH

1. An arbitrary reference direction used with grid navigation. The direction of the 180th geographical meridian from the north pole is used almost universally as grid north. 2. The northerly or zero direction indicated by the grid datum of directional reference.

grid_north datum north_pole direction grid meridian north pole reference

HEIGHT OF TIDE

Vertical distance from the chart sounding datum to the water surface at any stage of the tide. It is positive if the water level is higher than the chart sounding datum. The vertical distance from the chart sounding datum to a high water datum is called RISE OF TIDE.

height_of_tide water surface chart water_level distance level datum rise vertical chart_sounding_datum high high_water stage

HYDROGRAPHIC DATUM

A datum used for referencing depths of water or the heights of predicted tides

hydrographic_datum water datum predicted_tides

NATIONAL TIDAL DATUM EPOCH

The specific 19-year cycle adopted by the National Ocean Survey as the official time segment over which tide observations are taken and reduced to obtain mean values(e.g., mean lower low water, etc.) for tidal datums. It is necessary for standardization because of apparent periodic and apparent secular trends in sea level. The present National Tidal Datum Epoch is 1960 through 1978.

national_tidal_datum_epoch time cycle datum ocean tide apparent epoch low mean observations secular survey tidal

HORIZONTAL GEODETIC DATUM

The basis for computations of horizontal control surveys in which the curvature of the earth is considered It consists of the astronomical and geodetic latitude and the astronom- ical and geodetic longitude of an initial point (origin); an azimuth of a line from this point; the parameters (radius and flattening) of the reference ellipsoid; and the geoidal separation at the origin. A change in any of these quantities affects every point on the datum. For this reason, while positions within a system are directly and accurately relatable, those points from different datums must be transformed to a common datum for consistency. The horizontal geodetic datum may extend over a continent or be limited to a small area.

horizontal_geodetic_datum control line system datum horizontal longitude astronomical azimuth continent curvature earth

TOTAL HEAT

The sum of sensible heat and latent heat between an arbitrary datum point and the temperature and state under consideration.

total_heat sensible_heat latent_heat temperature heat datum state

INDIAN SPRING LOW WATER

A tidal datum originated by G.H. Darwin when investigating the tides of India. It is an elevation depressed below mean sea level by an amount equal to the sum of the ampli- tudes of certain constituents as given in the Tide and Current Glossary published by the National Ocean Survey. Also called INDIAN TIDE PLANE, HARMONIC TIDE PLANE.

indian_spring_low_water current level datum ocean tide elevation harmonic mean mean_sea_level sea_level tidal

LEDGE

On the sea floor, a rocky, projection or datum outcrop, commonly linear and near shore

ledge datum linear projection shore

LEVELING

A survey operation in which heights of objects are determined relative to a specified datum

leveling datum operation relative survey

Related Terms

Related questions