incremental shaft encoder—A component of a water level gauge for converting length to a shaft angle on a rotating disk. The position of the rotating disk is determined by single or dual optical or magnetic sensors to provide an electrical output. No electro-mechanical components or gears are used, so extremely low torque is required to move the float wheel, wire, and float mechanism.

Indian spring low water—A datum originated by Professor 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 amplitudes of he harmonic constituents M2, S2, K1, and O1.

Indian tide plane—Same as Indian spring low water.

inequality—A systematic departure from the mean value of a tidal quantity. See diurnal inequality, parallax inequality, and phase inequality.

inertial flow—A solution of the relative hydrodynamic equations of motion in which only the horizontal component of the Coriolis and centrifugal forces are balanced. This anticyclonic flow results from a sudden application and release of a driving force which then allows the system to continue on under its own momentum without further interference. The period of rotation is 2B / 2S sin ø, where S = 0.729,211 x 10-4 radians s-1 and ø = latitude.

internal tide—A tidal wave propagating along a sharp density discontinuity, such as a thermocline, or in an area of gradually changing (vertically) density.

International Great Lakes Datum (1985) [IGLD1985]—Mean water level at Rimouski/Pointe-au-Pere, Quebec, on the Gulf of St. Lawrence over the period 1970 through 1988, from which geopotential elevations (geopotential differences) throughout the Great Lakes region are measured. The term is often used to mean the entire system of geopotential elevations rather than just the referenced water level. See low water datum (1).

International Hydrographic Organization (formerly Bureau)—An institution consisting of representatives of a number of nations organized for the purpose of coordinating the hydrographic work of the participating governments. It had its origin in the International Hydrographic Conference in London in 1919. It has permanent headquarters in the Principality of Monaco and is supported by funds provided by the member nations. Its principal publications include the Hydrographic Review and special publications on technical subjects.

intertidal zone—(technical definition) The zone between the mean higher high water and mean lower low water lines.

interval—See lunitidal interval and lunicurrent interval.

inverse barometer effect—The inverse response of sea level to changes in atmospheric pressure. A static reduction of 1.005 mb in atmospheric pressure will cause a stationary rise of 1 cm in sea level.

Irminger Current—A North Atlantic Ocean current setting westward off the south-west coast of Iceland.

isanostere—An isopleth of either specific volume anomaly or thermosteric anomaly.

isobar—An isopleth of pressure.

isobaric surface—A surface of constant or uniform pressure.

isohaline—An isopleth of salinity. Constant or uniform in salinity.

isopleth—A line of constant or uniform value of a given quantity. See isanostere, isobar, isohaline, isopycnic,  and isotherm.

isopycnic—An isopleth of density. Constant or uniform in density.

isotherm—An isopleth of temperature.