hachure
Any series of lines used on a map to indicate the general direction and steepness of slopes. The lines are short, heavy, and close together for steep slopes; longer, lighter, and more widely spaced for gentle slopes.

halftone
A picture in which the gradations of light are obtained by the relative darkness and density of tiny dots produced by photographing the subject through a fine screen.

high water
Maximum height reached by a rising tide. The height may be due solely to the periodic tidal forces or it may have superimposed upon it the effects of prevailing meteorological conditions. Use of the "high tide" is discouraged.

high water line
Intersection of the land with the water surface at an elevation of high water.

high water mark
Line or mark left upon tidal flats, beach, or along shore objects indicating the elevation or the intrusion of high water.

hydrographic survey
Survey of water area, with particular reference to submarine relief, and any adjacent land. See: oceanographic survey

hydrography
Science that deals with the measurement and description of the physical features of the oceans, seas, lakes, rivers, and their adjoining coastal areas, with particular reference to their use for navigation.

hydrology
Scientific study of the waters of the Earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of ground water.

hypsographic map
See: map, hypsographic

hypsography
Topography referred to the national geodetic vertical datum of 1929. The science or art of describing heights of land surfaces with reference to this datum.

hypsometric map
See: map, hypsometric

hypsometry
Science or art of determining terrain relief, by any method.

imagery
Visible representation of objects and (or) phenomena as sensed or detected by cameras, infrared and multispectral scanners, radar, and photometers. Recording may be on photographic emulsion (directly as in a camera or indirectly after being first recorded on magnetic tape as an electrical signal) or on magnetic tape for subsequent conversion and display on a cathode ray tube.

infrared scanner (thermal mapper)
Instrument that detects infrared radiation and converts the detected energy to an electrical signal for recording on photographic film or magnetic tape.

isogonic chart
Chart showing isogonic lines properly labeled with their magnetic declination.

isogonic line
Line joining points on the Earth's surface having equal magnetic declination as of a given date.

isopleth map
See: map, isopleth

Kelsh plotter
Double-projection plotting instrument utilizing swinging lamps to transmit light through contact- size diapositives (positive transparencies).

land use classification system
Coding system of categories and subcategories designed for use on a map to designate land or water use.

land use map
See: map, land use

landmark
Monument of material mark or fixed object used to designate a land boundary on the ground: any prominent object on land that may be used to determine a location or a direction in navigation or surveying.

latitude
Angular distance, in degrees, minutes, and seconds of a point north or south of the Equator.

lead line
Line weighted with lead for making depth soundings in water.

levee
Artificial bank confining a stream channel or limiting adjacent areas subject to flooding; an embankment bordering a submarine canyon or channel, usually occurring along the outer edge of a curve.

level surface
Surface which at every point is perpendicular to the plumbline or the direction in which gravity acts.

leveling
Surveying operation in which heights of objects and points are determined relative to a specified datum.
line copy (line drawing) Map copy suitable for reproduction without the use of a screen; a drawing composed of lines as distinguished from continuous- tone copy.

line map
See: map, line

longitude
Angular distance, in degrees, minutes, and seconds, of a point east or west of the Greenwich meridian.

low water:
Minimum height reached by a falling tide. The height may be due solely to the periodic tidal forces or it may have superimposed upon it the effects of meteorological conditions.

low water line
Intersection of the land with the water surface at an elevation of low water. Not to be confused with mean low water line.