HACHURES—A method of portraying relief byshort, wedge-shaped marks radiating from highelevations and following the direction of slope tothe lowland.HALF SECTION—A sectional view that passeshalfway through the object; used when the shapeof one half is identical to the other half.HATCHING—Sections lines that are drawn onthe internal surface of sectional views; may beused to define the kind or type of material ofwhich the sectioned surface consists.HEIGHT OF INSTRUMENT—1. (Spiritleveling) The height of the line of sight of aleveling instrument above the adopted datum.2. (Stadia surveying) The height of the center ofthe telescope (horizontal axis) of transit ortelescopic alidade above the ground or stationmark. 3. (Trigonometrical leveling) The height ofthe center of the theodolite (horizontal axis) abovethe ground or station mark.HELIOTROPE—An instrument composed ofone or more plane mirrors so mounted at thestation being sighted upon that the sun’s rays canbe reflected to any one observing the station.HEPTAGON—A polygon of seven sides.HEXAGON—A polygon of six sides.HIDDEN LINES—Thick, short, dashed linesindicating the hidden features of an object beingdrawn.HORIZONTAL ANGLE—The angle formed bytwo intersecting lines on a horizontal plane.HORIZONTAL CONTROL—Control thatdetermines horizontal positions only, with respectto parallels and meridians or to other lines ofreference.HORIZONTAL DATUM—In plane surveying,the grid system of reference used for thehorizontal control of an area; defined by thecasting and northing of one station in the area,and the azimuth from this selected station to anadjacent station.HORIZONTAL DIRECTION—A direction in ahorizontal plane.HORIZONTAL DISTANCE—A distancemeasured along a level line. It is commonlythought of as the distance between two plumblines. The distance may be measured eitherhorizontally or inclined, but the inclined distanceis always reduced to its horizontal length.HORIZONTAL LINE—A line tangent to a levelsurface, or a line lying on a horizontal plane.HORIZONTAL PLANE—A plane perpendicularto the direction of gravity.HORIZONTAL POSITION—The grid positionof a horizontal control point.HORIZONTAL REFRACTION—A naturalerror in surveying that is the result of thehorizontal bending of light rays between a targetand an observing instrument. This refraction isusually caused by the differences in density of theair along the path of the light rays, resulting fromtemperature variations.HORN CENTER—Device used to prevent thecompass needle from making holes in a drawing.HUB—A wooden stake or pipe set in the groundwith a tack or other marker to indicate the exactposition. A guard stake protects and identifies thehub.HYDROSCOPIC MOISTURE—The films ofadsorbed moisture that may be present in airdriedsoil. Hydroscopic moisture may be driven off byoven-drying. (See also adsorbed moisture.)ILLUSTRATION BOARD—Smooth white paperwith a cardboard backing, used for signs or chartsor mounting of maps, photos, or drawings thatrequire a strong backing.IMAGE PLANE—See PLANE OF PROJEC-TION.INDIA INK—Drawing ink consisting of apigment (usually powdered carbon) suspended inan ammonia-water solution.INFINITE DISTANCE—An indefinite unmea-surable distance; for example, parallel lines aresaid to converge at infinity.INSCRIBED FIGURE—A figure that iscompletely enclosed by another figure.AI-7
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