INTERSECTION—A method of locating thehorizontal position of a point by observationsfrom two or more points of known position, thusmeasuring directions that intersect at the stationbeing located. A station whose horizontal positionis located by an intersection is known as anintersection station.IRRATIONAL NUMBER—Real number thatcannot be expressed in the ratio of two integers;for example, 3, n.IRREGULAR POLYGON—A nonequilateralpolygon.ISOMETRIC AXIS—Axis used in isometricprojections and drawings. Each line in the axisforms an angle of 1200 with the adjacent line,easily constructed with a straightedge and a30°/60° triangle.ISOMETRIC DRAWING—Same as an isometricprojection except that the dimensions of the objectdrawn are scaled and not projected.ISOMETRIC PROJECTION—A single viewprojection of an object showing three dimensions.The object is inclined so all faces make the sameangle with the plane of projection, making all linesand surfaces foreshortened in the same ratio. Thisallows one scale to be used throughout.ISOSCELES TRIANGLE—A triangle having twoequal sides.LATERAL FACES—Faces or surfaces formingthe sides of a solid figure; also known as lateralsurfaces.LATERAL SURFACES—See LATERAL FACES.LATITUDE—In plane surveying, the amountthat one end of a line is north or south of the otherend. As the plane coordinates of a point areknown as the casting and northing of the point,the latitude is the difference between the northingsof the two ends of the line, which may be eitherplus or minus. (See DEPARTURE.)LAW OF COSINES—A law of mathematics thatstates that, in any triangle, the square of one sideis equal to the sum of the squares of the othertwo sides minus twice the product of these twosides multiplied by the cosine of the angle betweenthem. This statement may be expressed in formulaform as follows:a2=b2+c2–2bccos Abz=a2+c2–2accos Bcz=a2+b2–2abcos CLAW OF SINES—A law of mathematics thatstates that the lengths of the sides of any triangleare proportional to the sines of their oppositeangles. It is expressed in formula form as follows:LAW OF TANGENTS—A law of mathematicsthat states that, in any triangle, the differencebetween two sides is to their sum as the tangentof half the difference of the opposite angles is tothe tangent of half their sum. For any pair ofsides—as, side a and side b—the law may beexpressed as follows:LEADER LINES—Thin unbroken lines used toconnect numbers, references, or notes toappropriate surfaces or lines.LEGEND—A description, explanation, table ofsymbols, and so on, printed on a map or chartfor a better understanding and interpretation of it.LEVEL—1. Pertaining to a level surface; 2. Tomake horizontal at the point of observation;3. An instrument for leveling.LEVEL DATUM—A level surface to whichelevations are referred. The generally adoptedlevel datum for leveling in the United States ismean sea level. For local surveys, an arbitrarylevel datum is often adopted and defined in termsof an assumed elevation for some physical mark(bench mark).LEVEL LINE—1. A line in a horizontal plane;2. A line over which leveling operations areaccomplished.LEVEL NET—Lines of spirit leveling connectedtogether to form a system of loops or circuitsextending over an area. This is also called avertical control net.AI-8
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