(measured upward from the horizon) is termedan angle of elevation, and a negative altitude(measured downward from the horizontal) istermed an angle of depression. 2. Altitude issometimes used to apply to elevation above adatum; for example, the altitude of an airplane.ANGLE—A figure formed by two lines or planesextending from or diverging at the same point.ANGLE OF DEPRESSION—A negative altitude.ANGLE OF ELEVATION—A positive altitude.ANGLE OF INCLINATION—A vertical angleof elevation or depression.ANNUAL VARIATION—The annual change inthe magnetic declination.ANTILOG—The result when a logarithm isconverted to a number.ARC—A portion of the circumference of a circle.ARCHITECT’S SCALE—Scale used whendimensions or measurements are to be expressedin feet and inches.ARITHMETIC—The art of computation by theuse of positive real numbers.ASPHALT—A dark brown to black cementitiousmaterial in which the predominating constituentsare bitumens that occur in nature or are obtainedin petroleum processing. Asphalt is a constituentin varying proportions of most crude petroleums.AUXILIARY PLANE—A plane (NOT one of thenormal planes) from which the auxiliary view isprojected.AUXILIARY VIEW—A view that is not on oneof the normal planes of projection. It is used toshow features of objects that do not appear- intheir true size and shape in the normal views.AXONOMETRIC—A single view of an objectdepicting all three dimensions. The projectionlines are parallel to each other and perpendicularto the plane of projection. The object is inclinedto the plane of projection, thereby allowing theviewer to see three dimensions.AZIMUTH (Surveying)—The horizontal direc-tion of a line measured clockwise from a referenceplane, usually the meridian; often calledFORWARD AZIMUTH to differentiate fromBACK AZIMUTH. In the basic control surveysof the United States, azimuths are measuredclockwise from south following the continentalEuropean geodetic practice. However, thispractice is not followed in all countries.AZIMUTH MARK—A marked point visiblefrom a survey station, the azimuth to which isdetermined for use in dependent surveys.BACK AZIMUTH—As the azimuth of the linefrom A to B is known as the forward azimuth,the azimuth of the same line from B to A is knownas the back azimuth.BACKSIGHT—1. In traversing, a backsight (BS)is a sight on a previously established traverse ortriangulation station, that is not the closing sightof the traverse. 2. In leveling, a backsight is areading on a rod held on a point whose elevationhas been previously determined and not theclosing sight of a level line.BALANCING A SURVEY—Distributing correc-tions through a closed traverse to eliminate theerror of closure and to obtain an adjusted positionfor each traverse station.BASE COURSE—The layer of material im-mediately beneath the surface or intermediatecourse. It may be composed of crushed stone,crushed slag, crushed or uncrushed gravel andsand, or combinations of these materials. It alsomay be bound with asphalt.BASE LINE—A surveyed line established withmore than usual care as the known length of atriangle side for computing other triangle sides.BASE CONTROL—Horizontal or verticalcontrol, the positions of whose stations have beenaccurately coordinated and correlated, forminga framework to which other surveys are adjusted.BEARING—The direction of a line within aquadrant, with respect to the meridian. Bearingsare measured clockwise or counterclockwise fromnorth or south, depending on the quadrant.BENCH MARK-A relatively permanent object,natural or artificial, bearing a marked point whoseAI-2
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