(measured upward from the horizon) is termed
an angle of elevation, and a negative altitude
(measured downward from the horizontal) is
termed an angle of depression. 2. Altitude is
sometimes used to apply to elevation above a
datum; for example, the altitude of an airplane.
ANGLE—A figure formed by two lines or planes
extending from or diverging at the same point.
ANGLE OF DEPRESSION—A negative altitude.
ANGLE OF ELEVATION—A positive altitude.
ANGLE OF INCLINATION—A vertical angle
of elevation or depression.
ANNUAL VARIATION—The annual change in
the magnetic declination.
ANTILOG—The result when a logarithm is
converted to a number.
ARC—A portion of the circumference of a circle.
ARCHITECT’S SCALE—Scale used when
dimensions or measurements are to be expressed
in feet and inches.
ARITHMETIC—The art of computation by the
use of positive real numbers.
ASPHALT—A dark brown to black cementitious
material in which the predominating constituents
are bitumens that occur in nature or are obtained
in petroleum processing. Asphalt is a constituent
in varying proportions of most crude petroleums.
AUXILIARY PLANE—A plane (NOT one of the
normal planes) from which the auxiliary view is
projected.
AUXILIARY VIEW—A view that is not on one
of the normal planes of projection. It is used to
show features of objects that do not appear- in
their true size and shape in the normal views.
AXONOMETRIC—A single view of an object
depicting all three dimensions. The projection
lines are parallel to each other and perpendicular
to the plane of projection. The object is inclined
to the plane of projection, thereby allowing the
viewer to see three dimensions.
AZIMUTH (Surveying)—The horizontal direc-
tion of a line measured clockwise from a reference
plane, usually the meridian; often called
FORWARD AZIMUTH to differentiate from
BACK AZIMUTH. In the basic control surveys
of the United States, azimuths are measured
clockwise from south following the continental
European geodetic practice. However, this
practice is not followed in all countries.
AZIMUTH MARK—A marked point visible
from a survey station, the azimuth to which is
determined for use in dependent surveys.
BACK AZIMUTH—As the azimuth of the line
from A to B is known as the forward azimuth,
the azimuth of the same line from B to A is known
as the back azimuth.
BACKSIGHT—1. In traversing, a backsight (BS)
is a sight on a previously established traverse or
triangulation station, that is not the closing sight
of the traverse. 2. In leveling, a backsight is a
reading on a rod held on a point whose elevation
has been previously determined and not the
closing sight of a level line.
BALANCING A SURVEY—Distributing correc-
tions through a closed traverse to eliminate the
error of closure and to obtain an adjusted position
for each traverse station.
BASE COURSE—The layer of material im-
mediately beneath the surface or intermediate
course. It may be composed of crushed stone,
crushed slag, crushed or uncrushed gravel and
sand, or combinations of these materials. It also
may be bound with asphalt.
BASE LINE—A surveyed line established with
more than usual care as the known length of a
triangle side for computing other triangle sides.
BASE CONTROL—Horizontal or vertical
control, the positions of whose stations have been
accurately coordinated and correlated, forming
a framework to which other surveys are adjusted.
BEARING—The direction of a line within a
quadrant, with respect to the meridian. Bearings
are measured clockwise or counterclockwise from
north or south, depending on the quadrant.
BENCH MARK-A relatively permanent object,
natural or artificial, bearing a marked point whose
AI-2
