NORMAL LINE—A line that is parallel to two
planes of projection and perpendicular to the
third. A line or plane that forms a 900 angle with
another line or plane is normal to that line or
plane.
NORTHING—One of the two values indicating
the position of a point on a grid system. The
northing coordinate is abbreviated N. (See GRID
COORDINATES.)
OBLIQUE PROJECTION—A single view of an
object showing three dimensions (length, width,
and height).
OBLONG—A nonequilateral rectangle.
OBTUSE ANGLE—An angle greater than 90°.
OCCUPIED STATION—A traverse or triangu-
lation station over which a theodolite or an
engineer transit is set up for the measurement of
angles at this station. Also, a station at which
angles have been so measured.
OCTAGON—A polygon of eight sides.
OFFSET LINE—A supplementary line close to,
and usually parallel to, a main survey line to which
it is referenced by measured offsets. When the line
for which data are desired is in such a position
that it is difficult to measure over it, the required
data are obtained by running an offset line in a
convenient location and measuring offsets from
it to salient points on the other line.
OGEE CURVE—Any curve composed of two
consecutive tangent circular arcs that curve in
opposite directions; also known as reverse curve.
OPEN TRAVERSE—A traverse that starts at a
point of known or assumed position and ends at
a point whose relative position is unknown with
respect to the starting point.
ORDER OF ACCURACY—A mathematical
ratio defining the general accuracy of the
measurements made in a survey. The orders of
accuracy for surveys are divided into four classes
named first-order, second-order, third-order, and
fourth-order.
ORGANIC SOIL—Soil that contains mineral
grains and a more or less conspicuous admixture
of vegetable matter.
ORIENT—To establish the correct relationship
in direction with reference to the points of the
compass; to bring into correct relationship in
direction with reference to the points of the
compass.
ORTHOGRAPHIC PROJECTION—The pro-
jection of height, width, and depth of an object
into various single planes so as to depict the true
size and shape of the object as seen from each
individual plane, each plane showing only two
dimensions, thereby necessitating a minimum of
two planes to show all three dimensions.
PARALLAX—The apparent displacement or the
difference in apparent direction of an object as
seen from two different points not on a straight
line with the object. In testing the focus of a
telescope, the head of the observer must move
from side to side or up and down while sighting
through the eyepiece. Any apparent movement of
the cross hairs in relation to the object image
means that parallax is present. Parallax can be
practically eliminated by careful focusing.
PARALLEL OF LATITUDE—A line on the
surface of the earth having the same latitude at
every point.
PARALLELEPIPED—A solid figure whose base
is a parallelogram.
PARALLELOGRAM—A quadrilateral with each
pair of opposite sides parallel.
PARTIAL AUXILIARY VIEW—An auxiliary
view in which only the features of an object that
are specifically desired are shown.
PARTIAL SECTION—A sectional view con-
sisting of less than a half section; used to show
the internal structure of a small portion of an
object; also known as a broken section.
PAVEMENT STRUCTURE—All courses of
selected material placed on the foundation or
subgrade soil, other than any layers or courses
constructed in grading operations.
PENTAGON—A polygon of five sides.
PERCENT—Portion in one hundred parts.
PERIMETER—The sum of the sides of a
polygon.
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