PERMANENT BENCH MARK—A bench mark
of as nearly permanent character as it is
practicable to establish; usually designated bench
mark.
PERSPECTIVE—A single-view drawing of an
object showing three dimensions. Lines and
surfaces are foreshortened to make it appear
optically correct. Perspective drawing is used
when the end product is to be of an illustrative
nature.
PHANTOM LINE—Thin broken line consisting
of one long and two short evenly spaced dashes;
used to indicate the alternate position of a moving
part.
PICTORIAL PROJECTION—Any projection
that shows three dimensions in a single view as
it would be seen by the eye.
PLAN/PROFILE PAPER—Upper half (plan) is
plain white paper used to draw the plan view;
lower half is gridded and used to draw profiles.
PLANE COORDINATES—See GRID COOR-
DINATES.
PLANE OF PROJECTION—The theoretical,
transparent plane placed between the point of
sight and the object in any type of projection. The
observer sees the features of an object as they lie
on the plane of projection; also known as image
plane.
PLANE SURVEY—A survey in which the effect
of the curvature of the earth is almost entirely
neglected, and computations of the relative
positions of the stations are made using
the principles of plane geometry and plane
trigonometry.
PLASTIC LIMIT (PL)—The lower limit of the
plastic state, expressed as the minimum moisture
content, at which a soil can be rolled into a thread
one-eighth in, in diameter without crumbling.
PLASTICITY—The property of a fine-grained
soil that allows it to be deformed beyond the point
of recovery without cracking or changing volume
appreciably.
PLASTICITY INDEX (PI)—The numerical
difference between the moisture content at the
liquid limit and the moisture content at the plastic
limit.
PLUMB LINE—The line indicated by a plumb
bob cord. The direction in which gravity acts.
PLUNGE THE INSTRUMENT—Turn the
instrument telescope end-for-end about its
horizontal axis.
POINT OF SIGHT—The position of the observer
in relation to the object and the plane of
projection in any type of projection; also known
as station point.
POLYGON—A plane figure that is bounded by
straight-line sides.
POROSITY—The ratio, expressed as a per-
centage, of the intergranular space in a given soil
mass to the total volume of the soil mass.
POSITION— 1. Data that define the location of
a point with respect to a reference system. The
coordinates that define the location of a point on
a grid. 2. A circle position.
POWER—The number of times, as indicated by
an exponent, a number occurs as a factor; for
example, 24 = 2<2.2.2=2 to the 4th power=16.
POZZOLAN—A siliceous or aluminous material
that in itself possesses little or no cementitious
value but will, in finely divided form and in the
presence of moisture, chemically react with
calcium hydroxide at ordinary temperatures
to form compounds possessing cementitious
properties.
PRECISION—The degree of refinement in the
performance of an operation or the degree of
perfection in the instruments and methods used
when making measurements. Precision relates to
the quality of the operation by which a result is
obtained and is distinguished from accuracy that
relates to the quality of the result.
PROJECTION CHART—Chart showing the
comparison of scheduled progress and actual
progress.
PROJECTION LINES—The imaginary lines
from the eye of the viewer to the points on the
object in any type of projection; also known as
lines of sight.
PROPORTION—An equation in which two
ratios are set equal to each other; for example,
3/4 = 9/12,
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