Figure 5-4.-Primary (principal) planes of projections.
is normally placed between the point of sight and
the object. For the purpose of studying any type
of projection, it must be assumed that the planes
of projection are in fixed positions. Once the
object is placed in a definite imagined position,
it should never be changed. If a different view of
the object is desired, the location of the point of
sight is changed.
The PROJECTION LINES (or LINES OF
SIGHT) are the imaginary lines from the eye of
the viewer (point of sight) to points on the object
(fig. 5-2). By the use of projection lines, points
on the object are projected on the image plane.
These points are the points at which the
projection lines appear to pierce the image plane.
By the projection of the prominent points, lines,
and surfaces of an object, a complete view of
that object can be projected on the plane of
projection.
The relationship between the point of sight
(station point), the plane of projection (image
plane), the projection lines (lines of sight), and
the manner in which they are used for each
individual type of projection will be discussed in
the following sections.
ORTHOGRAPHIC PROJECTION
When you are called upon to draw a
three-dimensional object or figure, it is customary
to represent the parts and forms on the
flat plane of the drafting paper in such a
manner that all features are shown in their
true dimensions and in their true relationship
with other features on that part of the object.
To do this, you must draw a number of
views of the object from different angles.
Projecting these essential views into a single
plane is known as ORTHOGRAPHIC PRO-
JECTION. The term orthographic is derived
from the word orthos meaning perpendicular or
right-angular.
Multi-view Projection
When an object is viewed through a
plane of projection from a point at infinity,
an accurate outline of the visible face of the
object is obtained (fig. 5-3). However, the
projection of one face usually will not provide
an overall description of the object; other
planes of projection must be used. Establishing
an object’s true height, width, and depth requires
front, top, and side views, which are called
the PRINCIPAL PLANES OF PROJECTION.
Figure 5-4 shows the three principal (or
primary) planes of projection, known as the
VERTICAL, HORIZONTAL, and PROFILE
PLANES. The angles formed between the
horizontal and the vertical planes are called
the FIRST, SECOND, THIRD, and FOURTH
ANGLES, as indicated in the figure. Cur-
rently, however, for technical reasons, only
the use of first- and third-angle projection is
practical.
FIRST-ANGLE PROJECTION.— A fine
example of first-angle projection using a cube is
5-3
