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ANGLES IN OBLIQUE | Up
Engineering Aid 3 - Beginning Structural engineering guide book | Next
SKETCHING |
Figure 5-54.-Transferring an angle in oblique projection.
Figure 5-55.-Cavalier projection of a circle on a receding
surface.
size; an angle on any other surface will not. The
upperpart of figure 5-54 shows a two-view multi-
view projection of a block. It has a 30-degree
angle on the top face and another on the front
face. In the cavalier projection below, the angle
on the front face still measures 30 degrees; that
on the top face measures only about 9 degrees.
You transfer the top face angle by locating the
end points of the line by measurements along
regular lines.
CIRCLES IN OBLIQUE.— In an oblique
projection, a circle on the surface parallel to the
plane of projection will appear as a circle. A
circle on any other surface will appear as an
ellipse, as shown in figure 5-55. The upperpart
of this figure shows a two-view multi-view
projection of a block with a circle on its upper
face. The lower part of this figure shows a cavalier
projection in which the circle appears as an ellipse.
Each of the conjugate (joined together) diameters
of the ellipse is equal to the diameter of the circle.
PERSPECTIVE PROJECTION
AND PERSPECTIVE DRAWING
PERSPECTIVE PROJECTION (fig. 5-2) is
obtained when the projection lines converge to a
point that is at a finite distance from the plane
of projection. Each projection line forms a
different angle with the plane of projection,
giving the viewer a three-dimensional picture of
the object. This type of projection, however,
cannot accurately convey the structural features
of a building; hence, it is not adequate for
working drawings.
On the other hand, of all the three-
dimensional single-plane drawings, PERSPEC-
TIVE DRAWINGS are the ones that look
the most natural. At the same time, they
are also the ones that contain the most errors.
Lines that have the same length on the object
have different lengths on the drawing. No
single line or angle on the drawing has a
length or size that has any known relationship
to its true length or size when projected through
perspective projections.
Perspective drawing is used only in drawings
of an illustrative nature, in which an object
is deliberately made to appear the way it looks
to the human eye. Most of the drawings you will
prepare will be drawings in which accuracy,
rather than eye appearance, will be the chief
consideration. Consequently, you will not be
concerned much with perspective drawing.
If you are required to prepare perspective
drawings, refer to Illustrator Draftsman,
NAVEDTRA 10472, or civilian publications, such
as Architectural Drawing and Light Construction
and Architectural Graphic Standards.
5-28
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