Orthographic Projections
Orthographic Projections
Parallel projection technique
Projectors normal to projection plane
Projection Plane
Projector
s
NOTE: CAD Systems typically use orthographic projection
Orthographic Projection Categories
Axonometric
Three object faces visible
No principal axes
parallel/perpendicular to projection
plane
Multiview
Only one object face
visible
One object face (i.e., two
principal axes) parallel to
projection plane
Axonometric Projections
Classed according to angles made by principal axes
when projected onto projection plane
Trimetric – no equal angles
All principal axes foreshortened unequally
Dimetric – two angles are equal
Two principal axes are equally
foreshortened
Isometric – all angles equal (iso equal,
metric measure)
All principal axes foreshortened equally
Axonometric Projection Classes
Axonometric Projection Classes
Top View
Front View
Front View
Axes Only
Isometric Scaling
Lines parallel to any
principal axis are
foreshortened equally
( scaled proportionally)
Measurements can be
made parallel to any of
the three principal axes
NOTE: In a trimetric projection, measurements along a given
principal axis are proportional; however, measurements along
different principal axes are not scalable
Isometric Projections
All 3 principal axes equally foreshortened
same scale
Can be visualized as looking down the
diagonal of a cube
Trimetric Projection Isometric Projection
Isometric Drawings
It can be shown that, in an isometric projection:
Each axis is foreshortened
to approximately 82% of its
true length
To correct for this, an
isometric projection can be
plotted at a scale of 1/.82
(~1.22). This convenient
approximation of an
isometric projection is
called an isometric
drawing
Isometric Viewing
An isometric projection is obtained by rotating
the view:
About a vertical axis (45 ± 90n)º (n is an
integer), then
Out of the horizontal plane, ±35.26º
Front
Front, Side
Top, Front, Side
Isometric Sketching
In sketching an isometric view, principal axes
are aligned as shown below:
Orthographic Projections