Lecture 6 (06.08.12):
Theory of Multi-view Orthographic
Projections
Dr. Sharad Gokhale
Civil Engineering Department, IIT Guwahati
208, M-Block, Academic Complex
Email: sharadbg@iitg.ernet.in
Telephone #: 2419
Multi-view Orthographic
Projections
Terms and definition
• Projection – image or the act of obtaining an
image of an object
• In technical drawing – we call it a view
• Method – we use projection method to obtain
a view of an object
Orthographic Projections
• It is a technical drawing in which different views of
an object are projected on different reference
planes
• Different Reference planes (principal planes):
– Horizontal Plane (HP),
– Vertical Frontal Plane (VP)
– Side Or Profile Plane (PP)
• Different views:
– Front View (FV),
– Top View (TV),
– Side View (SV)
Projection System
Perspective
Projection System
Projection lines
Plane of Projections (POP)
Perspective
Parallel
Projection System
Projection lines
Plane of Projections (POP)
Perspective (Convergent
projection)
Three basic elements:
i. Object
ii. Observer
iii. POP
Parallel Projection
Projection of an Object
The outline on the plane of projection shows how the object appears to the
observer. In orthographic projection, projections from all points of the
object extend parallel to each other and perpendicular to the plane of
projection.
Y
X
VP
2nd
1st Quad.
Quad.
Y
Observer
X Y
HP
X
3rd Quad.
4th Quad.
This quadrant pattern,
If observed along x-y line ( in red arrow direction) will exactly appear as shown on
right side and hence, It is further used to understand illustration properly.
Methods of Drawing Orthographic Projections
First Angle Projections Method
Here views are drawn
by placing object
Third Angle Projections Method
Here views are drawn
by placing object
in 3rd Quadrant.
in 1st Quadrant
( FV above X-Y, TV below X-Y )
( TV above X-Y, FV below X-Y )
Symbolic presentation of both methods
with an object standing on HP (ground) on it’s base.
FV
X
TV
Y
NOTE:HP term is used in 1st angle method
&
ground term is used
in 3rd angle method of projections
TV
X
Y
FV
G
L
Planes
PRINCIPAL PLANES
HP AND VP
Profile Plane (P.P.)
AUXILIARY PLANES
Auxiliary Vertical Plane
(A.V.P.)
A.V.P.
⊥ to HP & ∠ to VP
Auxiliary Inclined Plane
(A.I.P.)
Planes & Views (first angle method)
This is a pictorial set-up of all three planes. Arrow direction is a
normal way of observing the object. But in this direction only VP and a
view on it (FV) can be seen. The other planes and views on those can
not be seen.
Procedure to solve above problem:To make those planes also visible from the arrow direction,
i) HP is rotated 900 downward, ii) PP, 900 in right side direction.
This way both planes are brought in the same plane containing VP.
PP
VP
Y
FV
LSV
Y
X
X
TV
HP
HP IS ROTATED DOWNWARD 900
AND
BROUGHT IN THE PLANE OF VP.
ACTUAL PATTERN OF PLANES & VIEWS
OF ORTHOGRAPHIC PROJECTIONS
PP IS ROTATED AWAY IN RIGHT SIDE 900
DRAWN IN
AND
FIRST ANGLE METHOD OF PROJECTIONS
BROUGHT IN THE PLANE OF VP.
First angle projection
FOR T.V.
IN THIS METHOD,
THE OBJECT IS ASSUMED TO BE
SITUATED IN FIRST QUADRANT
MEANS
ABOVE HP & INFRONT OF VP.
OBJECT IS INBETWEEN
OBSERVER & PLANE.
PP
VP
FV
LSV
Y
X
TV
HP
ACTUAL PATTERN OF
PLANES & VIEWS
IN
FIRST ANGLE METHOD
OF PROJECTIONS
THIRD ANGLE
PROJECTION
FOR T.V.
IN THIS METHOD,
THE OBJECT IS ASSUMED TO BE
SITUATED IN THIRD QUADRANT
( BELOW HP & BEHIND OF VP. )
PLANES BEING TRANSPERENT
AND INBETWEEN
OBSERVER & OBJECT.
TV
X
Y
LSV
FV
ACTUAL PATTERN OF
PLANES & VIEWS
OF
THIRD ANGLE PROJECTIONS
Orthographic projections
- points, lines, planes, and solids
• To draw projections of any object, one must
have the following information
– Object (with it’s description, well defined)
– Observer (always observing perpendicular to respective
reference plane)
– location of object (means it’s position with reference to
HP & VP)
• Terms ‘above’ & ‘below’ with respective to HP and terms
‘infront’ & ‘behind’ with respective to VP form 4 quadrants.
• Objects can be placed in any one of these 4 quadrants
UNDERSTANDING PROJECTIONS
To make and interpret drawings you need to know how to
create projections and understand the standard arrangement of
views.
You need to be familiar with the geometry of objects and
be able to visualize a 3D object that is represented in a
2D drawing.
Views of Objects
The system of views is called
multi-view projection. Each
view provides certain definite
information. e.g. a front view
shows the true shape and size
of surfaces that are parallel to
the front of the object.
Principal Dimensions
The three principal dimensions of an object are width,
height, and depth.
The front view shows only the height and
width of the object and not the depth.
In fact, any principal view of a 3D object
shows only two of the three
principal dimensions; the third is found
in an adjacent view.
Height is shown in the rear, left-side,
front, and right-side views.
Width is shown in the rear, top, front, and
bottom views.
Depth is shown in the left-side, top, rightside, and bottom views.
Envision the object in a Glass Box
To understand the standard arrangement of views on the sheet of paper
To draw the views on a sheet of paper, imagine the
six planes of the glass box being unfolded to lie flat.
Note the six standard
views (front, rear, top,
bottom, right side, left
side).
The Glass Box Unfolded
Lines extend around the glass box from one view to another on the planes of
projection. These are the projectors from a point in one view to the same point in
another view.
The Orthographic Projection
The front, top, and right-side views of the object shown now without
the folding lines.
Necessary Views
The top, front, and right-side views, arranged
together, are called the three regular views
because they are the views most frequently used.
A drawing should contain only the views needed to clearly and
completely describe the object.
View Selection
Select the most descriptive views
Use minimum number of views to
describe the object
How to project Side Views?
• Projecting across meter line
• Projecting through arcs
• Projecting through 45 degree projectors
Projecting across meter line
X1
LHSV
FV
45o
X
Y
Meter line
TV
Y1
Projecting through 45o projectors
X1
LHSV
FV
X
Y
45o projectors
TV
Y1
Projecting through arcs
X1
LHSV
FV
X
Y
Arcs
TV
Y1
Three basic views (FV, TV, SV) will provide
complete information about the real object
Top view
Top View
VIEWS OF SURFACES
The three orientations that a plane surface can have to
the plane of projection are normal, inclined, and
oblique.
A plane surface that is
perpendicular to a plane of
projection appears on
edge as a straight line
Standard Views of Primitive Solids
Ex. 1 & 2: Draw the three
principal views of the
objects shown In fig 9.12
(a) and 9.13 (b)
Ex. 3 & 4: From the pictorial
view of the objects shown
In fig 9.14 (a) and 9.15 (b)
Draw FV, TV, RHSV