ProjectionProjection If straight lines are If straight lines are

drawn from various drawn from various points on the contour points on the contour of an object to meet a of an object to meet a plane, the object is plane, the object is said to be projected said to be projected on that plane. The on that plane. The figure formed by figure formed by joining, in correct joining, in correct sequence,sequence, the points the points at which these lines at which these lines meet the plane, is meet the plane, is called the projection called the projection of the object. of the object.

ProjectionProjection Projection means an Projection means an

image or act of image or act of obtaining the image of obtaining the image of an object. The image an object. The image is referred to as a is referred to as a ‘view’.‘view’.

The lines from the The lines from the object to the plane object to the plane are called the are called the projectors.projectors.

Concept of ProjectionConcept of Projection Look at an object kept in Look at an object kept in

front of an illuminating front of an illuminating bulb as shown in the bulb as shown in the figure. Light rays from the figure. Light rays from the bulb strike the object and bulb strike the object and create its shadow on the create its shadow on the screen. The image thus screen. The image thus obtained is represents a obtained is represents a ‘view’ or the projection of ‘view’ or the projection of the object. the object.

Fig. 1

Concept of ProjectionConcept of Projection The ‘view’ appears The ‘view’ appears

larger than the object larger than the object as the light rays are as the light rays are divergent.divergent.

In such a case the In such a case the projection is called projection is called oblique projection.oblique projection.

Concept of ProjectionConcept of Projection If the light-rays are If the light-rays are

parallel to each other and parallel to each other and perpendicular to the perpendicular to the plane, the ‘view’ or image plane, the ‘view’ or image formed would appear formed would appear equal to the actual size of equal to the actual size of the object.the object.

In such a case, the In such a case, the projection is called projection is called orthographic projection.orthographic projection.

Ortho means Ortho means perpendicular.perpendicular.

Orthographic ProjectionOrthographic Projection When the projectors are When the projectors are

parallel to each other and parallel to each other and perpendicular to the perpendicular to the plane of projection, the plane of projection, the ‘view’ or projection is ‘view’ or projection is called orthographic called orthographic projection.projection.

Imagine that a person Imagine that a person looks at the block shown looks at the block shown in the figure from a in the figure from a theoretically infinite theoretically infinite distance, so that the rays distance, so that the rays of light emerging from his of light emerging from his eyes are parallel to one eyes are parallel to one another and another and perpendicular to the front perpendicular to the front surface F surface F

Concept of ProjectionConcept of Projection The projection system The projection system

used in engineering used in engineering drawing, similar to that drawing, similar to that shown in Fig.1, is depicted shown in Fig.1, is depicted in Fig.2.in Fig.2.

The light source is The light source is replaced by a person replaced by a person looking towards the object.looking towards the object.

The lines of sight of the The lines of sight of the observer create a ‘view’ of observer create a ‘view’ of the screen.the screen.

The screen is referred to The screen is referred to as the Plane of Projection as the Plane of Projection (POP).(POP).

The lines of sight are The lines of sight are called projection lines or called projection lines or projectors. Thus observer, projectors. Thus observer, object and POP are the object and POP are the three basic elements of a three basic elements of a projection systemprojection system

Fig. 2

Orthographic ProjectionOrthographic Projection The view of this block will The view of this block will

be the shaded figure, be the shaded figure, showing the front surface showing the front surface of the object in its true of the object in its true shape and proportion.shape and proportion.

If these rays of sight are If these rays of sight are extended further to meet extended further to meet perpendicularly a plane perpendicularly a plane (marked V.P.) set up (marked V.P.) set up behind the block, and the behind the block, and the points at which they meet points at which they meet the plane are joined in the plane are joined in proper sequence, the proper sequence, the resulting figure (marked resulting figure (marked E) will also be exactly E) will also be exactly similar to the front face.similar to the front face.

Orthographic ProjectionOrthographic Projection This figure E is called the This figure E is called the

projection of the block on projection of the block on the plane V.P.the plane V.P.

The lines from the block The lines from the block to the plane are also the to the plane are also the projectors.projectors.

As the projectors are As the projectors are perpendicular to the perpendicular to the plane on which the plane on which the projection is obtained, projection is obtained, this is called the this is called the orthographic projection of orthographic projection of the front surface F of the the front surface F of the block on the plane V.P.block on the plane V.P.

Orthographic ProjectionOrthographic Projection The projection is shown The projection is shown

separately in Figure (b) at separately in Figure (b) at right.right.

It shows only two It shows only two dimensions of the block, dimensions of the block, viz. height H and width W.viz. height H and width W.

It does not show the It does not show the thickness.thickness.

Thus we find that only Thus we find that only one projection is one projection is insufficient for the insufficient for the complete description of complete description of the block or for that the block or for that matter, any object.matter, any object.

Orthographic ProjectionOrthographic Projection Let us further assume Let us further assume

that another plane that another plane marked H.P. is hinged marked H.P. is hinged at right angle to the at right angle to the first plane, so that the first plane, so that the block is in front of V.P. block is in front of V.P. and above H.P.and above H.P.

The projection on H.P. The projection on H.P. (Figure P) shows the (Figure P) shows the top surface of the top surface of the block.block.

Orthographic ProjectionOrthographic Projection If a person looks at If a person looks at

the block from above, the block from above, he will obtain the he will obtain the same view as the same view as the figure P.figure P.

It shows the width W It shows the width W and the thickness T of and the thickness T of the block.the block.

It however, does not It however, does not show the height of the show the height of the blockblock

W

T

Orthographic ProjectionOrthographic Projection One of the planes is One of the planes is

now rotated or turned now rotated or turned on the hinges so that on the hinges so that it lies in the extension it lies in the extension of the other plane.of the other plane.

This can be done in This can be done in one of the two ways:one of the two ways:

(i) by turning the V.P. in (i) by turning the V.P. in the direction of the the direction of the arrows A orarrows A or

(ii) by turning H.P. in the (ii) by turning H.P. in the direction of the arrows direction of the arrows B.B.

W

T

Orthographic ProjectionOrthographic Projection The H.P. when turned The H.P. when turned

and brought in line with and brought in line with the V.P. is shown by the V.P. is shown by dashed lines.dashed lines.

The two projections can The two projections can now be drawn on a flat now be drawn on a flat (2-dimensional) sheet of (2-dimensional) sheet of paper in correct relation paper in correct relation with each other.with each other.

When studied together, When studied together, they supply all they supply all information about the information about the shape and size of the shape and size of the block. block.

Orthographic ProjectionOrthographic Projection Thus any solid may be Thus any solid may be

represented by means of represented by means of different orthographic different orthographic views.views.

Different views of an Different views of an object can be obtained by object can be obtained by viewing the object from viewing the object from different directions.different directions.

Any orthographic view Any orthographic view gives only two gives only two dimensions.dimensions.

Any two orthographic Any two orthographic views together give all the views together give all the three dimensions. three dimensions.

Planes of ProjectionPlanes of Projection Plane of Projection (POP) Plane of Projection (POP)

is a plane on which a is a plane on which a particular view is particular view is projected.projected.

We need different POPs We need different POPs to draw different views.to draw different views.

Two such planes Two such planes perpendicular to each perpendicular to each other are employed for other are employed for the purpose of the purpose of orthographic projections orthographic projections and are called the and are called the reference planes (RPs) or reference planes (RPs) or Principal Planes.Principal Planes.

Principal Planes of ProjectionPrincipal Planes of Projection Horizontal Plane (H.P.):Horizontal Plane (H.P.):

A plane parallel to the A plane parallel to the ground (or horizon) is ground (or horizon) is called Horizontal Plane called Horizontal Plane (HP) or the Horizontal (HP) or the Horizontal Reference Plane (HRP).Reference Plane (HRP).

Vertical Plane (V.P.):Vertical Plane (V.P.):

A plane perpendicular to the A plane perpendicular to the ground and intersecting ground and intersecting the horizontal plane is the horizontal plane is called Vertical Plane (VP) called Vertical Plane (VP) or Frontal Reference or Frontal Reference Plane (FRP). Plane (FRP).

Principal Planes of ProjectionPrincipal Planes of ProjectionApart from these two, Apart from these two,

there is one more there is one more Principal Plane:Principal Plane:

Profile Plane (P.P.):Profile Plane (P.P.):

A plane perpendicular to A plane perpendicular to the HP and the VP the HP and the VP and intersecting both and intersecting both of them, is called of them, is called Profile Plane (PP) or Profile Plane (PP) or Profile Reference Profile Reference Plane (PRP).Plane (PRP).

Principal Planes of ProjectionPrincipal Planes of Projection It is important to note It is important to note

that thee RPs are that thee RPs are imaginary only.imaginary only.

They are assumed to They are assumed to be transparent so that be transparent so that the observer can look the observer can look through them.through them.

Auxiliary Plane of ProjectionAuxiliary Plane of Projection

A plane inclined to the A plane inclined to the HP and perpendicular HP and perpendicular to the VP is called to the VP is called Auxiliary Inclined Auxiliary Inclined Plane (AIP).Plane (AIP).

A plane inclined to the A plane inclined to the VP and perpendicular VP and perpendicular to the HP is called to the HP is called Auxiliary Vertical Auxiliary Vertical Plane (AVP).Plane (AVP).

AVP

Orthographic ViewsOrthographic Views Each of the RPs act as Each of the RPs act as

the POP for the the POP for the corresponding view., corresponding view., e.g.: e.g.:

1.1. The H.P., for the view The H.P., for the view from above or the top from above or the top view (TV). Top view is view (TV). Top view is also called plan.also called plan.

2.2. The V.P., for the view The V.P., for the view obtained from the front, obtained from the front, i.e. Front View (FV) or i.e. Front View (FV) or elevation. elevation.

Orthographic ViewsOrthographic Views3.3. Side Views: When the Side Views: When the

observer looks at the object observer looks at the object from side, i.e., from his left-from side, i.e., from his left-hand side or right-hand side, hand side or right-hand side, the view obtained is called the view obtained is called side view side view (SV). SV is seen (SV). SV is seen on the PP.on the PP.

I.I. Left-Hand Side View: When Left-Hand Side View: When the observer views the object the observer views the object from his left-hand side, the from his left-hand side, the view obtained is called view obtained is called leftleft--hand side view hand side view (LHSV).(LHSV).

II.II. Right Hand Side View: When Right Hand Side View: When the observer views the object the observer views the object from his right-hand side, the from his right-hand side, the view obtained is called as view obtained is called as right-hand side view right-hand side view (RHSV).(RHSV).

Orthographic ViewsOrthographic Views4.4. Bottom View: When Bottom View: When

the observer looks to the observer looks to the object from below, the object from below, the view obtained is the view obtained is called called bottom view bottom view (BV) (BV) or or bottom planbottom plan..

5.5. Rear View: When the Rear View: When the observer looks to the observer looks to the object from back, the object from back, the view obtained is called view obtained is called rear view rear view (RV) or (RV) or back back view view or or rear elevationrear elevation..

The Four QuadrantsThe Four Quadrants When the planes projection When the planes projection

are extended beyond the line are extended beyond the line of intersection, they form four of intersection, they form four quadrants or dihedral angles quadrants or dihedral angles which may be numbered as in which may be numbered as in the figure. the figure.

The object may be situated in The object may be situated in any one of the quadrants, its any one of the quadrants, its position relative to the planes position relative to the planes being described as ‘ above or being described as ‘ above or below HP’, and in front of or below HP’, and in front of or behind VP.behind VP.

The planes are assumed to be The planes are assumed to be transparent.transparent.

The projections are obtained The projections are obtained by drawing perpendiculars by drawing perpendiculars from the object to the plane, from the object to the plane, i.e., by looking from the front i.e., by looking from the front and from above. and from above.

The Four QuadrantsThe Four Quadrants They are shown on They are shown on

the flat surface by the flat surface by rotating one of the rotating one of the planes as already planes as already explained.explained.

It should be It should be remembered that the remembered that the first and the third first and the third quadrants are always quadrants are always opened out while opened out while rotating the planes.rotating the planes.

Methods of Projection- First Angle Methods of Projection- First Angle Projection MethodProjection Method

In this method, the object is In this method, the object is assumed to be situated in front assumed to be situated in front of the V.P. and above the H.P., of the V.P. and above the H.P., i.e., in the first quadrant and i.e., in the first quadrant and then projected on these then projected on these planes.planes.

The object lies between the The object lies between the observer and the POP.observer and the POP.

In this method when views are In this method when views are drawn in their relative drawn in their relative positions, TV comes below the positions, TV comes below the FV view.FV view.

Each projection shows the Each projection shows the view of that surface of the view of that surface of the object which is remote from object which is remote from the plane on which it is the plane on which it is projected and which is nearest projected and which is nearest to the observer. to the observer.

Methods of Projection- Third Angle Methods of Projection- Third Angle Projection MethodProjection Method

In this method, the object In this method, the object is assumed to be situated is assumed to be situated behind the V.P. and below behind the V.P. and below the H.P., i.e., in the third the H.P., i.e., in the third quadrant.quadrant.

The planes of projection The planes of projection are assumed to be are assumed to be transparent.transparent.

They lie between the They lie between the object and the observer.object and the observer.

Methods of Projection- Third Angle Methods of Projection- Third Angle Projection MethodProjection Method

When the observer views When the observer views the object from the front, the object from the front, the rays of sight intersect the rays of sight intersect the VP.the VP.

The figure formed by The figure formed by joining the points of joining the points of intersection in correct intersection in correct sequence gives the front sequence gives the front view of the object.view of the object.

The top view is obtained The top view is obtained in a similar manner by in a similar manner by looking from above.looking from above.

Methods of Projection- Third Angle Methods of Projection- Third Angle Projection MethodProjection Method

Each projection Each projection shows the view of that shows the view of that surface of the object surface of the object which is nearest to which is nearest to the plane on which it the plane on which it is projected. is projected.

Reference LineReference Line

Considering the front Considering the front view which is the view as view which is the view as seen from the front, seen from the front, independently, the HP independently, the HP coincides with the line xy. coincides with the line xy. In other words xy In other words xy represents H.P.represents H.P.

Similarly considering the Similarly considering the top-view which is top-view which is obtained by looking from obtained by looking from above, the same xy above, the same xy represents the V.P.represents the V.P.

Reference LineReference Line

Hence, when these Hence, when these two projections are two projections are drawn in correct drawn in correct relationship with each relationship with each other, xy represents other, xy represents both the HP as well both the HP as well as the VP.as the VP.

This line xy is called This line xy is called the reference line.the reference line.

Orthographic ProjectionsOrthographic Projections Further, in the first-angle Further, in the first-angle

projection method, the HP is projection method, the HP is always assumed to be so placed always assumed to be so placed as to coincide with the ground on as to coincide with the ground on or above which the object is or above which the object is situated. Hence, in this method, situated. Hence, in this method, the line xy is also the line for the the line xy is also the line for the ground.ground.

In the third-angle projection In the third-angle projection method, the HP is assumed to be method, the HP is assumed to be placed above the object. The placed above the object. The object may be situated on or object may be situated on or above the ground. Hence, in this above the ground. Hence, in this method, the line xy does not method, the line xy does not represent the ground. The line for represent the ground. The line for ground, denoted by letters GL, ground, denoted by letters GL, may be drawn parallel to xy and may be drawn parallel to xy and below the front view.below the front view.