Orthographic projection is the graphical method to represent engineering 3D models into the 2D planes. To draw three-dimensional object into the two-dimensional plane orthographic projection is used. First angle system and third angle systems both are the modes of orthographic projection drawings.
First angle :
The object is placed in the first quadrant above the horizontal plane. It is the real system of projections. The object is supposed to be kept in between the observer and the projection of planes. The right-side view of the object is to the left of front view and the top view is at the bottom of front view. It is popular and widely used in India and Europe.
Third angle :
The object is placed in the third quadrant below the horizontal plane. It is an imaginary projection method. The plane of projection lies between the observer and the object. The right-side view will be at the right of front the view and the top view is at the top of front view. It is popular and widely used in the United States and Australia.
Symbols of first angle and third angle projection :
Difference between first angle and third angle projection
|Plane of projection is assumed to be non-transparent.||Plane of projection assumed to be non-transparent.|
|Real projection system.||It is an imaginary projection system.|
|Assumed that an object is placed in the first quadrant.||Assumed that an object is placed in the third quadrant.|
|Mostly used in India, European countries and Canada||Mostly used in the United States and Australia|
|An object is kept between the observer and the plane of projection.||The plane of projection is between an object and the observer.|
|The right view is projected on to the left of the front view and the top view is projected at the bottom of the front view.||The right view is projected on to the right of the front view and the top view is projected at the top of the front view.|
Views placement for first angle and third angle method :
Why are second and fourth angle projection methods not used in the industry?
In the first angle and third angle, we assumed that the horizontal plane is rotated clockwise by 90 degrees for drawing purposes. Similarly, if we rotate the horizontal plane in second angle and fourth angle projection, horizontal and vertical views will overlap with each other, which creates confusion in their drawn views. So, we are using methods of the first angle and third angle projection.
It is just a matter of assumptions. The assumption that the horizontal plane rotates clockwise. If you rotate the horizontal plane counter clockwise, then the views will overlap in the case of the first angle and third angle method, and then the fourth angle and second angle projections are favourable.
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