Technical Graphics - Projection Systems - Orthographic

Deciding and Symbols

How do you know if you are looking at a 1st Angle or a 3rd angle Projection. It can be quite difficult to tell them apart, even for simple objects. Well there are two symbols that are used world wide and they are standard .

1st Angle Projection
3rd Angle Projection

In both cases you have the Elevation of a truncated cone, (the bit like a triangle). The End Elevation of the truncated cone is on the correct side for that projection system.

On good Orthographic Drawings you will find one of these symbols beside or on top of the Title Box. It would also be a good idea if you started using these symbols when you are doing your drawings.

Below you can see the dimensions for the symbols. You choose the diameter of the base of the cone to suit the size of your drawing page.

Section 1 : This Lesson shows you the imaginary Glass Box and then goes onto First Angle Projection.

Section 2 : This Lesson shows Third Angle Projection.

Section 3 : This Lesson shows you how to determine wheather you are working in First or Third Angle Projection.

Section 4 : In this Lesson you get to put all of your newly acquired knowledge to the test with a wide variety of problems and question relating to Orthographic Projection.