What you'll learn
Orthographic projection is the standard way engineers represent a three-dimensional object using flat, two-dimensional views. For CSEC Technical Drawing you need to understand what orthographic projection is, the difference between first-angle and third-angle projection, how the views relate to one another, and how to lay them out correctly. This guide covers the principle of orthographic projection, the main views, the two projection systems and their symbols, and how the views are arranged. By the end you should be able to explain orthographic projection and set out the front, plan and side views correctly.
Key terms and definitions
Orthographic projection — Representing a 3D object using separate 2D views seen at right angles.
Front view (elevation) — The main view, looking directly at the front of the object.
Plan view — The view looking straight down on top of the object.
End view (side elevation) — The view looking at the side of the object.
First-angle projection — A system where each view is placed on the far side from where it is viewed.
Third-angle projection — A system where each view is placed on the same side as it is viewed.
Projection symbol — A truncated-cone symbol showing which projection system is used.
Projection lines — Light construction lines used to line up the views.
Core concepts
The principle of orthographic projection
A single drawing cannot fully describe a 3D object, so orthographic projection shows it using several flat views, each looking at the object straight on from a different direction (at right angles). The main views are the front view (elevation), the plan view (from above) and the end view (side elevation). Together these views give all the information needed to understand the object's shape and size, because each shows two of the three dimensions.
The three main views
- The front view (elevation) is the most important view, chosen to show the most detail or the most recognisable shape of the object. It is drawn looking directly at the front.
- The plan view is drawn looking straight down from above, showing the object's shape as seen from the top.
- The end view (side elevation) is drawn looking at the side of the object, showing its depth and height.
The views are lined up with each other so that features match across them — this is done using light projection lines, which are erased or left faint on the final drawing.
First-angle and third-angle projection
There are two systems for arranging the views, and you must know both:
- In first-angle projection, each view is placed on the opposite side from the direction it is viewed. So the plan view (looking down) is placed below the front view, and the end view (looking from the left) is placed on the right.
- In third-angle projection, each view is placed on the same side as the direction it is viewed. So the plan view is placed above the front view, and the end view looking from the left is placed on the left.
Both systems show the same object; they simply arrange the views differently. It is essential to be consistent and to show which system is being used.
The projection symbols
Because the two systems arrange views differently, a drawing must show which one is used, so it is read correctly. This is done with a projection symbol — a small drawing of a tapered (truncated) cone shown in two views. The symbol looks different in first-angle and third-angle projection, telling the reader immediately which system the drawing follows. The symbol is placed in or near the title block.
Lining up the views
A key feature of orthographic projection is that the views are aligned so that measurements carry across from one view to another. The front view and plan view share the same width; the front view and end view share the same height; and the plan and end views share the same depth. Because of this, if you know a dimension in one view, it lines up in the related view. Using projection lines to carry these dimensions across ensures the views are consistent.
Choosing the front view
Choosing the right front view is important. It should be the view that shows the object most clearly — usually the one with the most detail or the most recognisable outline — and should be chosen so that the other views have as few hidden detail lines as possible. A well-chosen front view makes the whole drawing clearer and easier to read.
Orthographic projection versus pictorial drawing
It helps to understand how orthographic projection differs from pictorial drawings such as isometric or oblique views. A pictorial drawing shows the object in three dimensions in a single view, which looks realistic and is easy to visualise, but distances and angles are distorted, so it cannot be measured accurately. Orthographic projection, by contrast, shows the object in several separate flat views, which is less realistic to look at but shows every dimension accurately and to scale. This is why orthographic projection is used for working drawings that a builder or engineer must measure from, while pictorial views are often used to give a quick, clear impression of what the object looks like.
Hidden detail in orthographic views
Because orthographic views look at the object from one direction only, some features are hidden behind others. These hidden features are shown using dashed (hidden detail) lines, so that internal or rear features can still be represented in a view. For example, a hole that does not appear on the surface facing you would be shown with dashed lines in that view. Using hidden detail lines correctly allows all the important features to be shown across the views without needing to cut the object open (which is what a sectional view does instead). Knowing when to use hidden detail lines is an important part of producing a complete and correct orthographic drawing.
Worked examples
Example 1: Naming the views
Name the three main views used in orthographic projection. The front view (elevation), the plan view (from above) and the end view (side elevation). Together they show all three dimensions of the object.
Example 2: Placing the plan view
In third-angle projection, where is the plan view placed relative to the front view? In third-angle projection, the plan view is placed above the front view, because each view is drawn on the same side as it is viewed (looking down places the view at the top).
Example 3: The purpose of the projection symbol
Why is a projection symbol included on an orthographic drawing? Because first-angle and third-angle projection arrange the views on opposite sides, the projection symbol (a truncated cone) tells the reader which system has been used, so the views are interpreted correctly.
Example 4: Aligning dimensions
Explain how the front view and plan view are related. The front view and plan view share the same width, and are drawn directly in line vertically. This means a width measured in the front view lines up with the same width in the plan view, keeping the drawing consistent.
Common mistakes and how to avoid them
A very common error is mixing up first-angle and third-angle projection. Remember: first-angle places each view on the opposite side (plan below), while third-angle places it on the same side (plan above). Learn which is which and stay consistent.
Students often forget the projection symbol. Because the two systems look different, the symbol is essential so the reader knows which one is used — leaving it off makes the drawing ambiguous.
Another mistake is not lining up the views. The views must be aligned so dimensions carry across; drawing them out of line makes the drawing wrong and hard to read.
When choosing the front view, do not pick an unhelpful angle. Choose the view that shows the most detail and gives the clearest other views.
Finally, keep projection lines light and construction-like; they are used to line up the views, not to be part of the finished outline, which is drawn with thick lines.
Exam technique for "Solid Geometry: Orthographic Projection"
Be ready to name and describe the three views (front, plan, end) and to explain the difference between first-angle and third-angle projection, including where each places the plan and end views. A quick sketch showing the layout helps.
Know that the projection symbol (truncated cone) indicates the system used, and be prepared to identify or draw it. Explain that the views are aligned so dimensions carry across, and use projection lines to show this.
When producing a drawing, choose a sensible front view, keep the views aligned, and use the correct line types. Use precise terms — orthographic projection, elevation, plan, first-angle, third-angle — throughout, as accurate vocabulary earns marks.
Quick revision summary
- Orthographic projection shows a 3D object using flat 2D views at right angles: front view (elevation), plan (from above) and end view (side elevation).
- In first-angle projection, views go on the opposite side (plan below the front view); in third-angle, on the same side (plan above).
- A projection symbol (truncated cone) shows which system is used, and must be included.
- The views are aligned so dimensions carry across: front and plan share width, front and end share height.
- Choose the front view to show the most detail and give the clearest other views.
- Keep projection lines light; the finished outline is drawn with thick lines.