Last reviewed 16 Sept 2026 · 7 min read
Projection
A projection is the representation of a three-dimensional object on a two-dimensional plane (the plane of projection) by drawing projectors (lines of sight) from points on the object to the plane.
| Type | Projectors | Examples |
|---|---|---|
| Parallel projection — orthographic | Parallel to each other and perpendicular to the plane | Multi-view drawings, isometric (axonometric) projection |
| Parallel projection — oblique | Parallel to each other but inclined to the plane | Cavalier and cabinet projections |
| Central (conical) projection | Converge to a point (eye) | Perspective drawings |
Orthographic projection
Orthographic (multi-view) projection shows an object by two or more views on mutually perpendicular planes, each showing true lengths and shapes of surfaces parallel to that plane.
Principal planes
- Vertical plane (VP) — frontal plane; projection on VP = front view (elevation).
- Horizontal plane (HP) — projection on HP = top view (plan).
- Profile plane (PP) — perpendicular to both — side view (end view).
- The line of intersection of VP and HP is the reference line XY.
Quadrants
VP and HP divide space into four quadrants:
| Quadrant | Position of object |
|---|---|
| First | Above HP, in front of VP |
| Second | Above HP, behind VP |
| Third | Below HP, behind VP |
| Fourth | Below HP, in front of VP |
For drawing, the HP is rotated (about XY) to coincide with the VP. In the second and fourth quadrants, views overlap, so they are not used for projecting objects.
First-angle and third-angle projection
| Feature | First-angle projection | Third-angle projection |
|---|---|---|
| Object position | First quadrant | Third quadrant |
| Arrangement | Object between observer and plane | Plane between observer and object (planes assumed transparent) |
| Top view | Below the front view | Above the front view |
| Left side view (viewed from left) | Placed on the right of the front view | Placed on the left of the front view |
| Right side view | Placed on the left of the front view | Placed on the right of the front view |
| Usage | India (BIS SP 46), Europe | USA, Canada |
Projection symbols
The symbol is two views of a truncated cone (frustum): a front view (trapezium) and a side view (two concentric circles).
- The two symbols differ only in the relative position of the circles and the trapezium. Each is drawn according to its own rule for placing views: the side view goes on the opposite side (first angle) or the same side (third angle) as the end from which the cone is viewed.
- Learn the symbols from the standard figure in SP 46 rather than by memorising "left" or "right". The appearance depends on which end of the frustum is viewed.
The symbol must be shown in the title block.
Principal views
An object may be viewed from six directions: front, top, left, right, bottom and rear. Usually two or three views — front view, top view and one side view — are sufficient.
Choice of front view
- The view showing the most characteristic shape and maximum details of the object.
- The longest dimension placed horizontally; fewest hidden lines.
- Natural/functional position (e.g. a building's main elevation).
Relations between views
- Front view and top view share the same length (widths aligned vertically).
- Front view and side view share the same height (aligned horizontally).
- Top view and side view share the same depth (width from front to back) — transferred using a 45° mitre line or arcs.