Orthographic Projection of Points, Lines and Planes
What the exam tests
This domain tests first-angle projection on HP and VP, quadrant positions, projections of points, true lengths and inclinations of lines, traces, and the traces of planes. You must be able to place front and top views correctly relative to XY.
Core rules
- In first-angle projection the object lies between the observer and the plane: the top view is below XY and the front view is above XY.
- A point's front view shows its height above HP; its top view shows its distance in front of VP. Distances are measured perpendicular to XY.
- For a line, the top view gives the plan length and the front view gives the elevation length. True length appears only when the line is parallel to a plane.
- Use the rotation or auxiliary-plane method to find true length and inclinations; locate traces where the line meets HP or VP.
- A plane is represented by its traces and by the projections of its edges; find true shape by an auxiliary plane parallel to it.
Worked example
Point A is 30 mm above HP and 40 mm in front of VP. Draw XY. Project A's front view a′ 30 mm above XY, because height above HP is shown in the front view. Project A's top view a 40 mm below XY, because distance in front of VP is shown in the top view in first angle. The projector a′a is perpendicular to XY, and the two views lie on opposite sides of XY.
Traps
The most common error is placing the top view above XY, which is third-angle practice. Do not measure both distances from the same side of XY. For lines, apparent lengths are not true lengths; do not label an inclined view as true length unless it is parallel to the reference plane.