Orthographic projection: points, lines and planes
Orthographic projection: points, lines and planes
This domain tests first-angle projection: placing front views, top views and side views correctly, and projecting points, lines and planes from their distances to the principal planes.
Core rules
- In first-angle projection the object lies between the observer and the plane. The front view (elevation) is above the XY line and the top view (plan) is below it. The view from the left is placed on the right of the front view.
- Projectors are perpendicular to XY. Measure heights above HP for elevations and distances in front of VP for plans. A point above HP has its elevation above XY; a point in front of VP has its plan below XY.
- A line parallel to a plane projects true length on that plane. An inclined line is foreshortened in the view where it is not parallel. A line perpendicular to a plane projects as a point on that plane.
- A plane parallel to VP shows its true shape in the front view and an edge in the top view. A plane perpendicular to VP but inclined to HP shows an edge in the front view and a foreshortened shape in the top view.
- Use construction lines lightly, and label points with lower-case letters for plans and primed letters for elevations (a and a′).
Worked example
Point A is 30 mm above HP and 20 mm in front of VP. Draw XY. Mark a′ 30 mm above XY for the elevation. Mark a 20 mm below XY for the plan. Draw the projector through both points perpendicular to XY. Reason: the height controls the elevation and the distance in front controls the plan; the projector keeps the two views aligned.
Traps
Mixing second-angle and first-angle positions is the main error. Also check whether a distance is above HP or in front of VP before measuring, and never draw projectors at an angle. For an inclined line, do not label the foreshortened view as true length.