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Rotations: seeing the plane in true size

Rotating a plane flat means turning it around one of its traces until it lies on a projection plane. Once rotated, everything it contains appears in true size: distances, angles and flat shapes stop being foreshortened. The trace you turn around is called the hinge.

What you need to know

• Choose the hinge: the horizontal trace to rotate onto the horizontal plane, the vertical one for the vertical plane.
• Rotate a point: perpendicular to the hinge and carry the height (or distance) from it.
• Rotate a line from its traces.
• Use the rotation to measure a segment, an angle or draw a shape contained in the plane.

How it is solved

1. Choose the hinge and mark the trace of the plane you will turn around.
2. Draw the perpendicular to the hinge through the projection of the point.
3. On that perpendicular carry the height (if rotating onto the horizontal plane) or the distance (onto the vertical plane).
4. Join the rotated points to get the shape in true size.

How it appears in the exam

It is the tool for the “true size of the section” and for drawing polygons contained in a plane. It is also used to measure angles. The usual mistake is carrying the wrong distance (height instead of distance) or not drawing the perpendicular to the hinge.

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