Drawing Abilities Teacher
True Length
© J Lewis 2004
Right angle view
This shape is obviously a triangle with all the sides equal ( equilateral )
The height of the triangle is easy to see, about 28 units
© J Lewis 2004
Not at right angles
Here the triangle is propped up at an angle
Its height appears to be about 20 units, which is obviously not the correct value
The correct value can only be achieved by viewing the triangle at right angles
© J Lewis 2004
The Idea of True Length
The understanding of True Length is not achieved without some thought and
lots of practice
If a shape appears in any view, it cannot be guaranteed to show all the correct
dimensions…..
B
C
A
Elevation
We need to look at the Plan view and use some imagination and knowledge
If the lines are not at right angles to the direction of viewing then the dimensions
cannot be correct, the Elevation above may not be an equilateral triangle…
© J Lewis 2004
True Length of a Line
In the Elevation view, shown below, the line direction cannot be worked out
The Plan view is also needed to find the True Length
A
B
Plan
A
B
Elevation
© J Lewis 2004
True Length of a Line
The line AB we are trying to measure is not at right angles to the direction of viewing
So the line AB must be swung round about Point A in the Plan view until it is at right
angles to the direction of viewing
A
A
B
B
Plan
Plan
A
A
B
Elevation
B
B
Elevation
Direction of
viewing
© J Lewis 2004
True Length of a Line
The Point B must also be moved in the Elevation view
B
A
B
B
Plan
Plan
A
B
Elevation
B
A
Direction of
viewing
Direction of
viewing
A
B
Elevation
B
© J Lewis 2004
True Length of a Line
The True Length of AB can now be measured from the diagram as shown
B
A
B
Plan
Direction of
viewing
A
True length of AB
B
Elevation
B
© J Lewis 2004
True Length of a Line
True Length can now be measured for any line in any drawing by using this method
Go back to the triangle problem and consider the measurement of one edge BC
B
C
A
Elevation
Without the Plan view, it is impossible to go any further….
© J Lewis 2004
True Length of an Edge in a Pyramid
The Plan view shows that ABC is actually the side of an Egyptian pyramid
E
D
B
A
Plan
C
B
C
A
Elevation
© J Lewis 2004
True Length of an Edge in a Pyramid
Don’t panic – swing BC around B to be at right angles to the direction of viewing
D
E
C
B
A
Plan
C
Direction of
viewing
B
A
C
Elevation
© J Lewis 2004
True Length of an Edge in a Pyramid
The Point C must also be moved in the Elevation view
E
D
C
B
A
Plan
Direction of
viewing
C
B
A
C
Elevation
C
© J Lewis 2004
True Length of an Edge in a Pyramid
The True Length of BC can now be measured from the diagram as shown
E
D
C
B
A
Plan
Direction of
viewing
C
B
True Length of BC
A
C
C
Elevation
© J Lewis 2004
True Length of an Edge in a Pyramid
The true length of BC can be seen as the pyramid is rotated.
B
C
B
True
Length
C
© J Lewis 2004
True Length of other lines
Suppose we need to find the True Length of BF in the pyramid
Follow the rules that you’ve seen – you should find that the measured size of BC in
the Elevation view is the True Length of BF – think about it…..
E
D
B
F
Plan
A
C
B
A
F
Elevation
C
© J Lewis 2004
True Length of other lines
Suppose we need to find the True Length of BF in the pyramid
Rotate the pyramid so that BF is at right angles to the direction of viewing.
B
F
B
True
Length
F
© J Lewis 2004