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….. 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 © 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 © J Lewis 2004

True Length of a Line The Point B must also be moved in the Elevation view © J Lewis 2004

True Length of a Line The True Length of AB can now be measured from the diagram as shown © 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 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 © 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 © J Lewis 2004

True Length of an Edge in a Pyramid The Point C must also be moved in the Elevation view © 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 © 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….. © 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