Engineers and Scientists use a lot of measurements in their work. Prior to the actualization of any engineering project, it is necessary to make a sketch of an idea on paper and fine tune it to the best picture. To our knowledge, it was the sketch of a flying object made by Leonardo Da Vinci in the 15th century that Wilbur and Oliver Wright actualized in the 19th century to become aeroplanes. Before the construction of buildings, machines, equipment, cars, roads etc., the drawings are prepared for what is in mind. One can see at a glance that drawings bear no proportion to the actual objects. There must be a language that unites the object on paper to the actual object. This language is known as SCALE. Scale means the proportion or ratio between dimensions adopted for drawing on paper and the actual dimensions of the object.
Scales are commonly put in the form of representative fractions (R.F.). Preferred R.F. scales for drawings and maps in metric systems are:
| Maps | Representative Fraction |
| Detail plans | 1:1
1:10 1:20 1:33.33333 |
| Site Plans | 1:50
1:75 1:100 1:200 1:500 |
| Large-scale maps | 1:1000
1:1250 1:2000 1:2500 1:5000 1:10000 |
| Small-scale maps | 1:20000
1:25000 1:50000 1:100000 1:200000 1:500000 1:1000000 |
If a drawing is made in one of the scales above and it happens that the instrument to be used to put the actual dimensions on the site does not correspond to the scale used for the drawing, it is possible to convert the scale used for the drawings to the scale at hand and then to convert the given dimensions to the equivalent ratio.
Assuming a drawing was made with a 1:20 scale and we have a rule with 1:100 calibration on site, the ratio of scale 1: 20 to scale 1:100 is 100/20 = 5. If we have 5 cm in scale 1:20, to convert to scale 1:100, the equivalent value would be 5/5 = 1 cm. In that case, if we are using a rule in scale 1:100 for the measurement, measure out 1 cm instead. In a similar way, other scales can be converted and used.
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