Technical and engineering drawings consist of drawn objects or items that will be eventually produced, manufactured, or constructed in real life; the sizes and dimensions of objects are always expressed by using the units of a particular measurement system.
The two most widely used measurement systems are the “Metric System” (also known as the “International System of Units”), and the “United State Customary Units”; both measurement systems consist of a number of units.
Among the two measurement systems, the metric system is the standard that is mostly used around the world, especially for expressing the sizes and dimensions of the lengths, heights, and widths of objects on technical and engineering drawings.
Various professions use measurement systems in technical and engineering drawings to communicate and document their designs; some examples of professions include civil engineering, environmental engineering, mechanical engineering, architecture, landscape design, industrial design, and manufacturing.
1. The Metric System (International System of Units, or SI Units)
The present-day metric system is the “International System of Units” which is commonly referred to as “SI Units”—an acronym from the French phrase “le Système International d’Unités”.
The International System of Units is a measurement system that was established in 1960 after an international agreement was reached; it is presently the international standard used in expressing the sizes and dimensions of objects.
Although some countries still use U.S. Customary Units to a lesser or greater degree, all countries in the world have adopted the International System of Units.
The most widely used units of the Metric System (International System of Units) are the kilometer (mm), the meter (m), and the millimeter (mm). The centimeter (cm) and the decimeter (dm) are also among the units in the Metric System, but they are rarely used on technical and engineering drawings.
It’s quite common to see some industries using a dual dimensioning system to express the units of the sizes of the dimensions of objects on each of their drawings.
For example, they could use “millimeter” and “inch” together on one drawing, even though millimeter is a unit that belongs to the Metric System (International System of Units) and inch belongs to the U.S. Customary Units.
It has to be noted that using a dual dimensioning system to express the units of the sizes of the dimensions of objects can cause a bit of confusion because the sizes derived by using two different systems may contain rounding errors whenever one unit is converted to another.
Most creators of technical and engineering drawings use Metric System units on dimensions in order to maintain consistency between different units that belong to the same measurement system. In standard practice, the following Metric System units and relationships are often used:
1 kilometer (km) = 1000 meters = 10,000 decimeters = 100,000 centimeters = 1,000,000 millimeters
1 meter (m) = 10 decimeters = 100 centimeters = 1000 millimeters
1 decimeter (dm) = 10 centimeters = 100 millimeters
1 centimeter (cm) = 10 millimeters = 0.1 decimeter
1 millimeter (mm) = 0.1 centimeter = 0.01 decimeter.
2. The United States Customary Units
The United States Customary Units is a measurement system that was formalized in 1832 and has been commonly used in the United States since then.
The United States Customary System (USCS or USC) was derived from the English units that were being used in the British Empire before the United States became an independent nation.
The most widely used units in the United States Customary Units are the mile (mi.), the foot (ft.), the inch (in.), and the yard (yd.). The pica (P.) and the point (p.) are also among the units in the United States Customary Units, but they are rarely used.
Although technical and engineering drawings may use either measurement system (Metric System, or the United States Customary Units), they adhere to popularly accepted drawing standards.
The dimensions given in the United States Customary Units can be easily converted to Metric System units in decimal or fractional form. In standard practice, the following units and relationships are often used:
1 mile (mi.) = 1760 yards = 5280 feet = 1.609 kilometers
1 yard (yd.) = 3 feet = 0.9144 meters = 914.4 millimeters
1 foot (ft. or ′) = 12 inches = 0.3048 meters = 304.8 millimeters
1 inch (in. or ″) = 6 pica = 25.4 millimeters = 2.54 centimeters
1 pica (P.) = 12 points = 4.233 millimeters
1 point (p.) = 0.3538 millimeters.