Units of measurement

Checking the depth of the Toylander engine bonnet this morning,stated at 16 1/2" I had to get one of my Imperial tape measures out to see what it was in metric,that got me thinking on why we use the various systems we do?

My thanks to Wikipedia for the following,there is a lot more at http://en.wikipedia.org/wiki/History_of_measurement

Earliest known systems
The inhabitants of the Indus Valley Civilization (c. 3000–1500 BC, Mature period 2600–1900 BC) developed a sophisticated system of standardization, using weights and measures, evident by the excavations made at the Indus valley sites.[1] This technical standardization enabled gauging devices to be effectively used in angular measurement and measurement for construction.[1] Calibration was also found in measuring devices along with multiple subdivisions in case of some devices.[1]

The earliest known uniform systems of weights and measures seem all to have been created at some time in the 4th and 3rd millennia BC among the ancient peoples of Egypt, Mesopotamia and the Indus Valley, and perhaps also Elam (in Iran) as well. The most astounding of these ancient systems was perhaps that of the Indus Valley Civilization (ca. 2600 BC). The Indus Valley peoples achieved great accuracy in measuring length, mass, and time. Their measurements were extremely precise since their smallest division, which is marked on an ivory scale found in Lothal, was approximately 1.704 mm, the smallest division ever recorded on a scale of the Bronze Age. The decimal system was used. Harappan engineers followed the decimal division of measurement for all practical purposes, including the measurement of mass as revealed by their hexahedron weights. Weights were based on units of 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, 100, 200, and 500, with each unit weighing approximately 28 grams, similar to the English ounce or Roman uncia, and smaller objects were weighed in similar ratios with the units of 0.871.

Other systems were based on the use of parts of the body and the natural surroundings as measuring instruments. Early Babylonian and Egyptian records and the Bible indicate that length was first measured with the forearm, hand, or finger and that time was measured by the periods of the sun, moon, and other heavenly bodies. When it was necessary to compare the capacities of containers such as gourds or clay or metal vessels, they were filled with plant seeds which were then counted to measure the volumes. When means for weighing were invented, seeds and stones served as standards. For instance, the carat, still used as a unit for gems, was derived from the carob seed.