Standardizarea acestor unităţi s-a produs târziu, dar toate se raportau la dimensiunile unor prăjini, lănci etc. care puteau fi manipulate uşor.

small distances were measured with solid rods, presumably of wood. The word pole is synonymous with rod, but what of the word ‘perch’? This is a Middle English word, derived from Latin pertica (a pole or measuring rod), and French perche. It is surmised that the length of 5.5 yards (16ft 6 inches) is as long as could be handled without excessive difficulty while remaining straight.
http://www.settledistrictu3a.org.uk/nchtjournal/Journals/200...

The rod is a unit of length equal to 5.5 yards, 5.0292 metres, 16.5 feet, or 1⁄320 of a statute mile. A rod is the same length as a perch and a pole.
http://en.wikipedia.org/wiki/Rod_(unit)


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Note added at 31 mins (2010-06-09 09:08:15 GMT)
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v. şi:
Rods, poles and perches are different names for the same unit. Medieval ploughing was done with oxen, up to 4 pairs at a time. The ploughman handled the plough. His boy controlled the oxen using a stick, which had to be long enough to reach all the oxen. This was the rod, pole or perch. It was an obvious implement to measure the fields, such as 4 poles to the chain.
http://gwydir.demon.co.uk/jo/units/length.htm