Food for cogitation. My apologies.
1973 Manual, 1-18:
Sixth. All lines shall be plainly marked upon trees, and measured with chains, containing two perches of sixteen and one-half feet each, subdivided into twenty-five equal links; and the chain shall be adjusted to a standard to be kept for that purpose.
The same language appears in 1-26 of the 2009 Manual.
So these are 50-link, 33-foot chains. However,
Section 2-42, paragraph 2, p. 37:
The field notes of some early rectangular surveys in the southern States show the distance in “perches,” equivalent to poles. The term now commonly used for the same distance is the rod. There are some places where distances were recorded in 2 pole (perch) chains, where a full mile contains 160 perches.
If the “perch” is considered a unit of area, 16.5’ by 16.5’, there are 160 of those in a square mile. But is it not the 66-foot chain that, squared and multiplied by 10, makes the square mile? In the night as I wrestled mentally with these matters, a voice came to me, in tones reminiscent of the late Governor Jimmie Davis, thus:
It's a long lonesome line I'm traversing
And I'll measure each chain of the way
But the length of the chain is a mystery
That brings me up short every day.
Now I long for the strength
To discover the length
Of the line I survey with such strain
For pole, perch and rod
Stretch the road that I plod
Till I'm tangled in perches and chains.
It's one-sixty a mile for the perches.
I said so myself, so it's true.
But a colleague astride a lost corner
Said, "My friend, something's gone wrong with you."
He went on with a smile
"At three-twenty a mile
You have perches or poles, but your brains
Have doubled or halved
Whatever you have
And you're tangled in perches and chains."
I’d be better off tending to the tree lights. Happiness to all.
Henry
I've been skimming some stuff about the metrology of Stonehenge and the pyramids and such. A lot of fluff about megalithic yards, gravity cubits, etc. The meatier stuff was about how you could use a sundial and a pendulum (plumb bob) to calibrate a length of cord: You count the # of swings in an amount of time measured by the sundial, and shorten or lengthen until it is the right whole number of swings in a given time. If you scale the sundial/pendulum length smaller, accuracy suffers, and if you make it larger, the Earth's rotation starts to mess it up. The 16.5' length is about where accuracy and practicality converge. Right about now you can start pondering how many sessions it would take to get that all calibrated. Pretty easy with proportioning and a decimal system, but as far as we know these guys were just watching for the right number of swings and a whole number coincidence of those swings with time, adjust the cord and start over. Proprietary methods & job security!
From there you can use your calibrated cord to mark sections of a larger cord, or transfer the unit to another material, like wood (hence our "rod"). I reckon the weight of the string/cord/whatever you suspend the plumb bob with figures into the center of gravity, so you start with something light and transfer the unit to something more durable.
Another author talks about how Stonehenge and many other Celtic megaliths are laid out in whole numbers of rods, within 1-2% tolerance, and likely used 6-rod and 12-rod cords to do so, dig that megalithic double-chaining!
Finally we know why that plumb bob is so important:
No more excuses from your crew about "dead batteries" or "the pit bull ate our steel tape."
"Y'all better derive yourselves a standard unit of measure and get back here with some data!"