Sunday, July 19, 2020

The Gunter Chain

above a Gunter Chain

In researching older property deeds you will encounter strange measurement terms such as Chain, Perch, Rod, pole and furlong.  These are based on a Gunter Chain which is a surveyor’s tool named after Edmund Gunter who in 1620 described it.  It is an actual chain that is made up of 100 links and the total length is 66 feet.  The links are 7.92 inches long.  From this we have a quarter chain equals 1 rod (16.5 ft) also 1 pole and 1 perch, 10 chains (660 ft) equals a furlong, 80 chains equal one mile (5280 ft)  10 square chains equal one acre.  It is interesting that road right-of-ways and railroad right-a-ways are sometimes 66 feet or one chain in width. 

In 1763-67 Charles Mason and Jeremiah Dixon surveyed and marked most of the boundaries between Maryland, Pennsylvania and the Three Lower Counties that became Delaware.  Mason had brought along state-of-the-art equipment for the survey. This included a “transit and equal altitude instrument,” a telescope with cross-hairs, mounted with precision adjustment screws, to sight exact horizontal points using a mounted spirit level, and also to determine true north by tracking stars to their maximum heights in the sky where they crossed the meridian.   Mason and Dixon also brought a Hadley quadrant, used to measure angular distances; high-quality survey telescopes; 66-foot long Gunter chains along with a precision brass measure to calibrate the chain lengths; and wood measuring rods or “levels” to measure level distances across sloping ground. A large wooden chest contained a collection of star almanacs, seven-figure logarithm tables, trigonometric tables and other reference materials; Mason was skilled at spherical trigonometry.  Mason had acquired a precision clock so that the local times of predicted astronomical events could be compared against published Greenwich times. Each one-minute local time difference implies a 15-second longitude difference.
They started out with a team of five people but by the time they got to the end of the survey they grown to 115 people.  A classic outcome of having a government contract.  Included in the team were axmen to clear a line of sight, pack mule drivers to get the trees out of the way as well as cows for milk, chain carriers, instrument bearers and tent bearers. It was like a small army moving through the woods.  They didn’t travel light.
Mason described their journey to Middle Point in their Journal: "1764 June 18 The waggons set out from Newcastle.
19 Joined the waggons and arrived at Dover at night.
20 At Esquire White's. 21 At Mr. Brown's.
22 At the River Nanticoke; pitched our Tents on its Banks.
23 Engaged axmen, etc. The whole company including Steward, Tent keepers, Cooks, Chain carriers, etc. amounting to 39. Two Waggons, Eight Horses, etc.
24 (Sunday) 25 Crossed the River Nanticoke in canoes and went to Middle Point, fixed up the Transit instrument and began to produce an arch of a Great Circle in the direction last run."
So in 1764 at the end of June Mason and Dixon were in the Delmar area.  They would return to the area in September to make adjustments to their line.

Measuring with the Gunter chain and levels they had a team of axmen clear a line of sight about nine yards wide the entire way.  The open view and clear skies were necessary for astronomical observations, a key component of the survey work.  The team would walk the boundaries and identify current property owners and significant landmarks—often trees or streams. With the chainmen, the surveyor would then identify the starting and ending points of a particular line. The two chain carriers would take hold of either end while the surveyor used a compass or theodolite to mark the distance along a particular line of latitude. As they moved along, the leader marked the ground with an “arrow,” or metal pin, and the follower walked in that direction. Upon reaching the leader, the follower picked up the “arrow” and replaced it with a wooden stave. Together, they would determine if their line was straight, and if not, with the help of the surveyor, they would adjust it before moving forward. When the leader was less than one chain’s length away from the final point, the chain carriers would stop and count the links between the last stave and the final station to determine the exact measurement.

The 1751 Transpeninsular line that runs from Fenwick Island to slightly west of Delmar required the same effort.  A farmer however that just want his land surveyed would have a
lot less workers but most of the same technique would be employed.

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