NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Old stone walls record the changing location of magnetic north
From: Bruce J. Pennino
Date: 2019 Mar 12, 19:36 -0400
From: Bruce J. Pennino
Date: 2019 Mar 12, 19:36 -0400
Hello All:
I’m not a PLS ( Professional Land surveyor) but I’ve known many and done a
lot of work with them. I once asked a PLS about the bearings, magnetic or true,
of property lines here in Massachusetts. Like many things in life., he
said......”It depends....” Usually because of convenience and
custom, the original bearings of boundary lines were e just brought forward in
time when the property was transferred. Sometimes a note is given on the
drawings. It is more work for surveyors to determine true bearings,
especially in the 1800s through and up to recent years. So, mag bearings were
used. Attachments, called astro or solar attachments, were sold for
transits. See W. E. Gurley solar attachment. These allowed the
determination of a true longitude( true north” and then turning an angle to the
boundary line.
The American transit has a very good compass on it and maybe it was good to
+/- 1 degree when there were no local anomalies. Surveyors were
trained to be aware of closed traverses “ Not closing”
geometrically. The sum of the angles of a closed figure is (n-2)180.
Then angles were balanced using mathematical methods . Usually for small
surveys, boundary lines are “obvious”. Old property lines can still go to ......
“ the ash tree 22 ft from Brown’s NE foundation corner. Then you
walk 300 ft into the woods to a “corner” and find a circle of four pins
1-2 ft apart. Surveyors call this a “Pin cushion corner”. The more rural the
area the older the survey. The drawings were dated so corrections to establish
true could be estimated.
A standard surveyor’s story, “ Out west during the indian wars , a lot of
western railroads and land divisions were surveyed from offices in St. Louis,
Missouri.”
Many surveyors were “trained in the field”. George Washington was a
surveyor. Now, States have established very specific standards for all survey/
property lines. I forget, maybe distance standards in cities are accurate to 1:
25000 or maybe 50,000. I don’t know what accuracy is required on
angles.
Old stone walls should be left alone unless there is no other option.
Regards,
Bruce
From: Brad Morris
Sent: Tuesday, March 12, 2019 4:46 PM
Subject: [NavList] Re: Old stone walls record the changing location
of magnetic north
I found the article suitably lightweight, but the paper much more
specific. From it we find:
Seventeenth‐
to nineteenth‐century land surveys of large tracts of land (from 100 to 100,000
acres) were used that showed (i) simple boundary geometries (e.g.,
quadrilateral), (ii) the date of the survey, and (iii) magnetic bearings of
the property boundaries at the time of the land survey. LiDAR images of the
area were processed to search for stone walls along the surveyed boundaries.
For
those who wish to ignore the scribblings of a "science journalist",
and wish to pursue the details, may I suggest the actual
paper.
Jackson: Thanks for bringing this to
our attention. Walls which were built along boundries, which had their
magnetic bearing recorded, do indeed show correlation with the magnetic
drift.
Brad
