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    Re: Old stone walls record the changing location of magnetic north
    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.
    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.



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