by Terri & Roger Coolong, PLS, RPF
Many times after a survey is completed, the client will look at a plat and say "Why is the magnetic bearing on this line N 42° W? My deed says that it is supposed to be N 39° W!" Although the use of true north would be a more accurate orientation for boundaries, magnetic north has been used in most of our local maps and deeds. Magnetic attraction varies over time. This is due to the fact that the magnetic poles of the earth are not located at the poles around which the earth spins, and the magnetic poles also move back and forth over a period of years. The variation is usually called the declination, defined as the amount and direction by which the magnetic needle is off the true meridian. Declination across the United States varies, but in Maine is approximately 17 to 22 degrees to the west of North.
Chances are, the description in your deed was written many years ago, when the magnetic declination was different than it is now. Charts are available from the United States Geological Survey which tell you approximately what the declination was in your geographic area during any given year over the last 200 years. By adding or subtracting the difference in declination between the year the property was last surveyed and now, you can find roughly what your compass would have read at that past time. Any map recently prepared by a surveyor should indicate the basis of bearing, for instance "magnetic north in August 1995", true north, or grid north. This will assist future surveyors in knowing what differences to expect when resurveying the property. The directions we seek may vary from compass to compass because of the amount of magnetism in the needle. Newer techniques in machining, combined with purer materials for the housings, have reduced many of the variations that were found in older compasses.
Many of the staff compasses used for surveying in the early years of statehood were only marked in full degrees. A surveyor may have only read the bearing to the nearest degree, or might possibly have "eyeballed" it to the nearest 1/4 degree. Older compasses did not have the dampening properties of a newer one, so it took much longer for the needle to "settle", or stop moving back and forth. If the surveyor was in a hurry (often the case when miles and miles of line were surveyed in a day as is indicated in field notes filed at the State Archives), he may not have waited for the needle to stop moving completely, but just guessed at where it would stop based on the reading that it was moving back and forth around. Keep in mind that should one run a compass line and have an error of 1°, in a mile distance the error is 92.1487 feet. At that rate, one would have a 1.74 foot error every 100 feet!
Generally, the greatest magnetic force at a point is created by the earth's magnetic field, but other natural and man-made elements can have local influences too. The presence of iron deposits, reinforcing bars in roads and buildings, wire fences, underground utility pipes and electric lines all modify how a compass will behave in any given place.
With the use today of high precision GPS, we can orient a parcel to true north or grid north. True north is absolute north while grid north is slightly east of north according to the State of Maine grid. The advantages of high precision GPS are that the orientation is not influenced by outside forces as with compass use and only one original monument is needed to orient a retracement survey.
So the next time you are trying to follow an ancient compass bearing, be it on an old survey plan, in a deed, or on the secret treasure map left to you by great-uncle Blackbeard, remember to convert the declination to the current year, watch for local compass influences, and in the case of the treasure map, leave no stone unturned!