Old land surveys are often hard to fit on a modern map due to a phenomenon known as magnetic declination--the difference between true north (the axis around which the earth rotates) which is used for maps, and magnetic north (the place the needle on a compass points) used by land surveyors. To put it another way, the complex shape of the Earth's magnetic field means that there are few places where a compass needle will point directly north. Yes, you can still use the compass to navigate to the North Magnetic Pole, but your journey will not follow the most direct path as seen on this magnetic declination map of North America for 2010.1.
Only along agonic, or zero declination, lines are true north and magnetic north the same. One zero declination line runs fairly directly through North and South America, generally through the Mississippi Valley states in the United States. There's also an agonic line looping through Scandinavia, Eastern Europe, Greece, India, China and Australia. As you move east of the declination line, the compass needle points west of true north, with a westerly (negative) declination. West of the line, the compass needle demonstrates an easterly (positive) declination. In Seattle, Washinton, the declination is currently around 20° East (+20°), while the Canadian Maritimes are roughly at 20° West (-20°). As you approach the poles, declination can be 90° or more, rendering the compass fairly useless as a navigational aid.
The angle between magnetic north and true north, or the direction to the north geographic pole, has been adjusted for by surveyors going back to at least the seventeenth century. Does this mean the plats from that point forward are truly accurate? I sure wouldn't bet on it. Magnetic declination wasn't always calculated correctly, as in the case of the 1742 survey of Massachusetts' northern boundary. Nineteenth-century surveyor, Samuel Williams, calculated this mistake meant the "loss of 59,873 acres to New Hampshire, and of 133,897 acres to Vermont," which would have meant those people now living in Williamstown and North Adams, Massachusetts, should actually be Vermonters.2. There were plenty of other reasons error could creep in as well. Rough terrain meant surveyors had few sight lines and plenty of hazards they had to climb over or work around (the wooded swamps described in many of my eastern N.C. ancestors' deeds come to mind). Poor equipment and the presence of iron ore, which interferes with the Earth's magnetic field, were blamed for a mistake in the border between Tennessee and Georgia which survives to this day.3.
While we may not be able to determine all of the reasons that our ancestors' land surveys may be inaccurate, we can take the time when platting to adjust for what is known as declination error. Important for genealogists, magnetic declination not only changes with geographic location, but also over time. Because of this, knowing what declination value the surveyor was likely using at the time the land was surveyed allows us to rotate our final plat by the appropriate number of degrees to place it accurately (at least somewhat) on a modern map. The National Geophysical Data Center offers a handy U.S. Historic Magnetic Declination calculator for just this purpose. For locations on or near the zero declination line (which, keep in mind, has changed over time), this isn't a huge issue. But it is about being as accurate as we possibly can in our representations of our family history.
1. National Oceanic and Atmospheric Association, Magnetic Declination Map of North America for the year 2010; digital image, National Geophysical Data Center (http://www.ngdc.noaa.gov/geomag/img/DeclinationMap_US.png : accessed 5 June 2012).
2. Samuel Williams, The Natural and Civil History of Vermont, Volume 1 (Burlington : Samuel Mills, 1809), 22; digital images, Internet Archive (http://www.archives.org : accessed 5 Jun 2012).
3. Bill Carey, "Sure enough, Georgia-Tennessee border in the wrong place," Tennessee Magazine, December 2010, 8-9; online reprint, TN History for Kids (http://www.tnhistoryforkids.org/tennessee_magazine/geography : accessed 5 June 2012).