A reader's inquiry regarding impact of continental drift on present longitude.

A Reader's Inquiry

Regarding impact of continental drift on present longitude

I am an amateur astronomer. [Because] the stars are moving slowly, the charts of the sky are redrawn each 50 years (called an Epoch). The maps do not really look different, but the associated catalog that precisely lists the position of each star does show the changes.

I have heard about continental drift [and] recently learned that the rate is about 3 inches per year. And, England is moving east while North America is moving west. So, the separation rate is about 6 inches per year.

Here are my two questions:

1. As the land mass of Great Britain drifts, does the zero meridian of longitude drift with it? In other words, is the zero meridian defined by some permanent marker on the ground? (If zero meridian does not drift with the land mass, then is it defined by what line the sun is highest above on a certain date and time, kept correct by leap year and leap seconds?)

2. For people in North America, boundaries defined by longitude, such as the shared vertical borders of states or counties, would be drifting about 6 inches per year west from the zero meridian. It would not look different on most maps, but it could be important to a property owner. For boundaries defined around 1850, that would be 75 feet (or half of that if the zero meridian does not move with the land mass). If a deed is defined by reference to a permanent marker installed in 1850, then there is no confusion. But if the property line is defined as a particular longitude, how do map makers and surveyors compensate for continental drift?

A current "catalog" of boundaries defined by longitude could be published as Epoch 2000 or Epoch 2050. For the years in between, users would need to interpolate. Boundary lines could be defined based on longitude for a certain Epoch. Then drift before or after that would be assumed to move at the established rate. I have addressed changes in longitude because that is the larger of the changes in North America. There is probably a smaller latitude component of drift. (For other continents, both components may need equal consideration.)

Please provide whatever insight you can,
Lee McBride
Columbus, Ohio

Dave Doyle, chief geodetic surveyor for the National Geodetic Survey, responds:
You are certainly aware of the problems associated with coordinates due to continental drift. The motion of most of the United States is well estimated at about the .5 to .75 inch per year level (1-2 cm) relative to a global coordinate system. This system, called the International Terrestrial Reference Frame (ITRF), is a product of the International Earth Rotation Service (see www.iers.org/). In a classical sense, the Greenwich Meridian is still the convention for longitude, however, instead of a specific mark in the ground, the International Reference Meridian (IRM) is deduced from a global set of observations that involve various types of technologies, including (among others) Very Long Base Line Interferometry, Satellite Laser Ranger and GPS. With the significant changes in positioning technology available from GPS, the art and science of determining position astronomically is rapidly fading. It has not been used as a geodetic technique by the National Geodetic Survey (NGS) in more than 20 years.

As coordinates relate to boundaries"¦ this is always an interesting topic. The bottom line is that in the United States regardless of the coordinate definition, the physical monument controls. When you consider some of the early state boundary surveys, consider the equipment and infrastructure limitation of the surveyor. There were very few high quality geodetic surveying instructions in the country at that time. In addition, the surveyors were severely limited in performing astronomic observations by a lack of quality time standards, and the fact that a well-defined network of geodetic survey control networks was only just beginning.

Part of the responsibility of NGS is to determine the relationship between the national coordinate system, called the North American Datum of 1983, which is defined as fixed and stable on the North American plate and the ITRF, and to maintain and provide high-accuracy transformations between these systems. The obvious exceptions of California, western Oregon, Washington and southern Alaska notwithstanding, very few land surveyors ever worry about the change in coordinates due to continental drift. NGS maintains this relationship with a network of more than 400 continuously operating GPS reference stations (CORS) (see www.ngs.noaa.gov/CORS/), a subset of which is provided annually to IERS for the global solution.

There are numerous books and periodicals that will help educate you: With Compass and Chain by Silvio Bedini; Fundamentals of Boundary Surveying by Paul Gay; and The Measure of All Things by Ken Alder.

You can also look to the U.S. Geological Survey Professional Paper 909 "Boundaries of the United States and the Several States" found at www.usgs.gov; and "Geodesy for the Layman" and "Basic Geodesy" both found at www.ngs.noaa.gov.