Wednesday, April 15, 2009

The New Prime Meridian and the Old British Empire

Here my students are rightly celebrating the discovery of the "new" Prime Meridian, some 300 feet from the one at the Royal Observatory at Greenwich. But you ask, where are the markers? The signage? The hoopla that should go with the establishment of a new foundational marker?

Well, of course, there isn't any! And therein lies a tale. In the 19th century, Britain was the leader among trading nations and responsive to the infrastructure needs necessary to support that trading, including navigation. Remember the story of the navigational chronometer which made the calculation of longitude possible? It was invented by a Brit whose tale was told in the bestseller, Longitude. If you can measure the sun's declination at noon Greenwich time and know what the time difference is between where you are and Greenwich, then you can calculate your longitude. Latitude was always easier to calculate, so with the invention of the nautical chronometer, navigation become much, much more accurate.

Thus the Prime Meridian, "0" longitude became important. It was the basis for most map making from the 1850's on except for the French (not unexpectedl). Visitors come in large numbers to stand astride this meridian, one foot in the eastern, the other in the western hemisphere.

But if on your pilgrimage to the PM you bring your GPS unit and hold it above this "0" meridian, you will discover, as I did a few years ago, that your GPS unit will not show "0"! This is indeed puzzling and returned a variety of answers when I raised the question why this was so on a list of geocachers. Looking back, some of the responses were quick and careless talking about an "acceptable margin of error." But the error necessary to produce the readings I got would have to be over 300 feet, not the 30 to 40 feet error that is part of all civilian system readings (as you own GPS will typically inform you).

Interestingly, the longitude shown on your GPS continues to approach "0" as you walk east of the Prime Meridian. When I first noticed this, I followed the "drop" until it reached "000 00.0000 on my GPS. I looked over my shoulder. Sure enough, there all of the visitors still were huddled over the "Prime Meridian" about 300+ feet west of me. What was happening?

Well in fact it is a well documented and predictable event. It has to do with the way in which the surface of the globe is mapped to provide the world-wide accuracy of "WGS84" the basis for our GPS units. It it a compromise which enables greater accuracy over-all than if the British system of mapping the earth's surface were used.

The result is that the WGS84 "Prime Meridian" is 300+ feet to the east where my class found it. They are making their own visual graphic of where the line would be, because it is not marked. The Observatory is loathe to grant that there may be other standards besides the famous one which attracts the visitors.

So which standard is the "real" one? (One of my students asked this with some indignation, as though there had perhaps been a lie or a cover-up!) In fact, they are co-existing different standards, caluculated in different ways. One, however, is superseding the other. WGS84 is used by airlines, ships, not to speak of cars with their increasing use of navigational aids. It is now the defacto standard of the world. It is the "new" Prime Meridian. But my students had to create their own signage at Greenwich, since Greenwich is reluctant to move with the times.

For an excellent article on this that provides more than I do, incudling a description of a precession of "new" prime meridia leading up the famous one of 1851 see Tom Standage's 1998 piece called "The Real Millennial Bug."

Here's my GPS on the "Old" Prime Meridian, by the way. It's a bit hard to read but shows something like 0.00151 of decimal degrees longitude rather than "0.00000" decimal degrees.

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