When the standard prime meridian (0° lon) wasn't a standard

Topics: General Topics
Coordinator
Jan 17, 2008 at 11:05 PM
I was looking for a list of common prime meridians, and came across this article in the New York Times, which illuminates a little of the process of choosing Greenwich to be the international standard prime meridian, which is where longitude measure begins.

A COMMON PRIME MERIDIAN.; THE INTERNATIONAL CONFERENCE NOT YET ABLE TO AGREE (may require registration)

In case you're new to GIS, the choice of which meridian to pick to start measuring degrees longitude from is arbitrary. This is unlike the equator, which isn't arbitrary, since the equator is defined by a plane which is orthogonal (perpendicular) to the axis of rotation of the earth (or any rotating spheroid). Before Greenwich was settled on as the international standard prime meridian, a cartographer would choose a meridian as the prime meridian, and base map measurements east and west relative to it. After a common one was chosen, this practice subsided, and is virtually unheard of today.
Developer
Jan 19, 2008 at 7:53 PM
For those of you who read "The Da Vinci Code" (haven't we all by now? :-), he often refers to the old prime meridian that went through Paris and straight through Le Louvre.

Proj.NET comes with a set of static methods for creating various prime meridians in CoordinateSystems.PrimeMeridian
Proj.NET (and thus SharpMap) actually supports changing the prime meridian as part of the reprojection.

Btw. if time isn't arbitrary (at least the entire world agrees on what time it is), I would argue that Greenwich isn't really arbitrary (since Greenwich is opposite of the DateLine)