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French charts start longitude where?

Jan 1, 2003

I enjoyed very much the piece in Chartroom Chatter on an odd 400° compass ("Revolutionary compass found," Jan/Feb 2000, Issue No. 103). Sounds like Willem Bruyns of Amsterdam has deciphered the mystery of the French 400° compass! In reading about the compass, I was put in mind of a personal brush with this 400° phenomenon almost a quarter century ago.

It was the summer of 1976, and I had just arrived at my new State Department post in Morocco. Coming from many years in the Far East, I found Morocco new and mysterious. As a sometime celestial navigation instructor who had been landlocked too long, one of the joys I hoped to experience there was the opportunity to take my sextant to the nearby beach and shoot the setting sun on a real horizon, and polish up some sight-reduction programs I'd written for my HP-65 programmable calculator.

On my first day in the office I noticed a lovely old French military chart of Morocco on my wall, published circa 1935. Inspecting it more closely, I noticed that the lines of longitude and latitude were off: way off from where Morocco should be. Knowing the French to be excellent cartographers, I knew this had to be some kind of a mistake.

Finding no explanation, I called in Danielle, my very French secretary, and shared my dismay with her. She looked at the map summarily, pointed a long finger at one of the lat/long notations and said, "Monsieur Trayfors, you see these little marks that look like degrees?" After acknowledging that I indeed saw them she added, rather disdainfully, "Well, these are not degree marks. They are grads." With that she marched out of my office.

Well, hell, I didn't know what grads were, but I remembered that there was a grad button on my HP-65 calculator. After a bit of experimenting, I determined that a grad equals 1/400th of a circle, while a degree equals 1/360th of a circle. Okay, I thought, no problem. So I converted the lat/long on the chart to degrees. Surprise! The latitude came out precisely correct, but the longitude markings were still way off. How could this be? I took a coffee break to try to figure out why.

Again tackling the problem, I examined every part of that old French chart very carefully. And I found it! Nestled in the corner of the chart and almost unreadable was the notation, "Longitude ouest de Paris!"

At that point I lost it, and went into uncontrollable laughter. Longitude west of Paris, it said! Where in Paris? The Eiffel Tower? The cartographer's alma mater? I never got to the bottom of this riddle. Maybe one of your readers can solve it.