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    French measure. was:Re: why is it that
    From: George Huxtable
    Date: 2010 Jan 13, 19:21 -0000

    John Huth wrote- "The French had many interesting issues about units of
    measure. "
    With a touch of nostalgia, I can recall a posting of mine, back in 2001 in
    the days of Nav-L, on this topic, which went as follows-
    ... on one occasion about 15 years ago we were making our way South
    towards Belle-Isle, South Brittany (Western France).
    We had good detailed admiralty chart coverage, but because of the way the
    coast trended, and the way the corners of the charts matched up, we were,
    ever so slightly, crossing a corner which our charts didn't cover.
    Although I knew we were in deep water without hazards, I always feel a bit
    uneasy in that situation. No doubt it's the same feeling that Magellan used
    to get...
    My wife, with sharper eyes than mine, had spotted the lighthouse at the
    North tip of Belle-Isle, which was on one of our charts. For some time
    there had been another offshore lighthouse in sight to our East. This we
    knew marked the Plateau des Birvideaux, a shoal patch we were well clear
    of, but it, too, was off our admiralty charts. However, it was shown on our
    1:200,000 one-sheet Michelin Road map of Brittany, which we always carry
    for inland expeditions, and which, to its credit, shows many other features
    as well as roads. We had the 1983 edition.
    Now there was a chance to establish our precise position by compass
    bearings on the two lighthouses. But how do you plot them, to get an
    intersection, when they aren't on a common chart? Well, simple really. Tape
    an extra sheet of blank paper to the appropriate edge of the chart, and
    plot in the missing lighthouse by extending the lat and long scales
    somewhat. The lat and long values would come from the Michelin road map,
    which is ruled with lat and long, though not on a Mercator projection.
    That was the idea, but it didn't work. The values we read off the lat and
    long scales of the road map just didn't fit at all. They were quite crazy.
    At this point my wife remembered she had been taught about grads at school,
    and deduced that this was the meaning of the "gr" markings on the scales of
    the Michelin road map. There are 100 grads in a right-angle, in place of 90
    degrees: a flawed attempt to decimalise angle measurements, and to match
    them to the kilometer. So all we had to do was divide the values in grads
    by 100, multiply by 90, and plot them as degrees on our extended chart. So
    we thought.
    Well, the latitude seemed be sensible, but still the longitude was way,
    way, out. At this point my wife made the crazy suggestion- "They French
    couldn't possibly still be referring their longitudes to Paris, rather than
    Greenwich, could they?" And so it turned out to be. French nautical charts
    had indeed reluctantly adopted Greenwich in the previous century, but their
    terrestrial mapmakers had made no such concession!
    At this point we abandoned the attempt to plot the missing lighthouse from
    its coordinates, and instead transferred it by estimating bearings from
    landmarks that were shown on the two charts. At last we were able to plot
    those compass bearings we had measured some time before, and establish just
    where we had been.
    By this time, however, it was all rather irrelevant. The process of sorting
    out the map scales had taken so long that by now we were well into the
    passage between Belle-Isle and Quiberon peninsula.
    I am pleased to report that by the time of the 1990 edition of that road
    map, Michelin had decided to enter at least the 19th century, by adopting
    degrees rather than grads and referring them to Greenwich rather than
    The moral of this story is to carry charts for wherever you plan to go. It
    also points out the value of carrying an intelligent mate.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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