Maps are 2D plans of geographical space. (On some, 3D information also appears, indicated by topographical contours or shading).
They are a highly conventional form of expression. Among the conventions:
| · | north is at the top of the map*
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| · | symbols denote type and size of features (for example, towns)
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| · | lines denote roads and political boundaries
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| · | grid lines run from left to right and from top to bottom
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| · | a scale denotes how many miles or kilometers an inch or centimeter represents
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For some, the reading of maps comes easily.
For others, the conventions, welter of information of various types, and abstract connection between representation of space on a flat surface and the realities of current whereabouts -- all these make mapreading extremely difficult.
For the first group, the toughest challenge is finding where on the map they are. Once that is determined, the way is made plain -- assuming (as you cannot do here in New England) that roads have signs on them.
For the second group, neither current location nor the way is available.
Where you are and where you've been
Together, your GPS unit and the GPSS software tell you where you are. A trail of tracking marks show where you have been most recently, and can help the second group begin to connect maps with reality.
Latitude and longitude
On maps, latitude runs horizontally and longitude runs vertically, creating a grid. You can plot your location on that grid - so many degrees North or South, and so many degrees East or West.
Latitude
Latitude, your relative distance north or south of the equator, came early, derived from a fairly complex mathematical interplay between the position of the north star at night and the position of the sun at noon on the same day.
Latitude, exactly similar to measurement on a two-dimensional circle, is traditionally expressed in degrees, minutes (1/60th of a degree each), and seconds (1/60th of a minute each).
What is it?
Imagine slicing the Earth's globe in half by cutting through the north pole through to the south pole. Turn the cut side toward you and you have a circle.
Then, you simply have place a (heat-resistant) protractor's origin at the center of the core of Earth and measure the angle of your north-south position relative to the 0 meridian, the equator.
Longitude
Longitude is a similar set of measurements of where you are, this time west or east of a prime meridian, except now the measurements are on the circle created by slicing the Earth through the equator.
If you combine latitude (represented by lines running up and down) with longitude (represented by lines running right to left), you end up with a grid. You are somewhere on the world's grid.
A bit of history of longitude
Reasonably accurate determination of longitude at sea has been with us only since the 18th Century. (In the 16th Century, longitude determination on land was pretty good, using telescopes and eclipses of Jupiter - don't ask me how - but there was no way to keep the instruments still on board a ship).
Round about 1710, the English Parliament set a prize for the accurate, repeatable determination of longitude, the winner to get some 10,000 or 20,000 pounds (sources vary). At that time, a poor man could live in London for a year on one pound. If you figure that subsistence in London today is around 20,000 pounds per annum, this is an equivalent of 350 to 700 million pounds today, more than an okay amount for a geek prize.
Thanks to developments in basic science around that time, the quest soon evolved into the demonstration of a clock accurate enough for longitudinal work during long sea voyages.
John Harrison, a carpenter turned clock maker, worked through 40 years and 5 versions before achieving an extremely high precision timekeeper that was accurate enough for longitude derivation in all climates and sea conditions.
He went broke and struggled almost continuously against opposition from the established scholars and scientists of his day, sort of a Galileo in a more modern age. Parliament waited ten years after the winning demonstration to cough up the prize money and did so only after King George III personally intervened. At least it's a better outcome than Galileo's, who died under house arrest.
Harrison was British, so the National Observatory at Greenwich, England, awarded itself the prime meridian. That's pronounced GREN-itch, by the way.
We are lucky. Had longitude been discovered more recently, say by General Electric, the General Electric Prime Meridian (registered trademark) would be through Schenectady, New York and we'd have to pay 25 cents each time we used it. Nobody but an American, and few of them, can pronounce Schenectady (skeh-NECK-ta-dee).
By Dave Gehman
© Copyright 2004, Robin Lovelock
Send changes, suggestions to Dave Gehman