Planet Earth: Maps, Longitude, Latitude, Equators Part 2

About the planet Earth, how cartography put a globe on a flat plane through the Mercator projections, types of maps.


Transferring of the curved lines of the meridians and parallels to flat paper so that straight lines can be used as compass directions is done by a system generally referred to as a Mercator projection--named after Gerardus Mercator, a mathematician, geographer, and engraver who died in 1594. In his system of projection, all parallels and meridians are at right angles to each other (not curved as they would be on a globe). The converging meridians of longitude are pulled apart until they are straight and parallel. Mercator compensated by also extending the distances between parallels at an increasing rate from the equator to the poles. Lands and seas were distorted by the Mercator method, particularly in the polar regions. In straightening the meridians, Mercator produced a grid of straight lines on a map, making it of great value for accurate compass directions. Professor Edward Wright of Cambridge University improved the Mercator projection system in 1590 and this adaptation was generally accepted around 1630 and continues to serve air and sea navigators and is said to be unequaled as a navigational aid. Other types of projection are now in use, but all have some degree of distortion.

Maps of the world reveal that less than 30% of the earth is land; the rest is water. Safe navigation on water today is dependent upon a hydrographic (or nautical) chart, which shows the depths of water, positions of submarine cables, coastal landmarks, anchorages or prohibited anchorages, locations of buoys, lights or other beacons, and other information. An oceanographic chart is used to provide information about the distribution of physical and chemical properties in the sea (temperature, salinity, etc.), geology, meteorology, and marine biology.

On land, topographic maps are used by all nations to reveal the elevation of various areas. The average elevation of the U.S., except for Alaska and Hawaii, is about 2,500' above sea level. A topographic map is a detailed record of survey of a land area, with geographic positions and elevations of natural and man-made features. By means of contour lines and other symbols, topographic maps show the shape of the land--the mountains, valleys, and plains--in measurable form. They show the network of streams and rivers and other water features in their true relationship to the land, and the principal works of man in their relative size and actual position. Surprisingly, only a small portion of the earth's surface has been mapped to the degree of accuracy required today for industrial, military, scientific, and recreational purposes. (Only about 40% of the land in the U.S. is covered by topographic maps.)

Geological survey maps, using a topographic map as a base, reveal what kinds of rock exist within the borders of a country. Geologists crisscross the land, examining rock formations on its surface and making studied projections of those underground.

Geological maps are vital in determining the location, depth, and dimensions of valuable bodies of rock such as building stones or ores.

Today there is a growing use of "photogeology"--aerial and orbital (space) photography to identify and map geologic formations, certain characteristics of water, and earth faults indicating where earthquakes may occur. Long-range sensing devices in aircraft or satellites orbiting the earth can detect temperature differences in various types of rock formations. Geologic mapping plays a vital role in determining where structures are placed--and in how they are designed, built, and maintained--by analyzing the rocks and soils that surround them.

Consequent earthquakes, floods, and other disasters can make a map obsolete. Nature can cause changes in a shoreline, requiring revisions to make currently used maps accurate.

Maps reveal much to a nation about the world around it--like the little known fact that the western coast of Alaska is less than 50 mi. from the Soviet Union.

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