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Tuesday Term of the Week | Grid

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A grid is an essential element of measurement used in mapmaking to locate objects. It defines a two-dimensional, or “X-Y” space and allows positions to be precisely defined within that space. Think of placing an imaginary piece of graph paper over an area of land on the earth. If you align the horizontal lines so they are pointing east-west, and the vertical lines north-south, you can specify any single location as an (X,Y) coordinate on your imaginary page.  A “grid” is made from of all of the imaginary lines on your virtual page, with the distance between the lines set to any measurement of your choosing.  Typically, when locating a point anywhere on earth, the “X” coordinate used is the Longitude and the “Y” coordinate the Latitude.

For a grid to be meaningful, there must be an origin point – in other words, a starting location where the coordinate is (0,0).  For a standard map of the earth, the ‘X’ origin is at the longitude of zero, also known as the Prime Meridian. This is a line which passes through Greenwich, England, stretching half-way around the earth from the North Pole to the South Pole. The ‘Y’ origin is the Equator, which encircles the earth mid-way between the two poles. That places “origin” coordinate of (0,0) off the coast of Africa in the Atlantic Ocean, just under 400 miles south of the nation of Ghana and nearly 700 miles west of Gabon.  With the origin established and measuring in degrees, one can now locate New York City for example.  It is found at approximately 74 degrees west and 40.75 degrees north of the origin point. Specified as an (X,Y) coordinate, New York City is therefore located in the area around a point at (-74.00,40.75).