GEOGRAPHIC COORDINATES

You must use the concepts of direction and distance to locate points. Primitive man probably did this in relative terms, using aids such as the directions of the rising and setting sun, forward and backward, and left and right. He probably expressed distance in terms of travel time in relation to his own location. A universal system, however, must have some unique reference or starting point. If we designate such a point, then we can state the location of every other point in terms of direction and distance from it. The most widely accepted system of locating a point on the Earth's surface uses lines of latitude and longitude known as geographic coordinates. Coordinates allow us to provide an answer to the question "Where is it?"

LATITUDE

When you draw a grid on a globe, you must have a starting point. Unlike drawing a grid on a piece of paper where you can start in a corner or at the center, drawing a grid on a globe requires that you have a starting point that everybody accepts. The point of origin for latitude is the Equator. The Equator is an

Figure 3-3.-Degrees, minutes, and seconds.

imaginary line, with a numerical value of 0, running east and west around the center of the Earth.

Latitude locates a place relative to the Equator. Because the numbering of lines of latitude begins with 0 at the Equator and increases towards the poles, we must show whether the latitude of a place is north or south of the Equator.

The value of a line of latitude is determined by the angle formed by drawing a line from the Equator to the center of the Earth, and then back out to the surface of the Earth. See figure 3-2. Since the value of any angle would be constant all the way around the Earth, a line drawn on the Earth's surface connecting all the points that are formed by the angle would be parallel to the Equator. For this reason, latitude is commonly referred to as a parallel of latitude. Since 90 is straight up or down in relation to the Equator, the North and South poles are 90. Therefore, you have latitude running from 0 to 89 north or south of the Equator.

Each degree is subdivided into minutes('). For instance, between 48 and 49 north latitude, there are 60 minutes. If you were locating a point that was halfway between 48 and 49 north latitude, it would be at 48 degrees, 30 minutes north (4830'N). See figure 3-3. Each minute is subdivided into seconds ("). For instance, between 30' and 31') there are 60 seconds. So if you were locating a point that was one-quarter of the way between 30' and 31', it would be at 48 degrees, 30 minutes, 15 seconds north (4830'15'N). Again, see figure 3-3.

The military writes coordinates using a system called military notation without the symbols , ', or ". This system uses 6 numbers plus a letter to indicate north or south. The coordinate 4830'15'N would be written 483015N. When a position has less than 10 of latitude in its coordinate designation, a zero is added to the left of the degree number. In other words, latitude will have two digits. Seven degrees of latitude appears as 07 in the designation. Likewise, two digits designate minutes and seconds: for example, 030704N or 801708S.

NOTE: In geographic coordinates, always write the latitude first.

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