Locational References

A central and important use for maps is to identify location, to tell you where you are. Only after you know where you are can you figure out how to get somewhere else. People have devised many ways to describe location on the earth. One of the oldest and most important is the system of latitude and longitude. Together, latitude and longitude create an "earth grid" that allows us to precisely locate the position of any point on the earth's surface.

Points of reference

Every system of location must be based on points of reference that everyone knows. For example, say that you are going out to lunch with someone in this lab, and you are trying to describe how to get to your apartment. What if you said "I live two blocks east of a pine tree?" Would they be able to find you? Probably not, since there are so many pine trees around town. Instead, you need a reference point that everybody knows. For example, what if you said to a longtime resident of your local area, "I live two blocks east of the post office". Because he would know where the post office is located, it would be relatively easy for him to find you standing in front of your apartment.

Similarly, when we describe locations on the earth we also must use well-known, universally agreed upon, points of reference. In the latitude / longitude system we use four such references: 1.) the center of the earth, 2). the north and south poles, 3.) the equator and 4.) the Prime Meridian.

Latitude and longitude both measure from the center of the earth. The north and south poles mark where the earth rotates (turns) on its axis, the motion that makes a day. The equator is a line on the earth's surface, forming a circle, that marks the half-way point between the poles. A meridian is a line running on the earth's surface from pole to pole. There are many different meridians, the one that was chosen to be used as a world-wide reference point is called the Prime Meridian and it passes through southeastern England, near London. These four references points, or lines, are agreed upon by everyone on Earth and are used in the latitude / longitude system to describe the location of points on the earth's surface.

Latitude and longitude are both measures of the angle formed within the earth rather than a distance measurement on the outside of the earth. This angle is formed by the lines between the base reference, the center of the earth, and the specific location. Because these are angle measurements, the base unit is degrees. The earth, being roughly spherical, is divided into 360 degrees. Degrees are subdivided into minutes and seconds. One degree equals 60 minutes (1° = 60'), and one minute equals 60 seconds (1' = 60"). Think of it as being like time. Sixty seconds equals one minute time, and sixty minutes equals one hour, except that in arc it is called a degree. So, thinking of your watch, 15 seconds is 1/4 of a minute, 45 seconds is 3/4 of a minute, and 30 minutes is half of a degree (hour). The standard, traditional way of writing latitude and longitude is with degrees, minutes, and seconds, or what is called DMS. With the advent of computers, an alternate form has become more prevalent. The degrees, minutes, and seconds can be converted from this system where a whole is 60 units to the base ten format. In this format, all coordinates are in degrees, with the minutes and seconds being decimal equivalents. This form is called decimal degrees or DD. For example, 15° 45'N, 108° 30'E would be 15.75°N, 108.5°E. Advanced function hand calculators often perform this conversion, and websites are available that allow you to enter the values and will return the conversion.

Latitude

Latitude measures the number of degrees north or south of the equator which has a latitude of 0°. Lines of latitude (parallels) run east-west around the globe and are used to measure distances NORTH and SOUTH of the equator. As the base location, the equator is 0°. The latitude of the north pole, 1/4 of the way around the globe going in a northerly direction, is 90°N. This is the highest latitude possible. All latitudes except for the equator must be designated either north or south of the equator.

Longitude

To locate one point on the earth's surface, another angle measurement is needed. The angular measurements of latitude and longitude create a grid system or graticule that allow any specific point within it to be located. The longitude angle intersected with the latitude angle designates one particular point on the earth's surface.

Longitude measures the number of degrees EAST or WEST of the Prime Meridian which runs through the front door of the Royal Observatory, located in Greenwich (pronounced Gren-itch), England, but could have been placed anywhere. The Prime Meridian has a longitude of 0°. Lines of longitude (meridians) running north-south around the globe measure distances EAST and WEST of the Prime Meridian. Directly on the opposite side of the earth from the prime meridian is located the 180° meridian. This is the highest longitude possible. All other longitudes must be designated either east or west of the Prime Meridian. All meridians converge at the poles so exactly at the poles no longitude is given; the longitude of the poles is left blank. It is required here that latitude be given before longitude (generally preferred in non-computer applications).

Lines of latitude and longitude represent true geographic east-west and north-south directions despite the fact that on some maps the grid lines may be curved due to the problems of projecting the spherical surface of the earth onto the flat surface of a map. These aspects will be discussed later in this lesson.

When giving the latitude and longitude coordinates, be sure to identify if the location is north or south, east or west of the base or reference lines!! At these lines, no direction can be given, so do not put a letter. Also, leave the longitude of the poles blank.

Distance and the Graticule

We have just seen how you can use degrees of latitude and longitude to locate your position on earth. The problem is, a single degree of latitude or longitude describes a very large area on the earth's surface. How big is a degree of latitude? To figure this out, you need to know two facts. First, since the earth is shaped more or less like a sphere, it is 360 degrees around. Second, the earth has a circumference of approximately 25,000 miles. Use these two facts to calculate the length of a single degree of latitude (divide miles by degrees). The length of a single degree of latitude is about 69 miles.

What is the length of a degree of longitude? This question is a little more difficult. Remember that longitude describes distance east and west. This distance around the earth moving east or west varies depending on your latitude. At the equator, it is about 25,000 miles around the earth, so the length of a degree of longitude at the equator is 25,000 miles divided by 360 degrees or about 69 miles per degree. This is the same size as a degree of latitude. The circle formed by the parallel at 60 degrees north (still 360 degrees), has a much smaller circumference than does the equator, only about 12,500 miles. Using this distance, calculating the length of a degree of longitude at this location results in a distance of about 34.7 miles. At the North Pole, how far is it, traveling east to west along a parallel, around the earth? The distance is zero. Here you could travel around the earth simply by turning around. Since the size of circle (still 360 degrees) around the earth at this location is zero, the length of a degree of longitude at the Pole is zero miles divided by zero, that is, zero miles per degree.

To summarize, as you have seen, the size of a degree of longitude will vary enormously depending on the latitude. The maximum length occurs at the equator, about 69 miles per degree, and decreases as you increase in latitude until reaching zero miles per degree at the poles.

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