Page WEB12-1

Practice Problems

Problem W12.1. Where Are You? (Latitude and Longitude)

A primary purpose of any map is to establish the location of a point so it may be found at a later time. The most efficient way to establish a location on a map is to use a system of grid lines. These intersect at right angles, like a net or screen laid over the map.

U.S. topographic maps use several grid systems. Each has its advantages. But we will work only with the two in most common use:

  • Latitude/longitude. This grid system is used worldwide. For its starting points, the system uses the Equator (0 latitude) and Prime Meridian (0 longitude).
  • Township and range. This system is unique to the United States and is used to designate land ownership, mostly in the West, Midwest, and South.

Materials

  • globe

Procedure

1. Latitude. On the globe, find the rings that run roughly horizontal. These are the lines of latitude, which are imaginary rings around the Earth. Latitude lines are all parallel to one another, both north and south of the Equator (Figure W12.1). (It may help you to think “flatitude” because latitude lines appear “flat” or horizontal on a globe or map when north is at the top.)

2. For numbering the lines of latitude, the Equator is the starting point: 0° latitude. Lines of latitude increase from 0° at the Equator through 10, 20, etc., to 90° at the North Pole. They also increase from 0° to 90° southward to the South Pole. (This is confusing until you get used to it—again, look at the globe.)

3. Longitude. On the globe, find the rings that run roughly vertically around the globe. These are the lines of longitude, which also form imaginary rings around Earth. They all run north-south, passing through both the North Pole and the South Pole.

4. Find 0° longitude. This is called the Prime Meridian, and itpasses through Greenwich, England. (Why it passes through Greenwich is a fascinating story too long to tell here. But the 0 north-south line had to run through somewhere, so the world’s powerful nations, after much dispute, agreed on the Greenwich Observatory in England. Look it up on the Web!)

5. Longitude is counted from 0 to 180 eastward, halfway around the planet, to the middle of the Pacific Ocean. Longitude also is counted from 0 to 180° in the opposite direction, westward, halfway around the planet, to the middle of the Pacific. Whichever way you go, the 180 meridian is in the middle of the Pacific, halfway around the world from the 0 point at Greenwich, England. By international agreement, the International Dateline (where each new day “starts”) roughly corresponds to the 180 meridian (Figure WS12.1). On the globe, note how degrees increase from 0 to 180, converging at the International Dateline.

6. Using Latitude and Longitude to Pinpoint a Location. Anylocation on land or water can be specified by using latitude and longitude together. For example, New York City is between 70 and 80 west longitude and lies just above 40 north latitude line.

7. Because latitude and longitude lines are circles, they are described in degrees. Recall from your math experience that each degree contains 60 minutes, and each minute contains 60 seconds. Thus, precise locations are recorded as so many degrees (°), minutes ('), and seconds (").

WQ12.1. Using the globe, find the degrees (°) and minutes (') for the location of the Statue of Liberty in New York Harbor.

Figure WS12.1 Latitude and longitude.

Problem W12.2. Where Are You? (Township and Range)

Two hundred years ago, when the United States was brand new, people were exploring and developing the land. Disputes over property boundaries popped up immediately. Thus, in 1785, the government created the U.S. General Land Office to solve the problem. Their solution was the township and range grid system. Today, over two centuries later, much of the land bought and sold within the United States is still described using this system, particularly in the Midwest and West.

1. From the envelope at the back of your lab manual, remove the map. Unfold it and find the Clark Fork, Idaho map.

2. Townships are squares, 6 miles on each side. Thus they cover 6 miles by 6 miles, or 36 square miles. Along the edge of your map, you will see black lettering showing that the townships belong to three series: one is designated “T55N,” meaning that it is the 55th township north of some reference line. Above that you see “T56N” for the series of townships that lie just to the north of those in the T56N series. And near the top of your map is T57N.

3. We also need to designate the east-west location of a given township. This is done using a “range” designation. On the map, note the small black lettering just above and below the map itself. The map says that the easternmost townships belong to “R3E” or the third range east of some reference line. Other townships on the map belong to R2E.”

4. Each township is subdivided into 36 numbered sections, each measuring 1 square mile. The section lines and number are shown in red on topo maps. Find several section numbers on your map. The pattern that governs how these section numbers are aligned is shown in Figure W12.2.

5. Sections can be subdivided further into quarter sections (or occasionally halves). Each quarter section, in turn, can be subdivided into quarter sections (or halves), and this process can continue down to even finer levels (Figure WS12.2.).

6. Check that, on the Clark Fork Topographic Map, the University of Idaho Field Campus is in Section 1, T55N, R2E.

WQ12.2. In the Township and Range system, what is the location of the Fish Hatchery on the Clark Fork Quadrangle, north of the town of Clark Fork? (Answer to the nearest quarter section.)

Figure WS12.2 Township and Range Grid System.

Problem W12.3. Magnetic Declination. The top of a topo map is always geographic north (the “north” you normally think of). The magnetic declination (lower left margin) indicates magneticnorth, the direction on the map to Earth’s magnetic North Pole (which is in Canada, several hundred miles from the geographic North Pole).

Why are there two “norths”? Earth spins like a top, which means that it spins around an imaginary line called its axis. The points where the axis intersects Earth’s surface are its geographicpoles—the North Pole and the South Pole. But Earth is also a huge magnet, with north and south magneticpoles. The reasons for the magnetic field around the Earth are still not fully understood. But luckily for people who read maps outdoors to find their way around, the magnetic field has created a magnetic north pole near the geographic North Pole.

The north needle on a compass points toward magnetic north, not geographic north. This is important for navigation, because this difference between the direction a compass needle points and true north could throw you off many miles when traveling. The angle between “true” geographic north and magnetic north on the map is given in degrees.

Look at the either of the topographic maps in the map pocket of your lab manual. In the lower left margin is the magnetic declination diagram. It shows that, at the particular location of the map, magnetic north is so many degrees east or west of geographic north. In other words, if you tried to travel in a straight line to the geographic North Pole from any point on this map, your compass would be off by that many degrees to the east or west, leading you further and further off course as you traveled.

If you are wondering why this is important, think of transportation systems that use compasses for navigation. For example, ships and fishing boats must move on true bearings, not stray from their chosen course.

WQ12.3. On the Palmyra, New York Topographic Map in the back of your lab manual, what is the magnetic declination?