1. Chapter 3
Models of the Earth

2. Latitude and Longitude
This is the coordinate system that we use on Earth. It is measured in angular units: degrees, minutes, and seconds. There are 60 minutes in a degree, and 60 seconds in a minute.

3. Latitude
Latitude is the angular distance north or south of the equator. The equator is the middle location on the Earth's surface, and is located halfway between the North and South poles.

4. If a line is drawn from any point on the Earth's surface to the center of the Earth, the angle that line makes with the equator is that location's latitude. The diagram on the right shows the equator (highlighted in red), as well as other lines of latitude and longitude.

5. Measuring Latitude
The North Star (Polaris) is located directly above the North Pole. This means that the altitude of the North Star in the sky is equal to the latitude of the person observing it. If you are at 90 degrees North latitude (the North Pole), Polaris will be 90 degrees above the horizon (directly overhead).

6. As shown in the example to the right, this method will work for any location in the northern hemisphere. The girl in the diagram must be at 42 degrees North latitude.

7. Longitude
Longitude is the angular distance east or west of the Prime Meridian. The Prime Meridian is designated as 0 degrees longitude, and passes through Greenwich, England.

8. The diagram on the right shows the Prime Meridian (highlighted in red), as well as other lines of latitude and longitude.

9. Measuring Longitude
Solar noon is defined as being the time at which the sun reaches its highest point in the sky. Since the Earth rotates on its axis at a rate of 3600 per day, it rotates at a rate of 150 per hour (3600 divided by 24 hours). This means that the occurrence of solar noon will move from east to west at a rate of 150 per hour. Longitude can be calculated, if when solar noon occurs, the observer knows what time it is at the Prime Meridian. By determining how many hours difference there is between you and the Prime Meridian, you can calculate how many degrees of longitude away you are (using the formula of 150 per hour).

10. Fields
A field is a region with a measurable quantity at every location.

11. The picture to the right is an example of a field map
The picture to the right is an example of a field map. In this example, the numbers on the map represent the concentration of gasoline found at that location. So the map simply shows how much of something (in this case gasoline) is found at many locations.

12. Isolines
Isolines are lines that are drawn on a field map to connect all of the points on that map that have the same value.

13. The image on the left  is an example of a gasoline field map with the 40 isoline drawn in.
Notice that the line connects points that all have the same value. In this example, the circle connects all of the points with a value of 40. That means that every point on that line has a value of exactly 40.

14. This map has all of the isolines drawn in, at an interval of 10 (each line is 10 units apart) is shown to the right:

15. Contour lines are isolines that are drawn on an elevation map
Contour lines are isolines that are drawn on an elevation map. These lines connect all points with the same elevation, and give a great picture of the lay of the land. The resulting map is called a Topographic Map.

16. Close line =Steep Slope
The steepest slopes on the map can be seen by looking for lines that are very close together.

17. Contour lines bend upstream when crossing a river
A riverbed drops downward, so the contour line must bend uphill to stay at the same elevation. The highlighted lines show that the direction of the Mill River must be northeast.

18. Highest possible elevation of a hill
The highest elevation of a hill can be calculated by finding the last (highest) contour line on that hill, and then figuring out the next line that would be drawn. The highest possible elevation of the hill is just below the value of  that next line. The highest possible elevation of the hill indicated by the arrow is 239 meters.

19. Depressions are shown by small marks pointing inward off the contour line
The arrow is pointing to a depression, or hole. The contour line with the marks, or hatchers, has the same elevation as the line before it. In this case the hatchered line has a value of 140 meters, and the depression must be less than 140 meters.