Topographic Maps TOPOGRAPHIC MAPS

What are topographic maps? Topographic maps- maps that show changes in elevation of Earth’s surface Elevation- distance above sea level. Also show mountains, rivers, forests and bridges TA asks Ss to recall what elevation is (just discussed) Elevation is the height above sea level on the Earth’s surface.

Contour Lines Contour lines- connects points of equal elevation. TA asks students Is it possible for contour lines to cross? EA NO If students say yes. Ask them how that would be possible if each line represents an elevation. Can one point have two elevations? No!

What is a contour interval? Contour interval- the difference in elevation between two side by side contour lines What is the difference in elevation between these two lines? TA asks Ss. In the picture below how can we figure out the contour interval? What information will help us? EA the numbers. The numbers that are listed on the contour lines are known as Index Contours. Index contours just tell us the elevation at a certain contour. How can we use this information to help us determine the contour interval? Count the number of lines in between (count the one we start on, but don’t start the one we finish on) the two, find out the total elevation between them, then divide. What is the contour interval? 20 ft.

One Island…Two Views Looking at each map how many high points do we see? Two How are the high points represented on the map? Small closed circles

Identify the Elevation TA asks Ss What is the elevation at point A? EA 0 ft If elevation is a measure of distance above sea level what does an elevation of 0 ft tell us? EA we’re at sea level What is the difference in elevation between point A and point B? EA 10 ft. What is the distance between each consecutive contour line? 10 ft

Scale Scale is the relationship between horizontal distance on the map and distance on the ground.

Contour mapping rules

1. A contour line represents a single equal elevation, all points on the contour line have the same elevation. What is the elevation of point B? 10 What would the elevation of any point on that line be? 10

2. Contour lines do not cross other contour lines.

3. Where one closed contour line surrounds another, the inner contour line represents the higher elevation. Which line has the higher elevation?

What is slope? Slope- change in elevation over a given horizontal distance Formula for slope Slope = change in elevation horizontal distance Which hill has a steep slope? Gentle slope?

5a. On a contour map, closely spaced lines indicate a steep slope.

5b. Widely spaced lines indicate a gentle slope

5c. Uniformly spaced lines indicate a uniform (constant) slope. Notice that the contour lines for each are the same distance apart in each picture. There is the same change in elevation for each given distance.

Comparing Slope

6. A contour line that closes within the limits of a map indicates a hill, ridge or plateau.

6a. Highest possible elevation of a hill is just below the value of the next contour line that would be drawn. 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. The last line indicated on the map is 220 meters. The next line would be 240 meters (remember that the contour interval is 20 meters). Since there is no 240 meter line, the hill cannot be higher than 239 meters! The last contour line drawn is 220 m. The next line would be 240 m. Since there is no 240 m line, the hill cannot be higher than 239 m.

7. Depressions are shown by small marks (hachures) pointing inward off the contour line. When would you see an area were the elevation is lower than surrounding areas? EA: craters, mines, volcanic craters What would the elevation of the last contour line drawn be? 140 m The hachures have the same elevation as the last contour line. The depression is less than the last contour line, but not as small as the contour line before it. Elevation of the depression would be between 140 and 120 m. Elevation of the depression would be less than 140 m.

8. Where a contour line crosses a stream or valley, the contour bends to form a “V” to point upstream. Which direction is the river flowing? Towards the ocean, NE Contour lines will always bend to point upstream, toward the soruce of the river. Rivers flow from upstream to downstream, toward the mouth of the river. Since contour lines must remain at the same elevation, they must bend when they cross riverbeds. 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.