Topographic Maps. More than a Road Map

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Dylan Jackson
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1 Topographic Maps More than a Road Map

2 What is a Topographic Map? Scaled representation of features on the surface of the Earth such as roads, rivers, and plains Provides information about land elevations and landforms such as mountains, hills, and depressions

3 Map Scale Maps always have a scale printed somewhere on the map. For USGS maps it is at the bottom. On USGS the map portion below, the scale is 1:24,000. That means that 1 on the map is 24,000 on the ground or 1 on the map is 24,000 on the ground.

4 Map Scale There are also scales that can be used to determine distance for many different units using a ruler and the map scale. 1 mile 1 mile 1/2 mile 2000 feet 1 kilometer 200 feet

5 Contour Lines Elevation is indicated by contour lines. A contour line is a line of equal elevations. All points on a contour line have the same elevation. r.htm feet 700 feet

6 Contour Lines The elevation difference from one contour line to another is called the contour interval. 20 ft elevation decrease 20 ft elevation increase 20 ft elevation increase r.htm

7 Contour Lines The contour interval is usually printed on a topographic map. However, you can always calculate the interval from the lines with values.

8 Steep or flat? You can tell whether a region on a map is steep terrain (hill/mountain) or relatively flat (plain) by looking at contour lines. HOW? We need to calculate a terrain gradient. This is the change in elevation over the change in distance along the ground. Gradient = Elevation (ft) Ground Distance (mi)

9 Steep or flat? Elevation change 0.5 mile distance = 20 feet Gradient 20ft/0.5 mi = 40 ft/mi Flat Elevation change = 600 feet Gradient 600ft/0.5 mi = 1200 ft/mi Steep

10 Map Symbols Topographic maps usually have a separate sheet with an explanation of map symbols. There is no room on the map for all the symbols Sample page from USGS Topo Map Symbol Pamphlet

11 Map Symbols Marsh Secondary Highway Houses Forest Stream Intermittent Stream

12 Tracking Flowing Water How does water always flow? DOWNHILL How can we tell what direction (N,S,NW,ESE) is downhill?

13 Tracking Flowing Water You can look for elevation changes since water will flow from higher to lower elevations. Here the stream flows toward the northwest ft 2100 ft

14 Tracking Flowing Water You can look at the contour lines as they cross the stream. They will always point upstream. Downstream Upstream

15 Tracking Flowing Water You can also look at where two streams merge. The merge will form a V that points downstream. Direction of flow

16 Stream Gradients You can determine a stream gradient along any section of a stream just like a terrain gradient. 0.4 miles 2000 ft 2100 ft elevation stream gradient = distance along stream 100 ft = 250 ft/mi 0.4 mi

17 Picture walking along a beach exactly where the water meets the land The water surface marks an elevation we call sea level, or zero. As you walk along the shore your elevation will remain the same, you will be following a contour line.

18 If you stray from the shoreline and start walking down into the ocean, the elevation of the ground (in this case the seafloor) is below sea level. If you stray the other direction and walk up the beach your elevation will be above sea level.

Topographic Map RULES Contour lines do not cross each other.

Contour lines that are spaced close together represent steep slopes.

Contour lines bend toward upstream (opposite of stream flow) Concentric circles

19 Topographic Map RULES Contour lines do not cross each other. In fact, contour lines can never touch each other. Contour lines that are spaced close together represent steep slopes. Contour lines that are spaced far apart represent gentle slopes. Contour lines bend toward upstream (opposite of stream flow) Concentric circles represent either a peak -- highest point or a depression -- the lowest point. Read the map to determine whether the center point is a peak or the center of a depression.

20 Concentric Circles: Mountain or Depression?

http://reynolds.asu.edu/topo_gallery/images/hillmap.

21 Contours and Profiles You can generate a profile of a landform from the contour lines on a topographic map. Click on image to see contours develop Simple contour map

22 Profile and Aerial Views Draw a picture in the left box that represents a side or profile view. Draw a picture in the left box indicating right box that represents an overhead or aerial view.

23 Contours and Profiles From a contour plot we can draw an axis line. We can then drop lines straight down to a grid labeled with elevations to the correct value. Connecting the points generates a profile or cross-section of the map.

24 Aerial View Profile View

25 So how do you read a topographic map? Follow along as we discuss Points A-E on this map.

26 Point A =? ft Point A sits right on the 0 ft contour line. Since all points on this line have an elevation of 0 ft, the elevation of point A is zero.

27 Point B =? ft. Point B sits right on the 10 ft contour line. Since all points on this line have an elevation of 10 ft, the elevation of point B is 10 ft.

28 Point C ~?ft. Point C does not sit directly on a contour line so we can not determine the elevation precisely. We do know that point C is between the 10 ft and 20 ft contour lines so its elevation must be greater than 10 ft and less than 20 ft.

29 WHAT IS THE ELEVATION OF POINT C? 15 ft

30 Point D ~? ft. We are even less sure of the elevation of point D than point C. Point D is inside the 20 ft. contour line indicating its elevation is above 20 ft. Its elevation has to be less than 30 ft. because there is no 30 ft. contour line shown. But how much less?

31 There is no way to tell. The elevation could be 21 ft, or it could be 29 ft. There is no way to tell from the map. Point D = 25 ft.

32 Point E ~? ft Just as with point C above, we need to estimate the elevation of point E somewhere between the 0 ft and 10 ft contour lines it lies in between. Because this point is closer to the 10 ft line than the 0 ft. line we estimate an elevation closer to 10. In this case 8 ft. seems reasonable.

33 Point E ~ 8 ft

34 Topographic map examples Sometimes topographic maps show islands

35 Sometimes topographic maps show canyons

36 Sometimes they show mountains.

37 Sometimes they are complicated Sometimes they are simple

38 Sometimes they show a single landform in detail. Sometimes they show many landforms over a large region.

39 webrangers/activities/ readingmap/

Topographic Maps Knowledge of Maps What are they? What are they used for? Why are they important?

Topographic Maps Knowledge of Maps What are they? What are they used for? Why are they important? Topographic Maps Maps that show the change in elevation of land. Elevation is the height of land above