isolines
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Century College *
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1020
Subject
Geography
Date
Feb 20, 2024
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docx
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Uploaded by KidUniverse14742
Isolines (30 points)
Earth scientists use many types of maps to graphically depict data for a variety of variables. Scientists may use topographic maps
to show elevation above sea level of a surface as well as the general trend of the land (hills, depressions, valleys, etc.). In addition, these maps often show human-made features including roads and buildings. Figure 1 shows an example of a topographic map. Scientists also use weather maps
to show patterns in temperature, pressure, dew point, and many other variables. You will be working with these types of maps at times during the course. Figure 1: Part of the St. Paul West, MN USGS Topographic Map with a 10 foot contour interval
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To understand these maps, you need to know how to read and draw these maps. The most common
feature of these types of maps are called isolines
. An isoline is a line that connects points of equal value. Topographic maps use isolines called contour lines
. A contour line is a line of equal elevation. Anywhere a given contour line runs has the same elevation. The contour interval
is the vertical elevation difference between adjacent contour lines. For example, if the contour interval is 10
meters, a contour line adjacent to the 140-meter line will be either 130 or 150 meters above sea level.
Every fifth contour line is typically bolded for reference. These bolded reference lines are called index (or guide) contours.
A
benchmark (BM)
is a spot where a permanent plaque has been fixed
to the surface to denote the elevation at that point. Spot elevations
are also places where an exact elevation has been denoted on the map, but lack the permanent marker. Relief is the difference in elevation between two points on a map.
Common isolines seen on weather maps include isotherms
and isobars
. An isotherm is a line of equal temperature while an isobar is a line of equal pressure. Just like with contour lines, maps showing isotherms or isobars will have a consistent interval between those lines when plotted.
Isoline rules
There are several rules that isolines follow that will help when interpreting or drawing a map.
1)
Isolines always connect points of equal value.
Notice how all the points with the value of “5” are connected with a line on the Figure 2. All the points labeled “10” are connected with a separate line. Points between these lines are spot elevations, temperatures, etc.
Figure 2: Contour lines on a topographic map with a 5 foot contour interval
2)
Isolines are continuous.
They never end, branch, or cross one another. All isolines will form an enclosed loop. A line may end at the map’s edge, but would continue on the adjacent map.
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3)
Lines that are close together indicate a rapid change in the plotted variable
(for example: a steep slope, a rapid change in temperature, or a rapid change in pressure). Lines that are widely spaced indicate more gradual change in the variable.
4)
Lines forming an enclosed circle within a map indicates an area of higher value
(such as a hill or high pressure center) with lower values all around or an area of lower value with higher area all around. On topographic maps, depressions (enclosed low areas) are shown with enclosed circles with tick marks extending toward the center of the depression as shown in Figure 3.
Figure 3: Contour lines showing a depression (left) and hill (right)
5) When a contour line crosses a river, the line forms a V-shape (or U-shape) that points uphill.
Notice that the land slopes downhill from left to right in Figure 4. Rivers always flow downhill. The contour lines that cross the river in the figure, point to the left which is in the uphill direction. Water flows downhill out of the open end of the V-shape.
Figure 4: Contour lines (solid lines) crossing a river (dashed line)
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1. Figure 5 contains seven mistakes. Using the definitions and rules outlined on pages 5-7, identify and describe at least five separate mistakes seen on the map. Carefully label the mistake on the map with a letter (A, B, C, D, E) then clearly describe the mistake in the correct place below. A river is shown with the dashed line. If you can correctly identify the remaining two mistakes, you may earn one bonus point for each. Mistake A:
Mistake B:
Mistake C:
Mistake D:
Mistake E:
Mistake F (bonus):
Mistake G (bonus):
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Figure 5: Topographic map with seven mistakes
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2. What is a contour line?
Use the Figure 6 to answer questions 3-8. A spot elevation is shown as a reference. A stream is shown with the dashed line.
Figure 6: Topographic map with a 10-meter contour interval.
3. This map has a contour interval of 10 meters. What does this mean?
4a. Which direction is the main branch of the stream flowing?
b. How did you determine the direction of the flow (be specific)?
5. Label each
contour line on Figure 6 with its elevation in meters. 6.
List the elevation for each of the points labeled on the map. Be sure to label the correct unit (feet, meters, etc.) for the elevations.
A: __________
B: __________
C: __________
D: __________
E: __________
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7. What is the relief between points A and B?
8. Is the area around point A or point E steeper? How can you tell?
9.
Contour the land elevations in Figure 7 with a 10-meter contour interval. Label each contour line with its elevation. Figure 7: Topographic map with a 10-meter contour interval
182
194
199
210
219
188
190
204
218
227
197
209
216
222
229
191
196
224
230
234
Use Figure 8 to answer question 10-11.
10.
Contour the land elevations in Figure 8 with a 20-meter contour interval. Label each contour line
with its elevation.
11. Draw a dashed line where a stream is likely to exist.
Figure 8: Topographic map with a 20-meter contour interval
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674626651677687675
650602643674744699
620571634659691709
596565611635668682
560552580620651661
543550564599634657
Use Figure 9 to answer questions 12-16. Elevations are in meters.
Figure 9: Topographic map
Map scale: 1 cm = 0.5 km
12a. What is the relief between points A and B?
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b. What is the relief between points A and C?
13a. Using the map scale shown, what is the distance in kilometers between points A and B?
b. What is the distance in kilometers between points A and C?
14. If you were to walk from A to C, would the distance you walk be equal to, less than, or greater than your distance from question 13b? Why?
15. If you were standing at location B, would you be able to see your friend standing at location A (assuming you had a strong pair of binoculars or telescope!)? Why or why not?
16. If you were standing at location C, would you be able to see your friend standing at location A (assuming you had a strong pair of binoculars or telescope!)? Why or why not?
17.
Match the written description with the topographic feature designated with the contour lines.
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a. _____a gradual, symmetrical hill
b. _____a hill where the western slope is steeper than the eastern slope
c. _____a hill with a depression on top
d. _____a hill with two peaks, one being higher than the other
e. _____a hill with two equal peaks and a valley between
1.
4.
2.
5.
3.
18.
Match the following profile (skyline) views on the left to the correct contour lines on the right.
a._____
1.
b. _____
2.
c. _____
3.
d. _____
4.
e. _____
5.
Answer the questions below related to the weather map (Figure 10). Isotherms (in °F) are shown on the map with a 10°F interval. The current temperature in the Twin Cities is 20°F.
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Figure 10: Weather map with isotherms
19. Label each isotherm on Figure 10 with its correct value. 20. Estimate the temperature in the following locations. Be sure to label the unit that the temperature represents.
_______ at the most northernmost tip of Minnesota
_______ at the southernmost tip of Louisiana
_______ at the southwestern corner of Oregon
_______ at the northeastern corner of Ohio
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