Topographic Maps Notes

Earth ScienceEarth Science

Reading Topographic Reading Topographic MapsMaps

Definition of Topographic Maps

1. A graphical representation of the three dimensional shape of the earth’s surface that shows:

a) elevation b) shape

Features of Topographic Maps

(not on notes page…)TOPOGRAPHY (RELIEF):

- printed in brown - contour lines shows hills, mountains, plains, etc.

WATER FEATURES: - printed in blue- includes oceans, lakes, ponds, rivers, canals, etc.

CULTURE: - printed in black- human-make works such as roads, railroads, buildings, land boundaries, etc.

Features of Topographic Maps

Relationship between 3D and 2D representation

3D view

2D view

Key Terms:

Topography: general shape of the Earth.Elevation: distance above sea level.Contours: lines of equal value.Contour line: is an imaginary line on the surface of the earth connecting points of equal elevation.

Contour lines

Relationship between topographic features and contour lines

Contour interval (C.I.) is the difference in elevation between any two adjacent contour lines.

x

Relief: the difference between the highest and lowest elevation in a particular area.

ADD to your notes page: Contour index- extra dark line with numbers.

If we put contour lines on the island every 10 meters, it would look like this…

Now, let’s determine the elevation at several points of the island…

0 – sea level

10 meters ~15 meters

+20 meters, but < 30

~8 meters

General Features of Contour Lines

Topographic Maps can show:ELEVATION:

1.Contour lines connect points of equal elevation.

Topographic Maps can:Show if the land is STEEP or GENTLE:

2. Steep slopes are shown by closely spaced contour lines.

3. Gentle slopes are shown by widely spaced contour lines. Contour lines do not intersect, branch or cross.

Topographic maps can:

Identifying RIVERS and STREAMS and their FLOW:

4. When contour lines cross streams they bend upstream; that is, the contour line forms a 'V' with the apex at the intersection with the stream, and pointing in an upstream direction.

Topographic maps show:

If there is a HILL or DEPRESSION:

5. Closed contours appearing on the map as ellipses or circles represent hills.

6. Closed contours with hatchures, short lines pointing toward the center of the closure represent closed depressions.

TOPOGRAPHIC MAPS: DEFINITIONS

Freebies:#3 – gradient#4D – profile

Bottom of page #7 – solid, intermitted

For numbers 1 – 4 (a – d) Choose from these words:

Contour lineDepressions

ElevationIndex

IntervalTopographic map

For numbers 1 - 8 at the bottom, choose from these words:

Gentle hillhill

lowestnever

sea levelSteep hillupstream

Answers:1. Topographic map

2. elevation

3. Gradient

4. Contour line

a) Interval

b) Index

c) Depression

d) profile

1. Never

2. Hill

3. Steep

4. Gentle

5. Upstream

6. Sea level

7. Solid, intermitted

8. lowest

Write down: Rules to determine the contour interval (with multiple lines):

Find the difference between the two INDEX contours (a).

Count contour lines between the contour indexes, then add one (b).

NOW DIVIDE (a/b)

A. Contour Interval?

B.

C.

D.

E.

The difference in elevation between the two index contours (800 - 700) is 100. We cross five lines as we go from the 700 line to the 800 line (NOTE: We count the lines between the index contours, then ADD one. In this case, we have 4 lines + 1 = 5). Therefore is we divide the elevation difference (100) by the number of lines (5) we will get the contour interval. In this

case it is 20.

720 m

740 m

760 m

780 m

2. Determining a value of a depression… Determine the outer ring’s value. Outer ring is = to the lowest valued contour

line. In a depression – elevation DECREASES as

you go towards the center.

1. Determining a value of a hill… Determine the outer ring’s value. Outer ring is = nearest lowest valued contour

line PLUS contour interval. In a hill– elevation increases as you go

towards the center.

p

Row 110

30

10

30

200

Row 220180

2001400

5

+25

Row 3100

14005

35

2004400

Row 417

200

2400

100

400

Topographic Profiles

DEFINITIONS- A topographic profile is a diagram that shows the

change in elevation of the land surface along any given line (an area “looked from the side”).

- Gradient - The steepness of a slope as measured in degrees, percentage, or as a distance ratio (rise/run).

- Change in elevation between 2 points/length of

measured 2 points. When dividing, make sure the units are the SAME!!!

http://www.physicalgeography.net/physgeoglos/d.html#distance_ratio

Vertical exaggeration (VE)

Topographic profiles are usually constructed with some amount of vertical exaggeration (VE) to emphasize the details of topography.

The VE is determined by the following steps:

1 . Determine the number of feet represented by one inch on the vertical axis of the topographic profile. In Figure 7-7C one inch represents 100 ft or 1:1200.

2. The horizontal scale of the profile is the same as the map, 1:24,000.

3. The VE is calculated by dividing the horizontal scale by the vertical scale.

VE = horizontal (map) scale/vertical scale

VE = 24,000/1200

VE = 20x

The VE of 20 indicates that the topography illustrated in Figure 7-7C is exaggerated by 20 times that of the original map.

How to Make a Topographic Profile

This represents a very simple topographic map of a hill. The hill is steep on the left side (the contour lines are very close together) and has a gentle slope on the right side. The numbers represent the elevation of the contour lines. (*)

100200

300

400

What would the hill look like if you were to slice it from left to right? (*)

100200

300

400

500 feet

400 feet

300 feet

200 feet

100 feet

Normally, the Earth’s surface is not this blocky. In a topographic profile a line is drawn from these points (red dots) producing a smooth transition. (*)

Thus you have a topographic profile. This is what the hill would look like if you were to cut it along the profile line and look at it from the side. (*)

Topographic Profile

Topographic Profile

Constructing a Topographic Profile

A B

6400

The contour interval of this map is 40 feet. Every index contour would then be 200 feet.800

0

7000

760074007200

7800

68006600

A B

6400


0

7000

760074007200

7800

68006600

In this region the profile line cut across the 7800 foot line 4 times.

(*)

The last step is to simply connect the “dots”. (*)

The profile is finished. (*)

A B

6400


0

7000

760074007200

7800

68006600

This is a very classic example of a butte with steep sides and a very flat top. The profile that you see here is an exaggerated scale – the vertical scale is greater than the horizontal scale. The next example is where the vertical scale is closer to the horizontal scale. (*)

THE GRAND CANYON, ARIZONA

This view of the Grand Canyon is from the South Rim looking north into Bright Angel Canyon. This is what it looks like on a topographic map. (*)

THE GRAND CANYON, ARIZONA

This view of the Grand Canyon is from the South Rim looking north into Bright Angel Canyon. This is what it looks like on a topographic map. (*)

It may not be as majestic but it is full of information. (*)

Let’s take a look at this area from a topographic point of view. (*)

Cumberland, Pennsylvania/Maryland

This area is in the deeply eroded folds of the Appalachian Mtns. What type of drainage pattern is shown in this area? (*)

This a very good example of a TRELLIS PATTERN. The tributaries come in from the hills and meet the main channel at almost 90o. (*)

The tributaries are shown in blue and the main channel is shown in violet. What stage in the cycle of stream erosion do these streams display? (*)


Just for fun, let’s construct a topographic profile across the main stream from point A to point B. (*)

AA

BBTry to visualize what this profile would look like before you move on. (*)


AA BB

Using every INDEX CONTOUR – 100 foot interval … (*)

11001000900

800

700

600S

tream

Now just connect the dots and draw a smooth profile. The stream occupies the lowest point of the valley. (*)

Education

Topographic Maps Notes