Topographic Maps

Review: What is a Field Map?

A field map shows how variables (field values) change across an area, such as temperature, elevation, or air pressure.

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Field maps use lines, called isolines, to connect points with the same field value.

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Common isolines include:

• Isotherms - Connecting points of equal temperature.

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• Isobars - Connecting points of equal pressure.

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• Contour Lines - Connecting points of equal elevation.

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Field maps help people see patterns and make predictions about the environment.

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Isotherms

Isobars

Contour Lines

Topographic Maps

A topographic map is a detailed representation of the Earth’s surface, showing natural and artificial features using contour lines, symbols, and colors.

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Topographic maps are used for navigation, land surveying, hiking, and geological studies.

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Contour Lines connect points of equal elevation; all elevations are relative to sea level.

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Sea level is where the water meets the land and is always at zero elevation.

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Contour intervals tell you the elevation difference between lines.

What is the contour interval shown on this map?

Sea Level

5 m

Elevation Practice

When reading a topographic map, you must estimate the elevation depending on the location.

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Rules to Remember:

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• The elevation equals the line if the location is on the contour line.

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• If the location is between two contour lines, you need to make your best estimate depending on which contour line the location is closest to.

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• If the location is at a peak or the bottom of a depression, the elevation will be the number closest to the following possible contour line.

A

B

C

D

E

What are the elevations of the locations above?

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A = 2 m, B = 5 m, C = 7 m, D = 13 m, E = 19 m

Intermediate Vs. Index Contour Lines

A topographic map has two types of contour lines: Index and Intermediate contour lines.

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• Index contour lines (thicker lines) are labeled with their elevation.

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• Intermediate contour lines (thinner lines) appear between index lines and help visualize the terrain and determine the contour interval if it’s not shown on the map.

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Steps to determining the contour interval:

1. Subtract the difference between two consecutive index contours.

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• Count the number of intermediate contour lines between the index contours.

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• Divide the difference by the number of contours plus one.

Contour Interval Example

1. 750 m - 700 m = 50 m
2. 4 contours + 1
3. 50 m / 5 = 10 m

Contour Interval = 10 m

Circular Contour Lines

A topographic map will have two types of circular contour lines.

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• Closed circles represent hills or mountain peaks, with the smallest and innermost circles representing the highest point.

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• Depressions (holes or craters) will be marked with hachure lines (small ticks pointing inward).
• Please be careful when reading the elevations of a depression. The elevation decreases as you move toward the center.

Benchmarks

Locations where an exact elevation is known are symbolized with a benchmark.

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A benchmark will sometimes have a marker at the actual location. (as shown below)

Benchmark

A Young Mr. Amy In Hawaii

Map Scales

A scale on a topographic map represents the relationship between distances on the map and actual distances on the ground.

• Ex: 1 cm = 100 feet

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How to Use a Map Scale to Measure Distance:

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1. Use a ruler to measure the distance between two points on the map.

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• Compare the measurement to the graphic (bar) scale or convert using the ratio scale.

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• If measuring a curved path (like a trail or river):
• Use a piece of string or a flexible strip of paper to trace the path.
• Then, straighten it out and compare it to the scale.

1.5 km

Gradient

Reminder: The contour lines can also be used to determine the gradient of an area in two ways:

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Mathematically - Use the formula on page 1 of the ESRT

Gradient = Change in Field Value / Distance

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Visually - If you observe the spacing between the lines.

• If the isolines are closer together, the gradient is steeper or higher. (cliffs or mountains)

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• If the isolines are further apart, the gradient is gentler or lower. (plains or valleys)

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Calculate The Gradient Between Point X and Y

G = (380 ft - 300 ft) / 1.5 km

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G = 80 ft / 1.5 km = 53.3 ft/km

1.5 km

Steep Slope

Gentle Slope

Rivers

Rivers and other water bodies play a significant role in shaping the landscape, and their presence is reflected in topographic maps by making V-shaped contours.

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V-Shaped Contour Patterns

• Rivers create valleys, which appear as V-shaped contour lines on a topographic map.

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• The V always points upstream (toward higher elevation), and the river flows in the opposite direction (downhill).

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Example: The river is flowing NE on this map.

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N

What direction is this river flowing?

N

North East Flow