Standard 1.h: Read and interpret topographic and geologic maps.

Topographic contours indicating surface relief. Image taken from US Army training manual.

Solid yellow lines represent surface isobars, line of equal air pressure. Green dashed lines are geopotential heights, the altitude at which 500 millibars of air pressure are attained. These types of contour maps are used by meteorologists. The weather system shown is the Storm of the Century, a powerful winter storm called a nor’easter.

In this lesson, you will learn about the ways to show three dimensions on a flat map, how to read topographic maps and other contour map types, and how to draw contours.

• Detailed maps showing the elevations of hills and valleys of an area.

• Use lines, symbols, and colors to represent changes in elevation and features on Earth’s surface.

Contour Lines • Contour lines are used to represent the three dimensional surface of the world on a flat map. • Can also be called isolines or isopleths.

• A contour line connects points of equal elevation.

• Elevation refers to the distance of a location above or below sea level.

• Contour lines never cross.

• contour lines marked with elevation

Index Contours

Contour Intervals

• distance in elevation change between each contour line

The contour interval is 20 ft. Point elevations are:

A = 700 ft B = 740 ft C = 770 ft D = 820 ft

Uphill direction

Slow change in elevation. Contour lines far apart.

Rapid change in elevation. Contour lines Close together.

• Widely spaced contour lines show a gentle slope.

• When they are close together, the slope is steep.

Spacing between Contour Lines

• When the contour lines are close together at the top of a hill, the hilltop is pointed.

• When the contour lines are widely spaced, the hilltop is flat.

1. Contours are imaginary lines that join equal values of whatever is being mapped. For example, the 100 ft contour marks all areas on a map that are 100 ft in elevation

2. Contour lines are defined by a contour interval. The contour interval tells you the amount of elevation change between contour lines.

3. The contour interval for a map is constant unless otherwise noted.

4. When contour lines are close together, the change in elevation increases very quickly (steep).

5. When the contour lines are spread apart, the amount of elevation change is small (flat).

You are to hike to the top of Blake Island.

Which side of Blake Island would be the

steepest climb, the west coast or the north coast?

W

N

(The west coast because the contour lines are closely spaced. You have a very

large amount of change in elevation over a short

distance. Go to the north for a more gentle climb)

Contour maps allow you to interpret the “lay of the land”. From surveyors to soldiers, the ability to read the topography in a topographic map is essential. Review the following topographic maps and learn to identify the features shown.

The V’s point upstream in a draw. Where you see a draw in a topographic map you can reasonably assume there is or has been water flowing that led to the erosion and migration of the contour lines.

Match the letter and number

• Topographic (relief) maps

• Bathymetric (sea floor elevation) maps

• Isohyet (rainfall) maps

• Isopach (rock or sediment thickness) maps

• Isotach (wind speed) maps

• Isobaric (air pressure) maps

• Geopotential height (elevation of a given air pressure) maps

• Very common topographic map

• Entire US covered by these maps

• Size of each maps is 7.5 minutes by 7.5 minutes (30 minutes make 1 degree of longitude or latitude)

• Scale of these maps is 1:24,000 (1 inch = 2000 ft/24,000 in)

• Also known as a topographic quadrangle

Bathymetry map. Shows depth of seafloor.

Isobars (solid yellow lines) on an isobaric map. This is a strong low pressure system, which produced large amounts of snow and wind across the eastern US. In an isobaric map, where isolines are close together, winds are the strongest. Winds are strongest in Virginia, North Carolina, and West Virginia. Green dashed lines are geopotential heights, the altitude at which 500 millibars of air pressure are attained.

Isohyet map. Rainfall distribution on August 17th – 20th, 2002 (isohyets are in millimeters)

Isohyet map showing predicted rainfall in inches

Isopach lines (contour interval = 5 ft) show the thickness of a rock layer known to produce oil. The round symbols are oil well locations and the depths (6000 – 7000 ft) drilled to reach oil.

Isopach map showing thickness of the limestone comprising the Florida Aquifer

Colored areas represent isotachs, lines encompassing areas of equal windspeed. Contour interval = 10 knots. The red area is a fast region of wind comprising of the polar jet stream. It flows from west to east, something that you cannot infer from this map unless the mapmakers were to plot wind barbs, arrows to indicate the direction of wind flow. Green lines are geopotential heights (air pressure).

• These features are represented by different symbols.

• A map legend explains what the symbols on a map represent.

• Topographic maps and most other maps include both human-made and natural features that are located on Earth’s surface.

• A map scale is the ratio between distances on a map and actual distances on the surface of Earth.

• When using a map, you need to know how to measure distances.

• There are three types of map scales: verbal scales, graphic scales, and fractional scales.

– A verbal scale expresses distance as a statement, such as “One centimeter is equal to one kilometer.”

– A graphic scale consists of a line that represents a certain distance, such as 5 km or 5 miles.

– A fractional scale expresses distance as a ratio, such as 1:63 500.

What does it mean if a map says “Scale 1:100 000”?

This fractional scale means that one unit on the map represents 100 000 units on Earth’s surface. For example, one inch on the map would equal 100 000 inches on Earth’s surface.

This is a map of geopotential heights. Contour interval is 60 meters. Note that the wind barbs in this map convey the direction of wind flow, which is predominantly west to east at this altitude.