TOPOGRAPHIC MAPS

Topographic maps are graphic representations of the surface of the Earth. They provide:

a plan (bird's-eye) view of the land
scales to measure ground distances
bearings to indicate directions
coordinate systems to locate features
symbols for natural and cultural features
contour lines to show the elevation, slope, and relief of the landscape

SCALES

A scale defines the ratio between distance on a map and corresponding distance on ground.

fractional scale: numerical expression

eg: "1:1000" indicates that 1 unit of distance on map represents 1000 of the same units on the ground

verbal scale: written description

eg: "1 cm to 1 km"

graphic scale: calibrated bar or line

BEARINGS

Bearing: the direction between two points.
True north: the bearing toward geographic north, where axis intersects the Earth's surface
Magnetic north: bearing toward magnetic north pole

COORDINATE SYSTEMS

Two types of coordinate systems are found on most topographic maps:

latitude-longitude
township-range

The latitude-longitude system is universally used throughout the world. The township- range system is only used in western and southern states.

LATITUDE-LONGITUDE

The Earth's surface is divided by an imaginery coordinate grid. This grid is defined by two kinds of lines:

latitude (or parallels): run east-west
longitude (or meridians): run north-south

The location of any point on the Earth's surface can be indicated by the numbers (coordinates) of the line of latitude and line of longitude that cross it.

The coordinate numbers of latitude and longitude are angular measurements:

coordinate of any line of latitude: angle between line and equator
coordinate of any line of longitude: angle between line and prime meridian

The coordinate angles are measured in units of degrees, minutes, and seconds:

1 degree (°) = 60 minutes (')
1 minute (') = 60 seconds (")

Topo maps usually come in two sizes:

7.5' quadrangle: which extend 7.5' (1/8°) of latitude from north and south and 7.5' of longitude from east to west
15' quadrangle: which extend 15' (1/4°) from north to south and from east to west

MAP SYMBOLS

Topographic maps use a variety of symbols to show the location of physiographic and cultural features, the type and density of vegetation, and the elevation of the land.

The standard symbols (established by the United States Geological Survey) are shown in Table 5-1.

CONTOUR LINES

Topographic maps are distinctive from other maps in the use of contour lines to show the elevation, relief and slope of the land.

elevation: vertical distance from sea level
relief: difference in elevation between two points
slope: change in elevation over a given horizontal distance

Contour lines: imaginary lines connecting points of equal elevation.

Contour interval: difference in elevation between two adjacent contour lines:

usually constant on any given map
every contour line is a multiple of
the contour interval

Index contour: every fifth contour line is printed thicker, and then marked with its elevation for reference.

READING CONTOUR LINES*

1. Every fifth line in a series of contours in an index contour (see point A on Figure 5-7)

2. The elevations of specific points are noted by benchmarks (see points B and C)

3. The elevation of any point on a contour line is equal to the elevation of that contour line:

What is the elevation of point D?

4. The elevation of any point that lies between two contour lines is estimated on the basis of its relative distances from these lines.

What is the elevation of point E?

5. The spacing of contour lines is proportional to the slope of the land.

Slope	Contour Representation	Example* Fig.5-7
Level ground	No contours	F
Uniform slope	Equally spaced contours	G
Gentle slope	Widely spaced contours	H
Steep slope	Closely spaced contours	I
Vertical cliff	Merged contours	-

6. Every point enclosed by a solid contour line is topographically higher than the line itself. Solid contour lines enclose topographic highs (see point J).

7. Contour lines which cross rivers and valleys form Vs that point in the upstream direction (point L).

8. Hachured lines are used to mark the contours of closed depressions with no outlets such as ponds (see point K)

9. Every point enclosed by a hachured contour line is topographically lower than that line.

10. The outer hachured contour line around a topographic low has the same elevation as the closest solid contour line.

TOPOGRAPHIC PROFILES

Topographic maps provide a plan view of the shape of the Earth's surface.

A different perspective on the shape of the Earth's surface is the cross-section view, called a topographic profile.

The creation of a topographic profile is shown in Figure 5-8.

VERTICAL EXAGGERATION

Vertical exaggeration (VE): the distortion of the vertical scale of a topographic profile to emphasize the relief and slope of the land.

VE = vertical scale divided by horizontal scale

For example, the vertical scale in Figure 5-8c is 1"=100'; the horizontal scale is 1"=1000'.

STEREOPHOTOGRAPHS

To use stereophotographs:

1. Locate a prominent point or feature that is present in both images.

2. Place center of stereoscope directly over the line separating the two images, and adjust the lenses so that they are directly over the same point in each image.

3. Look through stereoscope and relax your eyes. The images will merge. If they do not, twist the stereoscope until they do.

Back to the Physical Geology Lab Homepage