Map And Compass.pdf

Finding Your Way with Map and Compass (FS03501, March 2001)

Finding Your Way with Map and

Compass

Fact Sheet 03501 (March 2001)

| From Near to Far: Distance | From Here to There: Determining Direction | A Word of Caution |

| For More Information |

A topographic map tells you where things are and

how to get to them, whether you're hiking, biking,

hunting, fishing, or just interested in the world

around you. These maps describe the shape of the

land. They define and locate natural and manmade

features like woodlands, waterways, important

buildings, and bridges. They show the distance

between any two places, and they also show the

direction from one point to another.

Distances and directions take a bit of figuring, but

the topography and features of the land are easy to

determine. The topography is shown by contours.

These are imaginary lines that follow the ground

surface at a constant elevation; they are usually

printed in brown, in two thicknesses. The heavier

lines are called index contours, and they are usually

marked with numbers that give the height in feet or

meters. The contour interval, a set difference in elevation between the brown lines, varies from map to

map; its value is given in the margin of each map. Contour lines that are close together represent steep

slopes.

Natural and manmade features are represented by colored areas and by a set of standard symbols on all

U.S. Geological Survey (USGS) topographic maps. Woodlands, for instance, are shown in a green tint;

waterways, in blue. Buildings may be shown on the map as black squares or outlines. Recent changes in

an area may be shown by a purple overprint. a road may be printed in red or black solid or dashed lines,

depending on its size and surface. A list of symbols is available from the Earth Science Information

Center (ESIC).

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Part of a 7.5-minute topographic map at 1:24,000 scale

Finding Your Way with Map and Compass (FS03501, March 2001)

| Back to the top | Main table of contents |

From Near to Far: Distance

Maps are made to scale; that is, there is a direct relationship, a ratio, between a unit of measurement on

the map and the actual distance that same unit of measurement represents on the ground. If, for instance,

1 inch on the map represents 1 mile (which converts to 63,360 inches) on the ground, the map's scale is

1:63,360. Below is a listing of the scales at which some of the more popular USGS maps are compiled.

A convenient way of representing map distance is by the use of a graphic scale bar. Most USGS

topographic maps have scale bars in the map margin that represent distances on the map in miles, feet,

and kilometers. The table below shows the corresponding area of coverage for each scale and the linear

distance that each scale represents in inches and centimeters.

Map Name

Series

Scale

1 inch

represents

1 centimeter

represents

Map area

(approximate

square miles)

Puerto Rico 7.5 minute

1:20,000

1,667 feet

200 meters

7.5-minute

1:24,000

2,000 feet

240 meters

40 to 70

7.5- by 15-minute

1:25,000

2,083 feet

250 meters (about)

98 to 140

Alaska

1:63,360

1 mile

634 meters (about)

207 to 281

Intermediate

1:50,000

0.8 mile

500 meters (about)

County

Intermediate

1:100,000

1.6 mile

1 kilometer (about)

1,568 to 2,240

United States

1:250,000

4 miles

2. 5 kilometers (about)

4,580 to 8,669

| Back to the top | Main table of contents |

From Here to There: Determining Direction

To determine the direction, or bearing, from one point to another, you need a compass as well as a map.

Most compasses are marked with the four cardinal points—north, east, south, and west—but some are

marked additionally with the number of degrees in a circle (360: north is 0 or 360, east is 90, south is

180, and west is 270). Both kinds are easy to use with a little practice. The illustrations on the reverse

side show how to read direction on the map.

One thing to remember is that a compass does not really point to true north, except by coincidence in

some areas. The compass needle is attracted by magnetic force, which varies in different parts of the

world and is constantly changing. When you read north on a compass, you're really reading the direction

of the magnetic north pole. A diagram in the map margin will show the difference (declination) at the

center of the map between compass north (magnetic north indicated by the MN symbol) and true north

(polar north indicated by the "star" symbol). This diagram also provides the declination between true

north and the orientation of the Universal Transverse Mercator (UTM) grid north (indicated by the GN

symbol). The declination diagram is only representational, and true values of the angles of declination

should be taken from the numbers provided rather than from the directional lines. Because the magnetic

declination is computed at the time the map is made, and because the position of magnetic north is

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Finding Your Way with Map and Compass (FS03501, March 2001)

constantly changing, the declination factor provided on any given map may not be current. To obtain

current and historical magnetic declination information for any place in the United States, contact:

National Geomagnetic Information Center

Phone: 303-273-8486

E-mail: jcaldwell@usgs.gov

Web site: geomag.usgs.gov .

National Geophysical Data Center

Phone: 303-497-6826

E-mail: info@ngdc.noaa.gov

Web site: www.ngdc.noaa.gov/ or

www.ngdc.noaa.gov/seg/potfld/geomag.shtml .

Taking a compass bearing from a map:

Draw a straight line on the map passing through your location and your destination and extending

across any one of the map borders.

Center the compass where your drawn line intersects the map border, align the compass axis N-S

or E-W with the border line, and read on the compass circle the true bearing of your drawn line. Be

careful to get the bearing in the correct sense because a straight line will have two values 180°

apart. Remember north is 0, east is 90, and so on.

To use this bearing, you must compensate for magnetic declination. If the MN arrow on the map

magnetic declination diagram is to the right of the true north line, subtract the MN value. If the

arrow is to the left of the line, add the value. Then, standing on your location on the ground, set the

compass so that "zero degrees or North" aligns with the magnetic north needle, read the magnetic

bearing that you have determined by this procedure, and head off in the direction of this bearing to

reach your destination.

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Finding Your Way with Map and Compass (FS03501, March 2001)

(1) Drawing a straight line over the map edge

(2) Reading the compass on the map

(3) Using the magnetic declination diagrams

| Back to the top | Main table of contents |

A Word of Caution

Compass readings are also affected by the presence of iron and steel objects. Be sure to look out

for—and stay away from—pocket knives, belt buckles, railroad tracks, trucks, electrical lines, and so

forth when using a compass in the field.

Information

For information on these and other USGS products and services, call 1-888-ASK-USGS, use the

Ask.USGS fax service, which is available 24 hours a day at 703-648-4888, or visit the general interest

publications Web site on mapping, geography, and related topics at

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Finding Your Way with Map and Compass (FS03501, March 2001)

mac.usgs.gov/mac/isb/pubs/pubslists/index.html . For additional information, visit the ask.usgs.gov Web

site or the USGS home page at www.usgs.gov .

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement

by the U.S. Government. This document has undergone official review and approval for publications

established by the National Mapping Division, U.S. Geological Survey. Some figures have been modified

or added to improve the scientific visualization of information.

U.S. Department of the Interior U.S. Geological Survey 509 National Center, Reston, VA 20192, USA

URL: http://mac.usgs.gov/mac/isb/pubs/factsheets/fs03501.html Contact: macweb@usgs.gov

Page Maintainer: USGS Mapping Applications Center

Last modified: 14:24:06 Thu 12 Apr 2001 USGS Privacy Statement Accessibility

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