Survival - Map And Compass, Zachomikowane, Survival i zdrowie
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Finding Your Way with Map and Compass (FS03501, March 2001)
Finding Your Way with Map and
Compass
Fact Sheet 03501 (March 2001)
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From Near to Far: Distance
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From Here to There: Determining Direction
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A Word of Caution
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For More Information
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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).
Part of a 7.5-minute topographic map at 1:24,000 scale
Finding Your Way with Map and Compass (FS03501, March 2001)
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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
71
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
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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
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:
Web site:
or
National Geophysical Data Center
Phone: 303-497-6826
E-mail:
Web site:
or
.
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.
1.
2.
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.
3.
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.
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
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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
Finding Your Way with Map and Compass (FS03501, March 2001)
. For additional information, visit the
Web
site or the USGS home page at
.
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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.
509 National Center, Reston, VA 20192, USA
URL:
Contact:
Page Maintainer:
USGS Mapping Applications Center
Last modified:
14:24:06 Thu 12 Apr 2001
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