Using Topo Maps

Finding Your Way with Map and Compass

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 man-made 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, which 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 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).
Topo Map example

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 in the same unit of measurement 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. A convenient way of representing map distance is by a graphic bar scale. Most USGS topographic maps have such a scale, or scales, in the margin. To use these scales, put a strip of plain paper down on your map, and mark the strip where it touches the two points. Then match this marked strip with the appropriate scale printed in the margin of the map and figure the distance from a series of comparisons with the scale. Read the distance on a curving road or fence line the same way. Mark a strip of plain paper at the ends of relatively straight sketches of road or fence, and then compare the marked strip with the scale.

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 degrees: north is 0 degrees; east is 90 degrees; south is 180 degrees; and west is 270 degrees). Both kinds are easy to use with a little practice. One thing to remember is that a compass does not really point north, not 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 between magnetic north and true north at the center of the map. Taking a compass bearing from a map:
(1) Draw a straight line on the map passing through your location and your destination and extending across any one of the map borders.
(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 degrees apart. Remember north is 0 degrees, east is 90 degrees, etc.
(3) To use this bearing, you must compensate for magnetic declination. If the MN arrow on the map 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.

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.