How to Read Topographical Maps

How to Read Topographical MapsTopographical maps display surface features of the Earth. In addition to bodies of water, roads, buildings and rail lines, they use contour lines to describe elevation. There are two distinct parts of a topographical map: the map face which displays a representation of the Earth's surface, and the map margin which presents information relating to the map face.

Instructions

Difficulty: Moderate

Understanding Margin Information

Things You’ll Need:
  • Topographic Map
Step 1
Locate the top margin of the map. On the top left margin is the name of the author or agency that created the map. On the top right margin is the title of the map.
Step 2
Find the bottom margin of the map. On the left side is the map production information. The datum for the map is typically located here. Datum is a reference value inputted into a GPS that allows both the GPS and map to use the same reference values, increasing accuracy of the device. Universal Transverse Mercator (UTM) zone and declination is also given in this section.
Step 3
Moving along the bottom margin to the right, the next item of information is a declination diagram. This diagram typically consists of a vertical line known as grid north, a line with a star at the end to indicate true north, and a line with an arrow at the end to indicate magnetic north. In addition to these lines is the angle between the lines; this diagram indicates the deviation between true north, grid north and magnetic north when navigating. Since magnetic north changes location at a variable rate, it is important to verify the declination for the current year. NOAA's National Geophysical Data Center (NGDC) provides this information online.
Step 4
Looking to the right of the declination diagram, at the center of the bottom margin is a fractional scale and a bar scale for the map. Scale is the ratio of how many units of horizontal Earth are represented by one horizontal unit on the map. For example, a ratio of 1:24,000 would mean that one inch on the map would represent 24,000 inches, or 2,000 feet, on the Earth. Below the scale is the contour interval, which indicates the vertical distance between contour lines on the map face.
Step 5
Continuing to the right of the bottom margin, the next piece of information is the quadrangle location. The quadrangle location relates the map to a larger reference area, such as where in a particular state the map is located.
Step 6
Observe the road classification key at the far right bottom margin. This key denotes how roads are represented according to their classification as primary highway, secondary highway, light duty improved road or unimproved road. Beneath the road classification is the date of the map and revisions. Dates are also listed in the production information on the bottom left margin.
Step 7
Note that additional information provided in the margin includes UTM coordinate values, section, township, and range numbers, and latitude and longitude all along the edge of the map face. In the corners can be found names of adjoining quadrangles; an adjoining quadrangle legend displaying the names of quadrangles on all sides of the map face is often posted in the margin.

Reading the Map Face

Step 1
Familiarize yourself with map symbols. A combination of colors, lines and symbols are used to denote natural and cultural features on a topographic map. For example, solid blue lines represent perennial streams while dashed blue lines with dots represent intermittent streams. The USGS publishes a list of symbols used on topographic maps.
Step 2
Notice the brown lines on the surface of the map. These brown lines are contour lines, which connect points of similar elevation and allow the detection of relief features on the land (topography). There are three types of contour lines: index contour lines are heavier brown lines that have numbers indicating the elevation of the contour line; light brown lines known as intermediate contour lines are placed at intervals indicated in the map margin between index contour lines; and very faint brown lines called supplementary contour lines, which are placed between intermediate contour lines.
Step 3
To ascertain land features, notice the arrangement of contour lines. A series of concentric circles increasing in elevation towards the center represents a hill. Contour lines placed close together represent a steep slope, while contour lines placed far apart represent a gentle slope. Contour lines very close to each other denote a cliff. "U"-shaped contours indicate a ridge with the bottom of the "U" pointing downhill. "V"-shaped contours indicate a valley with the bottom of the "V" pointing upstream. Saddles, ridges, depressions, fingers and rock outcroppings can be found by noticing the pattern of the contour lines.
Step 4
Estimate the slope of a land feature by dividing the vertical distance by the horizontal distance times 100. For example, a hillside on the map has eight contour lines spread over a distance of half an inch. If the contour interval is 40 feet, the vertical distance of the slope is 320 feet (eight contour lines x 40 feet per contour line = 320 feet). If the horizontal scale is 1:63,360 then the horizontal distance is (0.5 inch x 63,360 = 31,680 inches = 2640 feet = 1/2 mile). The slope can then be calculated as 12 percent (320 feet/2640 feet x 100 = 12%).
Step 5
Approximate the direction a slope is facing, known as aspect, by locating two points of the slope on different contour lines. Determine which point is at higher elevation. Draw an arrow from the higher point to the lower point. The arrow indicates the aspect of the slope relative to grid north.

Tips & Warnings

It is important to learn map reading close to home before venturing deep into back country. A good practice is to use a local topographic map to identify land features near your home.

Article Written By David Chandler

David Chandler has been a freelance writer since 2006 whose work has appeared in various print and online publications. A former reconnaissance Marine, he is an active hiker, diver, kayaker, sailor and angler. He has traveled extensively and holds a bachelor's degree from the University of South Florida where he was educated in international studies and microbiology.

Don't Miss a Thing!

All our latest outdoor content delivered to your inbox once a week.

FREE UPDATES

Subscribe

We promise to keep your email address safe and secure.