Basic Map Features

Often topographic maps are superimposed on a geologic map, so the features you need to look for in both are often the same. The basic information available at the top, bottom, and sides of a map can help you interpret these maps easily.

Title and Legend

The title of a topographic map is usually named after a major geographic feature within the map area, such as the Bright Angel Quadrangle, Arizona. On most U.S. Geological Survey (U.S.G.S.) maps, the title is found along both the upper and lower margins of the right-hand side of the map. The legend of a map is the key that explains the symbols used on the map. All U.S.G.S. topographic maps use the same symbols. Legends on topographic maps are usually limited to various symbols used for road classification.

Author(s)

All maps will at least have these listed, usually with the title or at the bottom.

Spatial coordinate markings or grids

Many maps will include latitude/longitude (Figure 10.1) or UTM information for precise locations on the map. Latitude and longitude coordinates on topographic maps and global positioning system (GPS) devices have different formats but are most commonly given in degrees, minutes, and seconds (DMS).

1 degree (°) = 60 minutes (‘) 1 minute (‘) = 60 seconds (“)

Topographic Map Base

We will dive more into this in another section, but this tells us what elevation is represented by each contour line in a topographic map so we can interpret elevation changes and easily find steep areas, hills, and valleys. The amount of elevation between contour lines is referred to as the contour interval and is usually printed on the map.

Scale

The scale of a map gives us the relationship between the true ground distance and the corresponding distance on the map. Topographic maps use three types of scale: ratio, verbal, and graphic. As part of this exercise, you will learn to convert from one scale to another.

•   Verbal Scale—A verbal map scale gives the scale in words. A verbal scale in the metric system might be 1 cm = 4 km, meaning one centimeter on the map represents 4 kilometers on the ground. A verbal scale in the English system might be something like 1 in = 5 mi., meaning one inch on the map represents 5 miles on the ground. You will not find a verbal scale on most topographic maps. Therefore, you must be able to calculate the verbal scale from the fractional scale.
• Graphic Scale —A graphic, or bar, scale is shown as a line or bar divided into units representing distances on the ground. The distance between two points on a map can easily be measured with a ruler, then you use the graphic scale to determine the true ground distance this map distance represents. Graphic scales are always found on topographic maps along the lower margin. Maps usually have several graphic scales, showing several different units in the English and metric systems. If a map is enlarged or reduced, the bar scale will also be enlarged or reduced by the same amount.
• Ratio Scale —The fractional scale of a map is the ratio between one unit distance on the map and the corresponding distance on the ground. For example, if the fractional scale of a map is 1:50,000, it means that one unit on the map corresponds to 50,000 of the same units on the ground. The units can be either in the English or metric system. In the metric system, a scale of 1:50,000 would mean one centimeter on the map corresponds to 50,000 centimeters on the ground. Using the English system, one inch on the map would correspond to 50,000 inches on the ground. Any unit of measurement may be used, but the units must be the same. Fractional scales are found at the lower margin of the map, usually near the graphic scales.
• Scale ConversionWorking with topographic maps requires you to convert from one type of scale to another.Common conversion units:

  1 meter (m) =	100 centimeters (m)
  1 kilometer(km) =	1000 meters (m)
  1 inch (in) =	2.54 centimeters (cm)
  1 foot (ft)=	12 inches (in)
  1 mile (mi) =	5280 feet (ft)

  To convert from a ratio to a verbal scale, simply divide by the conversion factors, and use these steps:

  1. List the ratio scale. For example 1:48000
  2. Convert to a verbal scale by turning the larger number into meters/kilometers/or miles.

  On the map	On the ground
  1 unit =	48000 units
  1 cm =	48000 cm ÷ 100 = 480 m 48000 cm ÷ 100000 = 0.48 km
  1 in =	48000 in ÷ 12 = 4000 ft
  1 in =	48000 in ÷ 12 ÷ 5280 = 0.76 mi

  If you need more practice or help, go to The Math You Need When You Need It.