Reading the Margins (Lat/Lon)

This section addresses how to read the information that is in the margins of a USGS topographic map. It starts with the upper left corner of the map and moves clockwise around the map.

Agency or Author Who Created Map (upper left corner of map)

The United States Department of the Interior Geological Survey is the agency that created the map below. This same information can also be found in the bottom left corner.

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Map Title (upper right corner of map)

This corner section provides the name of quadrangle, state (and sometimes the county) where the quadrangle is located, and map series. Quadrangles are often named after a prominent town or feature that is in the quadrangle. In the image below, the name of the quadrangle is “Lucky Peak” which is located in Idaho. The map series indicates how much land area is on the map; for example, in the image below, the Lucky Peak quadrangle is a 7.5 minute series which indicates it covers a four sided area of 7.5 minutes of latitude and 7.5 minutes of longitude.

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Road Classification (bottom right corner of map)

Road and trail symbols may be found in this legend as shown in the image below.

Revision Date (bottom right corner of map)

Some maps have a revision date, which is when the map was last updated. If the map is old, it may not be accurate. In the image below, the revision date is 1972. Refer to the “Map Production Information” block in the bottom left corner for additional information on map dates.

Quadrangle Location (bottom right corner of map)

The location of the quadrangle is pinpointed on a map of the state as shown in the image below.

Adjoining Quadrangle Legend (corners of map)

Names of adjoining quadrangles are frequently indicated in the corner margins of USGS topographical maps; Mayfield is the joining quadrangle in the image below.

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Some topographic maps will have an adjoining quadrangle legend. The image below shows the eight adjoining quadrangles for this map.

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Map Scale (bottom center of map)

The map scale indicates the ratio or proportion of the horizontal distance on the map to the corresponding horizontal distance on the ground.

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There are two types of scales on the topographic map:

1 – Fractional Scale

The fractional scale expresses the ratio of the map distance to the ground distance in like units of measurements. It is usually written as a fraction or ratio. For example, the map in Figure 2-5 has a map scale of 1:24,000 which means one inch on the map is 24,000 inches on the ground.

Typically, USGS produces maps using the 1:24,000 scale, but will also produce maps using 1:62,500 and 1:250,000 scale. The 1:24,000 scale provides larger and clearer details than the 1:250,000, but it does not cover as large an area.

The maps produced at a 1:24,000 scale (1 inch represents 24,000 inches or 2000 feet) are commonly known as 7.5-minute quadrangle maps; each map covers 7.5 minutes of latitude and 7.5 minutes of longitude, which is approximately 8 miles (north/south) and 6 miles (east/west). The primary scale used in Alaska topographic maps is 1:63,360 (1 inch represents 1 mile) due to the size of the state. The Alaska quadrangle map covers 15 minutes of latitude and varies from 20 – 36 minutes of longitude.

2 – Bar or Graphic Scale

A graphic scale or comparison scale is entirely different from the representative fraction scale. It usually compares map distances to the ground distance in different units of measurements.

Usually a graphic scale is a line marked off on a map indicating so many inches or millimeters equal to so many feet, kilometers, chains, or miles on the ground. A comparison scale of 1 inch to 2000 feet means that 1 inch on the map is proportioned to 2000 feet on the ground. We are comparing inches and feet which are different units of measurement.

Contour Interval (bottom center of the map)

Contour interval is the difference in elevation between two adjacent contour lines. In the image below, the contour interval is 40 feet. On USGS maps, contour intervals are usually 1, 5, 10, 20, 40, and 80 feet. If the contour interval is not printed on the map, it can be calculated (which is discussed later in this chapter).

North Arrow, Declination, and Map Production Information (bottom left corner of map)

It is common practice for maps to be oriented with true north at the top. Most USGS maps have a symbol of arrows pointing to the geographic North Pole (shown by a star), magnetic north (MN) and grid north (GN). Grid north shows the difference between geographic north (latitude/longitude) and the UTM grid.

In the image below, the magnetic north is 18.5 degrees east. The difference between the geographic North Pole and magnetic north is the magnetic declination for that map.

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If the declination is not indicated on the arrow diagram, it can be found in the “Map Production Information” which is in the lower left corner of the map. The map production information section provides additional information on how and when the map was created. Sometimes the magnetic declination is printed here.

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Datum and UTM Zone

The datum and UTM zone, which are extremely important when using a GPS receiver, can also be found in this block. Vertical and horizontal datums may be listed on the map; however, if the map lists only one datum then the vertical and the horizontal datum are the same.

Latitude and Longitude (edges of map)

Latitude and longitude lines are indicated with fine black tick marks along the edges of the map (Figure 2-8). Topographic maps do not show the latitude/longitude lines – just the tick marks. The numbers next to the tick marks indicate degrees (°), minutes (‘) and seconds (“). On 1:24,000 scale maps, latitude and longitude tick marks are indicated every 2.5 minutes.

  • Longitude tick marks are on the top and bottom edges of the map and latitude tick marks are on the right and left edges. Note that the degrees may be left off (as an abbreviation) and you may only see the minute and/or second designations.
  • Reference coordinates for latitude and longitude (degrees, minutes, and seconds) are black and located on the four corners of the map.
  • The intersection of latitude and longitude lines are noted by cross-marks (+).

When reading latitude/longitude, pay close attention to the units (degrees, minutes, seconds) because it is easy to misread them. Refer to Chapters 3 and 6 for additional information on latitude and longitude.

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Universal Transverse Mercator (UTM) (edges of map)

Prior to 1978, USGS topographic maps used blue tick marks along the edge of the map to illustrate where the UTM grid lines were located. Since 1978, USGS topographic maps actually show UTM grid lines (black) on the map and the coordinate values are in the margin. On USGS topographic maps, 7.5 quadrangle, the UTM grid lines are marked at 1,000 meter increments.

  • Abbreviated easting values, for example 336, are located on the top and bottom edges of the map.
  • Abbreviated northing values, for example 4164, are located on the right and left edges of the map.
  • Reference coordinates for UTM are located near the southeast and northwest corners of the map. Notice that the large bold numbers increase as you go north and east.

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Section, Township, and Range (edges of map)

Section, township, and range numbers are red.

  • Section numbers may be printed along the edge, but they are typically printed in the center of the section. In the image below, some of the section numbers include 15, 16, 17, 18, 19.
  • Township numbers are printed along the right and left edge of the map. In the image below, the township numbers are T.2S and T.3S.
  • Range numbers are printed on the top and bottom edge of the map. In the image below, the range numbers are R.1E and R.2E.

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[Continue to Interpreting the Contour Lines]

**Information adapted from the National Interagency
Incident Management System Basic Land Navigation
Manual, PMS 475 dated June 2007.