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Topographic Maps

Understanding map scales
Mapping definitions
How map areas are defined
Determining what map sheet you need

Topographic Map Basics

The Map and Geography Library offers an extensive collection of topographic map coverage for much of the world. The collection includes publications from the USGS as well as those published by foreign agencies. Topographic maps are used widely for applications such as hiking and camping, engineering, land conservation, etc. The feature that distinguishes a topographic map from any other type of map are contour lines that show lines of equal elevation. This method proves to be a highly effective way of portraying three-dimensional land forms on a two-dimensional piece of paper.

Understanding map scales
Mapping definitions
How map areas are defined
Determining what map sheet you need

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Understanding the concept of scale is useful when dealing with maps. Scale is the ratio of the distance on the map to the actual distance on the Earth's surface. A map of scale 1:24,000 means that one inch on the map is equivalent to 24,000 inches on the ground.

Large versus Small Scale
The concept of scale can be confusing because 1:24:000 maps are considered to be large-scale, while small-scale maps often have a scale of 1:1,000,000. In order to make sense of large versus small scale maps, it may be helpful to think of the ratio (i.e. 1:24,000 or 1:1,000,000) as a fraction. The larger the value of the fraction the larger the scale (i.e. 1/24,000 is larger than 1/1,000,000). With a large scale map it is possible to see more detail than with a small scale map.

Take a look at the difference in detail you can see of the Fermi National Accelerator Laboratory (Batavia, Illinois) in the following maps with various scales. The ring is approximately 1.25 miles in diameter.

Scale 1:250,000

Scale 1:100,000

Scale 1:24,000 *Please note: This image was constructed by combining images from two topographic quadrangles.

This table shows some common map scales available in the Map and Geography Library's collection with the distances they represent:

For more information about map scales and the USGS, see a factsheet they published.

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Contour Lines

Isolines that show equal elevation on a map at defined intervals.

Declination

A measurement of the degree to which a grid or magnetic north varies from true north. Declination is usually measured in mils, where one mil is equal to 1/6,400 of 360 degrees.

Graticules*

A network of lines on a map which represent the meridians of longitude and parallels of latitude.

Grid North**

North according to the grid imposed on the map. Because grids ignore the curvature of the Earth, this is not the same as true north.

Grid*

A network of uniformly spaced lines on the face of a map, intersecting at right angles and usually running north-south and east-west. Grids are often numbered and can be used to define position by rectangular coordinates.

Index Map

A map that accompanies a map set that shows the overall arrangment of maps in the set, showing their numbering scheme and areas of coverage. It is useful for finding the particular map needed without having to browse through the entire map set. Think of an index map like a visual book index. The index map shows you the contents of a map set so you can choose the specific page (or sheet) you need.

Magnetic North**

North according to the Earth's magnetic poles rather than its geographic poles. Declination from true north is given in mils where one mil equals 1/6,400 of 360 degrees.

Meridian

A starting point/line, usually a line of longitude, for a numbering system to section the earth into a grid.

Projection

A way of depicting the three-dimensional surface of the Earth onto a two-dimensional sheet of map paper. There are many types of projections all of which aim to have the least amount of distortion on as many properties of the map as possible, such as distance, angular measurement, area, or shape.

True North**

Geographic north, what most people think of when they hear the word north.

* Definitions taken from World Mapping Today by R.B. Parry and C.R. Perkins (1987, London, Butterworths) (Call number Q.025.286W893)

** Definitions taken from page 266 of Map Librarianship by Mary Larsgaard (1998, Englewood, Colorado, Libraries Unlimited, Inc.)

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Topographic maps are published in sets that cover a larger, named, geographic area. Each individual map within a set contains a unique identifier that may be comprised of numbers, letters, numbers and letters or a name that has significance in the area covered. The identifiers are typically determined according to one or more grid systems used by the map publisher. For instance, the USGS 7.5 minute topographic quad series, the grid is determined by sectioning the United States into rectangles (approximately) that are 7.5 minutes of latitude by 7.5 minutes of longitude. Each of these "quadrangles" is given an individual name that typically represents the most prominent feature of the area covered.

There are many such grid systems in existance in the mapping world. The U.S. Army Map Service published a set of 1:250,000 maps that use a map designation system where blocks north of the equator start with N and blocks south start with S. A typical block is numbered as follows: NB 39-12. The second letter is from A to Z, the letter determined by increasing distance from the equator. The next component of the designation gives information about how far the block is from a meridian. The area NB 39 is then further subdivided into 12 or 16 smaller blocks (depending on latitude), to which the final component of the map designation system refers. This system is still being used today by map publishers around the world, like the Pan American Institute of Geography and History (PAIGH) that published a set of topographic maps of Latin America.

Grid systems can be universal, like the Universal Transverse Mercator (UTM) Grid, they could be national, such as the Great Britain National Grid System or they could be determined on a map by map basis.

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Once you narrow your geographic search to a specific map set that has the type of information you need, it is necessary to find a particular map within the series. This can be accomplished through the use of an index map for the map set. The index map shows all of the individual maps within a set, overlaid onto the larger geographical area that the set covers. It is usually necessary to consult the index map in order to find the sheet number of the individual map you require. In the example below, you would request sheet #6 to see the map covering Cherbourg.

Example index map for a 1:100,000 series of maps of France.

The Map and Geography Library holds the index maps for all of its series in a file cabinet near the circulation desk in call number order. Please ask at the desk for assistance in locating and retrieving an individual map from a map series.

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The Map and Geography Library offers comprehensive coverage of all states by the United States Geology Survey . U.S. coverage includes the following scales: 1:24,000; 1:62,500; 1:100,000; 1:250,000 (1:24,000 scale is the most detailed, 1:250,000 the least detailed). There are individual state indexes for the 24,000 and 62,500 series. These maps provide information on topography, transportation, relief (differences in elevation), drainage systems, and other physical and cultural features.

The Library also has a comprehensive set of Canadian topographic maps at scales of 1:50,000 and 1:250,000. For more information on Canadian maps and geography visit the National Atlas Information Service of National Surveys, Mapping and Remote Sensing Organization, Natural Resources Canada.

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	University of Illinois at Urbana-Champaign Library Gateway Homepage Comments to: Map and Geography Library Updated: 12.05.2006 JMJ