Libraries
 

Contact Information
  
last updated: 5/24/2013

How to Read Maps

USGS Topographic Map Symbols. (PDF.)

Introduction to Topographic Maps (Idaho State University, Dept. of Geoscienes)

About the Universal Transverse Mercator (UTM) Grid (USGS), that other set of markings on a topo besides longitude and latitude.


Glossary of Map Related Terms

The terms and definitions included in this glossary are adapted from a brochure entitled "Topographic Maps" produced by Theodore D. Steger in 1986 at the United States Department of the Interior Geological Survey, as well as the Geological Survey's "Topographic Map Information and Symbols" legend produced in March of 1978.

Antarctica Mapping Program
A program established by the Geological Survey to create maps of Antarctica. The Antarctic maps are published at several scales, primarily 1:250,000, with shaded relief.
Colors: For the significance of colors used on maps, see the table under Symbols.
Contour Interval: The elevation difference represented by adjacent contour lines on maps. Contour intervals are dependent on ground slope and map scale. Small contour intervals are used for flat areas; larger intervals are used for mountainous terrain. Supplementary dotted intervals are used in selected, flat areas at less than the regular interval.
Contours
Imaginary lines following the ground surface at a constant elevation above or below sea level.
See also: Contour Interval, Index Contours, and Supplementary Contours.
Control Surveys
Control surveys present map features in correct relationship to each other and to the Earth's surface. Two kinds of control measurements are needed: horizontal and vertical.
  • Horizontal ground control is needed to establish and maintain correct scale, position, and orientation of the map. For this purpose, latitude and longitude of selected points within the area to be mapped are determined by field surveys.
  • Vertical control is needed to determine the correct position of contours which show the shape or elevation of the terrain. Thus, elevations of selected points above and below the zero level must also be determined in the field.
Horizontal and vertical control points become the framework on which map detail is compiled. An accurate framework also permits map detail to match from one sheet to the next.

Permanent, or monumented, control points are usually marked on the ground by metal tablets 2 to 4 inches in diameter set in rock or masonry or on driven metal rods; many are shown by symbols on the maps. One mark can serve both horizontal and vertical control purposes.

Because such data are useful for many nonmapping purposes, the results of control surveys are prepared for sale in tabulated lists. Each covers a 15-minute quadrangle. The Geological Survey and the National Ocean Survey are cooperating in publishing diagrams which show the location and order of geodetic control established by these and other Federal agencies. This information is overprinted on planimetric editions of the 1:250,000-scale maps.
Index Contours
Heavy contour lines, usually occurring every fourth to fifth contour (depending on contour level), that often are accompanied by elevation figures.
Map Margin
The space outside the quadrangle border line, which is used to identify and explain the map. The marginal information corresponds to the table of contents and introduction of a book, telling how the map was made, who prepared it, where it is located, when it was compiled, and (if so) when it was revised.

On a topographic map, the lines of longitude (meridians) represent true north and south with reference to the planimetric detail. Magnetic compass bearings, however, must be corrected for local variations as indicated by the declination diagram on the lower map margin. The angles of the diagram merely show the relative directions of true, magnetic, and grid north; therefore the angular values given in figures are to be used for corrections. Magnetic declination is continually changing (albeit very slowly), so that the value applies only for the year specified.

The current magnetic declination can be readily, and accurately determined in the locality. By orienting the map along a straight road or two distant planimetric features away from local magnetic interference, a correct magnetic north arrow can be drawn using a compass. It is possible that compass north will vary significantly across the map. There is no better way to obtain the local magnetic declination; anything else published is an approximation of this method.
Map Revision
Map revision is needed to keep pace with changes in man-made features such as new roads, buildings, reservoirs, and sometimes changes in the shape of the terrain. The rate and amount of change vary greatly from urban to remote areas; therefore maps are not revised at fixed intervals. The needs of map users are a first consideration in selecting maps for revision.

Revision methods vary but usually combine photogrammetric and cartographic procedures to bring the content up to date while maintaining the original accuracy of the map. The newest revision technique used by the Geological Survey consists in upgrading map content without field investigation. Changes interpreted from current aerial photographs are overprinted on the map in purple, so that the revised map also provides a historical comparison of development. The photorevision technique is a fast and efficient means of keeping maps as current as possible.
Map Scale
Defines the relationship between the measurements of the features as shown on the map and as they exist on the Earth's surface. Scale is generally stated as a ratio or fraction--1:24,000 or 1/24,000. The numerator, customarily 1, represents map distance, and the denominator, a large number, represents horizontal ground distance. Thus the scale 1:24,000 states that any unit, such as 1 inch or 1 centimeter on the map represents 24,000 of the same unit on the ground.
National Atlas for the United States
A hard-bound 431-page volume that describes America in a cartographic format using multicolored reference and special subject maps and an extensive gazetteer. Although out of print, selected separate maps from the Atlas and some new atlas maps are available.
National Park Series
The National Park Series at various scales covers national parks, monuments, and historic sites. Many of these maps are available with shaded-relief overprinting; the topography is made to appear three-dimensional by the use of shadow effects.
National Standards
Adopted in 1941, these standards define the horizontal and vertical accuracy of topographic maps. Maps that meet these standards bear an accuracy statement in the lower margin. The standards are:
Horizonal Accuracy No more than 10 percent of the well-defined map points tested shall be more than 1/50 inch (0.5 mm) out of correct position at publication scales of 1:20,000 or smaller. This tolerance corresponds to 40 feet on the ground for 1:24,000-scale maps and about 100 feet on the ground for 1:62,500-scalemaps.
Vertical Accuracy No more than 10 percent of the elevations of test points interpolated from contours shall be in error more than half the contour interval.
Orthophotomaps
Produced for selected quadrangles in the Topographic Map Series, these maps show land features primarily by color-enhanced photographic images which have been processed to show detail in true position. Orthophotomaps may or may not include contours. Because imagery naturally depicts an area in a more true-to-life manner than the conventional line map, the orhophotomap provides an excellent portrayal of extensive areas of sand, marsh, or flat agricultural areas.
Orthophotoquads
A basic type of photoimage map prepared in a quadrangle map format. They can be produced very quickly because they are printed in shades of gray without image enhancement or cartographic symbolization. Orthophotoquads are valuable as map substitutes in unmapped areas and as complements to existing line maps.
Photorevision
A fast and efficient map revision technique which involves upgrading map content without field investigation. Changes interpreted from current aerial photographs are overprinted on the map in purple, so that the revised map also provides a historical comparison of development.
Quadrangle Maps
Cover four-sided areas bounded by parallels of latitude and meridians of longitude. Quadrangle size is given in minutes or degrees.
Relief Shading
An overprint giving a three-dimensional impression.
Supplementary Contours:
Also known as supplementary dotted intervals, these lines are added to better depict areas of little relief where basic contours fall widely spaced, are shown as dashed or dotted lines.
Symbols:
The graphic language of maps. Shape, size, location, and color all have special significance.

On topographic maps published by the Geological Survey the colors of the symbols indicate the features they represent. The colors and their corresponding features are presented in the table below:
Black Man-made or cultural features, including roads, buildings, names, boundaries, and transmission lines
Blue Water or hydrographic features, such as lakes, rivers, canals, glaciers, and swamps
Brown Contour lines which show the shape and elevation (relief) of the land surface (the unique characteristic of topographic maps)
Green Used to show woodland cover, with patterns for scrub, vineyards, and orchards.
Red Emphasizes important roads, and is used to show public land subdivision lines, land grants, and fence and field lines.
Red tint Indicates urban areas, in which only landmark buildings are shown
Purple Shows office revision from aerial photographs.

Some map symbols are pictographs, resembling the objects they represent, but the brown contour lines are abstractions that have no counterpart in nature. Contour lines are an effective device for representing the vertical dimension on flat paper. Practice and imagination are needed by the map reader to visualize hills and valleys from the contour lines of a topographic map.

To understand the contour symbol, think of it as an imaginary line on the ground which takes any shape necessary to maintain a constant elevation above or below a reference level.

Index contours are emphasized by a heavier line. Supplementary contours, added to better depict areas of little relief where basic contours fall widely spaced, are shown as dashed or dotted lines. Figures in brown along the index contours designate their elevations above the reference level. The elevation of any point can be read directly if it coincides with a contour, or can be interpolated if it is between contours. Map users who are concerned with quantitative measurements of terrain features can determine the data from the contours.

Maps with shaded relief are useful to those who are more interested in the general appearance and shape of the land than in exact ground elevations. The pictorial effect of contoured topographic features is enhanced by simulating the appearance of sunlight and shadows. A conventional map overprinted with shading gives the illusion of a solid, three-dimensional land surface. The maps published in shaded-relief editions are listed in the indexes to topographic maps for each State. Usually they are published only for areas of special topographic or recreational interest.

All water features are printed in blue, but are generally Topographic Map: A line-and-symbol representation of natural and selected man-made features of a part of the Earth's surface plotted to a definite scale. A distinguishing characteristic of a topographic map is the portrayal of the shape and elevation of the terrain by contour lines.

The physical and cultural characteristics of the terrain, as determined by precise engineering surveys and field inspection, are recorded on the map in a convenient, readable form. Topographic maps show the location and shape of mountains, valleys, and plains; the networks of streams and rivers, and the principal works of man.

Topographic maps have many uses as basic tools for planning and executing projects that are necessary to our way of life. They are of prime importance in planning airports, highways, dams, pipelines, transmission lines, industrial plants, and countless other types of construction. They are an essential part of ecological studies and environmental control, geologic research, studies of the quantity and quality of water, and projects for flood control, soil conservation, and reforestation. Intelligent and efficient development of our natural resources depends on the availability of adequate topographic maps. Topographic maps are also utilized by outdoor enthusiasts, including hunters and hikers, to show relief features, wooded areas, and watercourses. All of the outdoors can be better understood and appreciated with the aid of topographic maps.
Topographic Map Series
The Topographic Map Series of the National Mapping Program includes quadrangle and other map series published by the Geological Survey. A map series is a family of maps conforming generally to the same specifications or having some common unifying characteristic such as scale. Adjacent maps of the same quadrangle series can generally be combined to form a single large map.

The principal map series and their essential characteristics are given in the following table:
 
Topographic Map Series
Series Scale One Inch Represents Standard Quadrangle Size (latitude & longitude) Quadrangle Area
(square miles)
7.5-minute 1:24,000 2,000 feet 7.5 x 7.5 min. 49 to 71
15-minute 1:62,500 about 1 mile 15 x 15 min. 197 to 282
Intermediate-scale quadrangle 1:100,000 over 1.5 miles 30 min. x 1 degree 1,145 to 2,167
U.S. 1:250,000 1:250,000 about 4 miles 1 degree x 2 degrees 4,580 to 8,669
International Map of the World 1:1,000,000 about 16 miles 4 degrees by 6 degrees 73,734 to 102,759
United States Board on Geographic Names
This board was established by law to resolve name controversies by determining the choice, spelling, and application on maps and documents published by the U.S. Government.
United States Geological Survey
The United States Geological Survey was established by Congress in 1879 to consolidate four earlier organizations that had been engaged in topographic and geologic mapping and in collecting information about the public lands. From the beginning of this work, it was evident that adequate classification of lands and conclusive geologic determinations depended on suitable base maps. A general plan was adopted in 1882 for the production of a series of topographic maps at three scales. Under this plan each map covers an area bounded by meridians of longitude and parallels of latitude and is called a quadrangle map.

The plan of 1882 had been expanded and modified over the years to meet the needs of a growing Nation and to satisfy numerous civil and military needs not contemplated in the 1879 Act. A principal objective of the Geological Survey's National Mapping Program is to provide multipurpose maps and related data of appropriate scale, content, and accuracy to satisfy modern requirements. A major element of this program is the series of topographic maps produced by the Geological Survey.


last updated: 5/24/2013