Map Accuracy

An inaccurate map is not a reliable map.

“X” may mark the spot where the treasure is buried, but unless the seeker can

locate “X” in relation to known landmarks, the map is not very useful.

The U.S. Geological Survey (USGS)

publishes maps and other products at

high levels of accuracy. Dependability is

vital, for example, to engineers, highway

officials, and land-use planners who use

USGS topographic maps as basic planning tools. 

As a result, the USGS makes every effort

to achieve a high level of accuracy in all

of its published products. An important

aim of its accuracy control program is to

meet the U.S. National Map Accuracy

Standards.

National Map Accuracy

Standards

To find methods of ensuring the accuracy

of both location (the latitude and longitude of a point) and elevation (the altitude above sea level), the American

Society for Photogrammetry and Remote

Sensing— an organization actively involved in the science of making precise

measurements from photographs (photogrammetry) and acquiring information

from aerial photographs and satellite

image data (remote sensing)— set up a

committee in 1937 to draft accuracy

specifications. Sparked by this work,

agencies of the Federal Government,

including the USGS, began their own

inquiries and studies of map accuracy

standards. In 1941, the U.S. Bureau of

the Budget issued the “United States

National Map Accuracy Standards,”

which applied to all Federal agencies 

that produce maps. The standards were

revised several times, and the current

version was issued in 1947. (The standards are printed on the reverse of this

factsheet.)

As applied to the USGS 7.5-minute

quadrangle topographic map, the horizontal accuracy standard requires that the

positions of 90 percent of all points

tested must be accurate within 1/50th of

an inch (0.05 centimeters) on the map. At

1:24,000 scale, 1/50th of an inch is 40

feet (12.2 meters). The vertical accuracy

standard requires that the elevation of 90

percent of all points tested must be

correct within half of the contour interval. On a map with a contour interval of

10 feet, the map must correctly show 90

percent of all points tested within 5 feet

(1.5 meters) of the actual elevation.

All maps produced by the USGS at

1:250,000 scale and larger are prepared

by methods designed to meet these

accuracy standards and carry the statement, “This map complies with National

Map Accuracy Standards.” Exceptions to

this practice involve areas covered by

dense woodland or obscured by fog or

clouds; in those areas, aerial photographs

cannot provide the detail needed for

precise mapping. The USGS tests enough

of its maps to ensure that the instruments

and procedures the Survey uses are producing maps that meet the U.S. National

Map Accuracy Standards.

How the Survey Maintains Map

Accuracy

In 1958, the USGS began systematically

testing the accuracy of its maps.

Presently, accuracy testing is performed

on 10 percent of the mapping projects at

each contour interval as a method of

controlling overall quality. It is rare for a

7.5-minute map to fail the test, but this

happens on occasion.

In testing a map, the USGS experts select

20 or more well-defined points; a typical

point would be the intersection of two

roads. Positions are established on the

test points by field teams using sophisticated surveying techniques to determine

positions from aerial photographs. Field

survey methods are the only tests 

accepted for official accuracy testing.

Positions must be obtained by surveys of

a higher accuracy. Vertical tests are run

separately to determine precise elevations. The mapped positions are checked

against the field and (or) photogrammetrically determined positions results. If the

map is accurate within the tolerances of

the U.S. National Map Accuracy

Standards, it is certified and published

with the statement that it complies with

those standards.

By such rigorous testing of some of its

maps, the USGS is able to determine that

its procedures for collecting map

information ensure a high level of map

accuracy.

Factual Errors

There are other kinds of errors in

mapmaking. Names and symbols of

features and classification of roads or

woodlands are among the principal items

that are subject to factual error. Mapmakers cannot apply a numerical value

to this kind of information; they must

rely on local sources for their

information. Sometimes the local

information is wrong. Sometimes names

change or new names and features are

added in an area. The USGS

cartographers and editors check all maps

thoroughly and, as a matter of

professional pride, attempt to keep

factual errors to a minimum.

“Errors” resulting from selection,

generalization, and displacement are 

necessary results of mapping complex

features at reduced scales. In congested

areas, large buildings may be plotted to

scale and the smaller buildings may have

to be omitted; in showing buildings of

irregular shape, small wings, bays, and

projections usually are disregarded, and

the outline is shown in general form. At

Map Accuracy Standards

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 171-99

November 1999
map scale, it may not be possible to

show each of several closely spaced

linear features in its correct position. In

such cases, one feature, such as a 

railroad, is positioned in its true location

and others, such as parallel roads or 

rivers, are displaced the minimum

amount necessary to make each symbol

legible or are omitted to make the

highest priority symbol legible.

United States National Map

Accuracy Standards

With a view to the utmost economy and

expedition in producing maps that fulfill

not only the broad needs for standard or

principal maps, but also the reasonable

particular needs of individual agencies,

the Federal Government has defined the

following standards of accuracy for

published maps:

1. Horizontal accuracy. For maps on

publication scales larger than 1:20,000,

not more than 10 percent of the points

tested shall be in error by more than 1/30

inch, measured on the publication scale;

for maps on publication scales of

1:20,000 or smaller, 1/50 inch. These

limits of accuracy shall apply to

positions of well-defined points only.

Well-defined points are those that are

easily visible or recoverable on the

ground, such as the following:

monuments or markers, such as bench

marks, property boundary monuments;

intersections of roads and railroads;

corners of large buildings or structures

(or center points of small buildings). In

general, what is well-defined will also be

determined by what is plottable on the

scale of the map with-in 1/100 inch.

Thus, while the intersec-tion of two

roads or property lines meeting at right

angles would come within a sensible

interpretation, identi-fication of the

intersection of such lines meeting at an

acute angle would not be practicable

within 1/100 inch. Similarly, features not

identifiable upon the ground within close

limits are not to be considered as test

points within the limits quoted, even

though their positions may be scaled

closely upon the map. This class would

cover timber lines and soil boundaries. 

2. Vertical accuracy, as applied to

contour maps on all publication scales,

shall be such that not more than 10

percent of the elevations tested shall be

in error by more than one-half the

contour interval. In checking elevations

taken from the map, the apparent vertical

error may be decreased by assuming a

horizontal displacement within the

permissible horizontal error for a map of

that scale. 

3. The accuracy of any map may be

tested by comparing the positions of

points whose locations or elevations are

shown upon it with corresponding

positions as determined by surveys of a

higher accuracy. Tests shall be made by

the producing agency, which shall also

determine which of its maps are to be

tested, and the extent of such testing.

4. Published maps meeting these

accuracy requirements shall note this fact

in their legends, as follows: “This map

complies with National Map Accuracy

Standards.”

5. Published maps whose errors exceed

those aforestated shall omit from their

legends all mention of standard accuracy.

6. When a published map is a considerable enlargement of a map drawing

(manuscript) or of a published map, that

fact shall be stated in the legend. For

example, “This map is an enlargement of

a 1:20,000-scale map drawing,” or “This

map is an enlargement of a 1:24,000-

scale published map.”

7. To facilitate ready interchange and use

of basic information for map construction among all Federal mapmaking

agencies, manuscript maps and published

maps, wherever economically feasible

and consistent with the use to which the

map is to be put, shall conform to

latitude and longitude boundaries, being

15 minutes of latitude and longitude, or

7.5 minutes, or 3.75 minutes in size.

How To Obtain More Information

For information on these and other

USGS products and services, call 

1-888-ASK-USGS, use the Ask.USGS

fax service, which is available 24 hours 

a day at 703-648-4888, or visit the

general interest publications Web site 

on mapping, geography, and related

topics at http://mapping.usgs.gov/

www/products/mappubs.html.

Please visit the USGS home page at

http://www.usgs.gov/.