Map Accuracy Standards - USGS

1 Map Accuracy Standards As applied to the USGS positions from aerial photographs. Field Map Accuracy quadrangle topographic map, the hori- survey methods are the only tests An inaccurate map is not a reliable map. zontal Accuracy standard requires that the accepted for official Accuracy testing. X may mark the spot where the trea- positions of 90 percent of all points Positions must be obtained by surveys of sure is buried, but unless the seeker can tested must be accurate within 1/50th of a higher Accuracy . Vertical tests are run locate X in relation to known land- an inch ( centimeters) on the map. At separately to determine precise eleva- marks, the map is not very useful. 1:24,000 scale, 1/50th of an inch is 40 tions. The mapped positions are checked feet ( meters). The vertical Accuracy against the field and (or) photogrammet- The Geological Survey (USGS) standard requires that the elevation of 90 rically determined positions results.

2 If the publishes maps and other products at percent of all points tested must be map is accurate within the tolerances of high levels of Accuracy . Dependability is correct within half of the contour inter- the National Map Accuracy vital, for example, to engineers, highway val. On a map with a contour interval of Standards , it is certified and published officials, and land-use planners who use 10 feet, the map must correctly show 90 with the statement that it complies with USGS topographic maps as basic plan- percent of all points tested within 5 feet those Standards . ning tools. ( meters) of the actual elevation. By such rigorous testing of some of its As a result, the USGS makes every effort All maps produced by the USGS at maps, the USGS is able to determine that to achieve a high level of Accuracy in all 1:250,000 scale and larger are prepared its procedures for collecting map of its published products.

3 An important by methods designed to meet these information ensure a high level of map aim of its Accuracy control program is to Accuracy Standards and carry the state- Accuracy . meet the National Map Accuracy ment, This map complies with National Standards . Map Accuracy Standards . Exceptions to Factual Errors this practice involve areas covered by National Map Accuracy dense woodland or obscured by fog or There are other kinds of errors in Standards clouds; in those areas, aerial photographs mapmaking. Names and symbols of cannot provide the detail needed for features and classification of roads or To find methods of ensuring the Accuracy precise mapping. The USGS tests enough woodlands are among the principal items of both location (the latitude and longi- of its maps to ensure that the instruments that are subject to factual error. Map- tude of a point) and elevation (the alti- and procedures the Survey uses are pro- makers cannot apply a numerical value tude above sea level), the American ducing maps that meet the National to this kind of information; they must Society for Photogrammetry and Remote Map Accuracy Standards .

4 Rely on local sources for their Sensing an organization actively in- information. Sometimes the local volved in the science of making precise How the Survey Maintains Map information is wrong. Sometimes names measurements from photographs (photo- Accuracy change or new names and features are grammetry) and acquiring information added in an area. The USGS. from aerial photographs and satellite In 1958, the USGS began systematically cartographers and editors check all maps image data (remote sensing) set up a testing the Accuracy of its maps. thoroughly and, as a matter of committee in 1937 to draft Accuracy Presently, Accuracy testing is performed professional pride, attempt to keep specifications. Sparked by this work, on 10 percent of the mapping projects at factual errors to a minimum. agencies of the Federal Government, each contour interval as a method of including the USGS, began their own controlling overall quality.

5 It is rare for a Errors resulting from selection, inquiries and studies of map Accuracy map to fail the test, but this generalization, and displacement are Standards . In 1941, the Bureau of happens on occasion. necessary results of mapping complex the Budget issued the United States features at reduced scales. In congested National Map Accuracy Standards , In testing a map, the USGS experts select areas, large buildings may be plotted to which applied to all Federal agencies 20 or more well-defined points; a typical scale and the smaller buildings may have that produce maps. The Standards were point would be the intersection of two to be omitted; in showing buildings of revised several times, and the current roads. Positions are established on the irregular shape, small wings, bays, and version was issued in 1947. (The stan- test points by field teams using sophis- projections usually are disregarded, and dards are printed on the reverse of this ticated surveying techniques to determine the outline is shown in general form.)

6 At factsheet.). Department of the Interior USGS Fact Sheet 171-99. Geological Survey November 1999. map scale, it may not be possible to closely upon the map. This class would How To Obtain More Information show each of several closely spaced cover timber lines and soil boundaries. linear features in its correct position. In 2. Vertical Accuracy , as applied to For information on these and other such cases, one feature, such as a contour maps on all publication scales, USGS products and services, call railroad, is positioned in its true location shall be such that not more than 10 1-888-ASK-USGS, use the and others, such as parallel roads or percent of the elevations tested shall be fax service, which is available 24 hours rivers, are displaced the minimum in error by more than one-half the a day at 703-648-4888, or visit the amount necessary to make each symbol contour interval. In checking elevations general interest publications Web site legible or are omitted to make the taken from the map, the apparent vertical on mapping, geography, and related highest priority symbol legible.

7 Error may be decreased by assuming a topics at horizontal displacement within the www/ United States National Map permissible horizontal error for a map of Accuracy Standards that scale. Please visit the USGS home page at With a view to the utmost economy and 3. The Accuracy of any map may be expedition in producing maps that fulfill tested by comparing the positions of not only the broad needs for standard or points whose locations or elevations are principal maps, but also the reasonable shown upon it with corresponding particular needs of individual agencies, positions as determined by surveys of a the Federal Government has defined the higher Accuracy . Tests shall be made by following Standards of Accuracy for the producing agency, which shall also published maps: determine which of its maps are to be tested, and the extent of such testing. 1. Horizontal Accuracy . For maps on publication scales larger than 1:20,000, 4.

8 Published maps meeting these not more than 10 percent of the points Accuracy requirements shall note this fact tested shall be in error by more than 1/30 in their legends, as follows: This map inch, measured on the publication scale; complies with National Map Accuracy for maps on publication scales of Standards .. 1:20,000 or smaller, 1/50 inch. These limits of Accuracy shall apply to 5. Published maps whose errors exceed positions of well-defined points only. those aforestated shall omit from their Well-defined points are those that are legends all mention of standard Accuracy . easily visible or recoverable on the ground, such as the following: 6. When a published map is a consid- monuments or markers, such as bench erable enlargement of a map drawing marks, property boundary monuments; (manuscript) or of a published map, that intersections of roads and railroads; fact shall be stated in the legend.

9 For corners of large buildings or structures example, This map is an enlargement of (or center points of small buildings). In a 1:20,000-scale map drawing, or This general, what is well-defined will also be map is an enlargement of a 1:24,000- determined by what is plottable on the scale published map.. scale of the map with-in 1/100 inch. Thus, while the intersec-tion of two 7. To facilitate ready interchange and use roads or property lines meeting at right of basic information for map construc- angles would come within a sensible tion among all Federal mapmaking interpretation, identi-fication of the agencies, manuscript maps and published intersection of such lines meeting at an maps, wherever economically feasible acute angle would not be practicable and consistent with the use to which the within 1/100 inch. Similarly, features not map is to be put, shall conform to identifiable upon the ground within close latitude and longitude boundaries, being limits are not to be considered as test 15 minutes of latitude and longitude, or points within the limits quoted, even minutes, or minutes in size.

10 Though their positions may be scal