I. United States National Map Accuracy Standards

With a view to the utmost economy and expedition in producing maps which fulfill not only the broad needs for standard or principal maps, but also the reasonable particular needs of individual agencies, standards of accuracy for published maps are defined as follows:

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 in [0.846 mm] measured on the publication scale; for maps on publication scales of 1:20,000 or smaller, 1/50 in [0.508 mm]. These limits of accuracy shall apply in all cases 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 markets, such as bench marks, property boundary monuments; intersections of roads, railroads, etc.; corners of large buildings or structures (or center points of small buildings); etc. In general what is well defined will also be determined by what is plottable on the scale of the map within 1/100 in [0.254 mm]. Thus while the intersection of two road or property lines meeting at right angles would come within a sensible interpretation, identification of the intersection of such lines meeting at an acute angle would obviously not be practicable within 1/100 in [0.254 mm]. 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. In this class would come timber lines, soil boundaries, etc.

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 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 point 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 on 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, where economically feasible and consistent with the uses 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 3/4 minutes in size.

U.S. Bureau of the Budget Issued June 10, 1941 Revised April 26, 1943 Revised June 17, 1947

II. Accuracy standards recommended for hydrographic

surveys

A. General Standards

1. Scale of survey

a. The scale adopted for a survey of a particular area should not be smaller than the scale of the existing or proposed chart of the area and preferably should be at least twice as large as that of the largest scale of the published or proposed chart of the area.

b. Ports, harbors, channels, and pilotage waters should be surveyed on a scale of 1:10,000 or larger.

C. Other waters used by shipping with possible shoals or other dangers to navigation should be sounded on a scale of 1:20,000 or larger.

d. Surveys of coastal and harbor approach areas to a depth of at least 20 m (11 fm) should be conducted on a scale of 1:50,000 or larger.

e. Offshore hydrographic surveys in depths greater than 20 m (11 fm) may be plotted on a scale smaller than 1:50,000 dependent on the importance of the area covered, the depth, and bottom configuration. The scale of the offshore plotting sheet should not be smaller than is necessary to provide a sheet of convenient size that will extend a short distance beyond the offshore limit of the survey and will, where feasible, include the stations necessary for control of the survey.

2. Interval of sounding lines at the scale of the survey a. Spacing of principal sounding lines:

1.0 cm (0.4 in) or less, as may be needed to thoroughly develop the area at the scale of the survey, except where depth and character of the bottom will permit wider spacing.

b.

Spacing of cross-check lines: 7.5 cm (3.0 in) or less. 3. Interval of plotted soundings Frequency along sounding lines:

Spacing should be less than the interval with peak and deep soundings shown, but this interval may be increased in areas of even bottom, and where the soundings are recorded on an echogram.

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4. Sampling of bottom characteristics

In general, sufficient sampling should be done to demarcate the limits where one general type of bottom changes to another.

In waters that may be used for anchoring, samples should be taken at regular intervals not to exceed 5 cm (2 in) at the scale of the survey. In other areas, shoaler or deeper, a spacing of 8 cm (3 in) is sufficient depending on the regularity of the bottom. Deep-water bottom samples, over 100 m (55 fm), are classed as oceanographic observations requiring special equipment and samples will be taken as required.

5. Spacing of position fixes

The spacing of position fixes on the survey sheets shall be from 2 to 4 cm (1-1.5 in).

6. Current observations

When velocity is expected to exceed 0.2 kn, both velocity and direction of currents shall be observed at entrances to harbors or channels, at any change in direction of channels, in anchorages, and adjacent to a pier or wharf area. It is also desirable to measure coastal and offshore currents when they are of sufficient strength to affect shipping. (Editor's note: Current observations for a circulating survey by NOS are made under more rigid and exacting specifications).

B. Specific Standards

1. Horizontal control

a. Primary shore stations

The location of primary shore control stations and electronic positioning stations shall be within the limits of accuracy for third-order control when the geodetic survey extends no more than 50 km (31 mi) from the point of origin or from stations of a geodetic net of higher order used as the origin. When the extent of the geodetic survey is in excess of 50 km the use of second-order control methods is desirable, and if the stations of an electronic positioning system are separated by distances in excess of 200 km (124 mi) ties shall be made to basic first-order control whenever possible.

Normally, a disagreement of cross-check lines with principal sounding lines of three times or more the allowable error stated above indicates error in either position, depth, or both, and should be further investigated.

b. Sweeping over wrecks, obstructions, and shoals The same accuracy as that specified for the measurement of depths (art. B.2.a.) to a depth of 30 m (16 fm). In depths greater than 30 m (16 fm) the same accuracies as for measurement of depth (art. B.2.a.) where the depth and equipment available permit these accuraries.

C. Reference of sounding to vertical datum Location and duration of tidal observations to be such that each sounding can be referred to the sounding datum with an error no greater than one-half that specified in art. B.2.a., above. Tidal reductions are not usually applied to oceanic soundings over 200 m (109 fm). 3. Current measurements

The velocity of the current at each station should be determined to the nearest 0.1 kn and the direction of the current to the nearest 10°.