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sistent throughout the mapped area. However, on small-scale maps covering large areas, the scale can change gradually from one point to another. Depending on the projection, the nominal scale shown may refer to the center of the map or to one or two lines within the map. In a series of maps on the same projection (such as a quadrangle series), the nominal scale may apply to points or lines outside the actual map sheet but within a block of adjacent maps (as along standard parallels of the State plane coordinate systems). Scale variations seldom concern map users working with paper prints and ordinary rulers because paper distortion is likely to be greater than any scale change due to the projection.

ACCURACY

Map accuracy is closely related to map scale. The National Map Accuracy Standards (app. 6) were devised to provide cartographers with practical criteria for certifying their maps. Note that the standards cannot be stated in positive terms because no practical amount of testing can assure them.

The standard for horizontal accuracy requires that no more than 10 percent of the well-defined map points tested be in error more than 0.02 in at the publication scale. The tolerance corresponds to 40 ft (12.2 m) on the ground for 1:24,000-scale maps and about 100 ft (30.5 m) for 1:62,500-scale maps. The standard for vertical accuracy requires that no more than 10 percent of elevations interpolated from the contour lines be in error more than half the contour interval.

The standards for hydrography (app. 6) adopted by the Federal Government are those agreed upon by the International Hydrographic Organization (Accuracy Standards Recommended for Hydrographic Surveys, IHB Special Publication 44, Monaco, 1968). The standards are comparable to the National Map Accuracy Standards; however, they apply to the accuracy of basic control and the vertical datum rather than the graphic product. The basic acceptable horizontal position error is 0.05 in (0.127 cm) on the map, and the acceptable vertical error is 1 percent of the water depth.

REVISION

In a sense, accuracy implies currentness. The compilation date on a map indicates whether its content is likely to be up-to-date. Certain types of maps generally need little revision, especially those showing geology or relief. Major revisions are mainly needed for manmade features. Although minor revi

sions may be needed on nearly all map plates, most of the work concerns such features as new roads, buildings, and reservoirs. Changes also occur in the shape of the shoreline as a result of weathering and erosion. Since rate and amount of change vary greatly from area to area, not all maps are revised at definite intervals and to the same extent. Maps are selected for revision according to the requirements of users for current maps that meet modern standards.

CHARTS

Maps are defined in the glossary and in the section "Maps." A chart is a map on which highly specialized data have been included to serve a specific purposemost commonly, aerial or marine navigation.

Various kinds and types of charts are prepared and published by NOS to promote safety in aerial and marine navigation. Because charts are important to the safety of life and property, accuracy sufficient for the intended purpose is basic in their data acquisition, design, and construction. Data invaluable for coastal activities are available as byproducts from the sources used in chart production.

TYPES

In addition to aeronautical and nautical charts, airport obstruction, isogonic, isopach, and tidal current charts also present data of value in the coastal

zone.

In designing and preparing charts for aerial navigation, features critical for the pilot and navigator are emphasized. Scale must be relatively small because of the flight speed. Terminal control area (TCA), sectional aeronautical, and world aeronautical charts are the most likely to contain coastal information of any value; they are produced at 1:250,000, 1:500,000, and 1:1,000,000-scale, respectively.

Nautical charts (app. 7, fig. 17) are classified according to the specific navigational phase each serves. For example, a sailing chart is appropriate for an ocean crossing, but a coast chart is needed as land is approached or for a voyage generally parallel to the coast. Harbor charts (app. 7, fig. 18) replace the coast charts as vessels enter restricted waterways with congested traffic and navigational hazards. Small-craft charts (app. 7, fig. 19) are convenient aboard small craft and other vessels where space is limited. The primary differences between these charts are scale and the features emphasized. Isogonic charts show lines of equal variation of

the magnetic compass. They are compiled on a suitable base map, usually at a small scale, to show the variation of magnetic bearing and its annual rate of change. The basic data are obtained from field observations.

Tidal current charts (app. 7, fig. 24) provide information about the direction and speed of the current in a specific area at intervals through the entire tidal cycle. They are currently limited to major harbors, but coverage is being extended to important coastal waters. These charts can be valuable in coastal management operations.

With few exceptions, the charts mentioned are basically line drawings, relying on colors and symbols to emphasize significant features. A new series contains an orthophotomosaic base to present information usually shown by lines and symbols. Landmarks and similar features important to navigators are emphasized by color or symbolization.

FORMAT

The different charts are issued in various formats, depending principally on the intended use and the environment.

Aeronautical charts (app. 7, fig. 35) of value to coastal activities cover differing areas and are normally issued in accordion-folded format. Terminal Control Area charts, available only for selected. major airports, cover the immediate vicinity. They vary little in physical size. Sectional aeronautical charts (scale 1:500,000) cover areas defined by geographic coordinates. They are printed on both sides, each covering 2° latitude and from 6° to 8° longitude. Because of the smaller scale (1:1,000,000), world aeronautical charts differ from sectional charts in the area covered. Each side covers 4° latitude and from 12° to 16° longitude. Airport obstruction charts cover approximately circular areas that include runway approach and departure zones, and are fairly uniform in dimensions. They are not published, but diazo copies are produced as needed.

Nautical charts vary widely in physical dimensions. They are designed to cover all or part of a body of water, or a section of coast between major ports or other important geographic features. When practical, coverage and dimensions of individual charts within each series are kept uniform for the convenience of the mariner. Depending on the area of coverage, isogonic charts vary widely in dimensions and are usually small scale. Tidal current charts are issued in bound volumes consisting of 12 or 13 diagrams prepared on the same base.

ACCURACY

Navigational charts are relied on for safety in two major transportation systems. Accuracy is therefore a major concern in production. Accuracy requirements vary with the kind of chart and are most critical for nautical charts.

Base maps that meet National Map Accuracy Standards provide the terrain data for aeronautical and nautical chart bases. Base maps for nautical charts, except in rare instances, are prepared by NOS from aerial photographs at a scale at least twice that of the final chart. When a new hydrographic survey is not scheduled, base topography is sometimes prepared directly at charting scale. Features critical to safe marine navigation are mapped to standards stricter than the national standards. For example, the shoreline (usually the mean high water line) and the mean low water line must be plotted within 0.5 mm (at map scale) of the true position, about 16 ft (4.9 m) on the ground at 1:10,000 scale, compared to 28 ft (8.5 m) under the national standards. Fixed aids to navigation and objects to be charted as landmarks must be plotted within 0.3 mm (about 10 ft.-3 m-at 1:10,000 scale) of true position. Similar accuracy is not required for aeronautical charts, but radio navigational aids and obstructions to safe operation of aircraft are located and charted with accuracy suitable for publication scale. Bases for all the other types of charts usually are prepared from the most suitable aeronautical or nautical charts.

SCALE AND CONTENT

Chart content depends on use and publication scale. At large scales more detail can be shown without congestion and legibility can be increased with larger symbols and type, but areal coverage is reduced. At small scales larger areas can be covered, but details must be omitted to avoid congestion.

Chart content of value in coastal activities varies with the kind of chart. In general, small-scale charts are suitable for general planning or for preparing gross inventories. For example, sectional aeronautical charts can be used for planning, but large-scale charts are of the greatest value to users. Scale can be changed by photographic or mechanical enlargement or reduction. However, enlargement is not recommended (except as an expedient) because errors made in producing the chart are enlarged so that the new larger scale chart is

not as accurate as its scale implies. Excessive photoenlargement can reduce legibility.

Some chart data may be common to several different kinds of charts. Shoreline, for example, is critically important to a ship's navigator and is always shown prominently on nautical charts. The shoreline is not as important to an aircraft navigator, and is therefore less prominently displayed on aeronautical charts.

REVISION

Obsolete contents can quickly destroy a chart's utility. Charts become obsolete mainly by the acts of man. Most aeronautical charts are revised semiannually; those of remote areas are revised less often. Changes usually affect airways data and navigational facilities and they therefore have little affect on the coastal zone. Nautical charts are usually revised annually for congested areas. Charts of remote areas are revised less often. Isogonic charts are recompiled every 5 years; isopach charts have no established revision schedule. Airport obstruction charts are revised every 3 years when resources permit-less frequently for some smaller airports with light traffic.

OVERPRINTS AND OVERLAYS

Overprinting is printing additional information on an already printed map. An overlay is a transparent or translucent photograph or drawing at the same scale as the base map made to register with it. The added information shows new details without changing the original.

USES

Overprints can be used to add new roads, buildings, dams, and reservoirs to original maps with a new press run. An extra color for revision data permits users to readily identify and evaluate changes (fig. 23). Overprints can also be used to direct attention to special data, such as by adding an arrow pointing to the particular feature.

Overlays can provide the same information as an overprint. However, they are used in conjunction with the original base map and do not become a permanent part of it. Sometimes several overlays are used in combination to show changes over a prolonged period.

Overlays are used in the planning and production of maps in the following typical ways:

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Compiling administrative data (costs, priori-
ties, and locations of field parties).
Planning control.

Planning, procuring, and annotating photo-
graphs.

• Editing compilation manuscripts.

• Layout of contour numbers or soundings. Name placement and type style.

The USGS Land Use Data and Analysis (LUDA) program is an example of the effective use of overlays. The 1:250,000-scale topographic maps are used as bases to produce overlays (film positives) showing classification of land uses throughout the country. These film positives can be used in conjunction with the base map, or they can be combined with selected plates from the base map and published as land-use maps.

Figures 24 and 25 show a base map and a corresponding overlay locating oil and gas fields.

EXAMPLES OF OVERLAYS FOR COASTAL
MANAGEMENT

Although overprints can be used in coastal mapping, overlays are usually more suitable. Examples

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FIGURE 23.-Revision by purple overprint. From the USGS Newport, R.I., 7.5-min topographic quadrangle map.

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ADVANTAGES AND LIMITATIONS

Overprints can be used to keep a map from becoming cumbersome, in cases where more than one overlay would be required. However, because an overprint is a permanent part of a map, the entire map may have to be revised if the overprint becomes undesirable. Overprinting requires at least one additional press run and register may not be exact because of printing on top of published maps whose size may have changed with time. Overprinting must be limited to data that will not cause cluttered detail or confusion.

The greatest advantage of overlays is that they avoid crowding map detail. Overlays, in keeping with the feature-separation system, show unlimited types of information related to the same base map and to each other. Production costs for small quantities of overlays can be less than that for overprinting published maps. Nevertheless, overlays usually re

VISUAL EXTRACTION

Whoever reads a map is extracting data. Whether he is conscious of the fact or not, a user selects information that he wants and disregards the rest. A person who reads a map to find his way visually interprets the data and chooses his route. Some readers may be interested in interpreting every aspect of specific map features, by mental calculation or with aids such as scales, protractors, compasses, cartometers, and planimeters.

FEATURE SEPARATION

As explained in the section, "Photogrammetric mapping techniques," press plates for multicolor maps are produced from color-separation guides. Each color can be broken down further. For example, one blue pressplate can be made from several blue guides one for lakes and ponds, one for streams, and one for swamps.

There can be greater separation, as the following list shows:

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