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show what happens in the intervals. Tinting or shading is sometimes used between isopleths.

There are various types of thematic maps, including geologic, forestry, soil, land use, slope, and historical. Geologic maps portray a number of different geologic conditions. Forestry maps show the size, density, kind, and values of trees in a given area. Soil maps portray soil conservation parameters. Land use maps depict land use by colors, letters, or numbers within small areas.

Slope maps use colors or shades to graphically represent different degrees of slope and are used in studies related to land use. Slopes can be determined mechanically by the distance between contours on a relief map, or they may be generated by computer printout from digital terrain data.

Thematic maps dedicated to the explanation of the past are called historical maps. They show such features as battlefields, military routes, and boundary changes. Although all maps are historical because of the time interval between compilation and publication or use, not all historical maps are thematic. Only those intended to illustrate statistics can be classified as thematic.

DIGITAL MAPS

The digital map is a recent addition to map classifications. It is a graphic produced automatically from a computer bank of map data. Often the user can retrieve selected information either in map or list form (see "Future outlook").

LINE MAPS

Any map produced from scribed, inked, or pasteon line copy is considered a line drawing or line map.

PHOTOMAPS

The photomap is an alternative to the line map-it shows nonselective details requiring photointerpretation by the user. Any aerial photographic image can be considered a photomap. However, although aerial photographs are map substitutes because they show surface features, they may contain serious scale distortions caused by camera tilt or topographic relief. Most photomaps include some cartographic enhancement to help the user-perhaps only marginal information, an overprinted line drawing, or names.

Aircraft tilt and relief cause certain image distortions and displacements on photographs. Distortions caused by camera tilt can be removed by simple

rectification. Distortions caused by relief can be removed by an Orthophotoscope or other differential-rectification system which produces orthophotographs, correct in scale and relative position.

Orthophotoquads and orthophotomaps have been developed using simple and differential rectification, which remove tilt- and relief-caused scale distortions. Both are made from an orthophotograph or an orthophotomosaic, and both contain marginal information including grid and projection lines or ticks. Orthophotoquads are black-and-white or color orthophotographic images in quadrangle format with a minimum of cartographic treatment. The major highways and a few principal places or features are labeled for orientation. The orthophotoquad user must interpret most features for himself. Orthophotomaps are hybrids that contain both orthophotoimagery and cartographic symbolization including color separation and enhancement. Color enhancement can often present features in colors more easily recognized than those nature provides. A variety of shades of green, blue, and brown accentuate such detail as saltwater encroachment, marshland limits, faults, and physical characteristics of prominent geologic features. With additional cartographic treatment (contours, elevations, boundaries, labels), orthophotomaps require approximately the same production effort as line maps, but the photoimagery is an advantage in the study of special interest areas by hydrologists, geologists, and other earth scientists.

FORMAT

Maps generally have a rectangular format that may include the entire area of a political division of interest. Generally, some map detail is shown beyond the political boundary to fill the entire area within. the outer drafted neatline. However, sometimes compilation ends at the political boundary, and the map then conforms to the shape of the political unit. A quadrangle map is bounded by meridians of longitude and parallels of latitude. Examples of standard quadrangle formats are 7.5'x7.5', 15'x15', 30'x1°, and 1°x2°. Adjacent quadrangles can be fitted together to form a large continuous map. Maps and charts of water areas generally are bounded by meridians and parallels.

CONTENT

General-purpose maps contain all or some of the following data:

• Control.

a. Horizontal and vertical stations. b. Spot elevations.

• Reference systems.

a. Geographic (latitude and longitude).

b. Universal Transverse Mercator grid.
c. State plane coordinate grids.

d. Public land system (township and section
lines and corners).

• Culture-roads, buildings, dams, utility lines, and other works of man.

• Drainage-swamps, rivers, streams, ponds, lakes, and oceans.

• Vegetation-woodland, scrub, orchards, and vine

yards.

• Relief-contours, hachures, form lines, color gradients.

CONTOUR INTERVAL

The contour interval is the difference in elevation between adjacent contours on a map. Selection of the interval is a basic consideration in topographic and bathymetric mapping, depending mainly on scale and to some extent on the intended use of the map. A large interval may be acceptable for small-scale and reconnaissance maps, but natural resource development usually requires an interval small enough to portray all significant details of the terrain. Too small an interval may cause contour crowding or show insignificant details; too large an interval may prevent adequate representation of the terrain. An interval that is appropriate at one scale may not be satisfactory at another.

Occasionally, selecting a contour interval is difficult because portions of the terrain or sea bottom need a smaller interval between contours more than other portions. For example, a 5,000-ft interval on a bathymetric map may miss a 3,000-ft mountain range within a 6,000-ft deep basin. Supplementary contours (dashed or dotted) are frequently shown between the regular contours in parts of a map where finer detail would otherwise be missed. USGS adopted standard contour intervals of 5, 10, 20, 40, and 80 ft for large- and intermediate-scale topographic maps. Standard intervals for the 1: 250,000-scale series are 25, 50, 100, and 200 ft. Other contour intervals are used for larger scales and for maps of Alaska. (See "Future outlook" for how conversion to the metric system affects these intervals.)

The National Ocean Survey has established standards for using contour intervals on bathymetric maps and nautical charts based on the accuracy of

measured depths, the spacing of the sounding lines, type of topographic detail encountered, and slope. In accordance with the International Hydrographic Bureau SP 44 and the NOS Hydrographic Manual (see app. 6.), continuous profile sounding lines are spaced at 1.0 cm (0.4 in) or less at the scale of the survey (not applicable to old surveys). Inshore at entrances to harbors, in areas adjacent to spits or rocky points, and in areas where major changes in bottom contours are found, the spacing is frequently reduced to 0.5 cm or less. The maximum allowable errors for depth measurements are:

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The scale of a map is one of its most important characteristics. Scale is the mathematical relationship between a unit distance on the map and its corresponding distance on the ground, and may be expressed as a representative fraction or ratio; thus 1/24,000 (or 1:24,000) indicates that any linear measurement on the map represents 24,000 times that measurement on the ground. The same scale may also be shown by giving a unit distance on the map and its equivalent on the ground; for example, 1 cm on the map represents 24,000 cm (240 m) on the ground.

The terms "small scale" and "large scale" are relative and refer to ratios with large and small denominators; for example, 1:24,000 is a larger scale than 1:250,000. Figures 20-22 show the Stafford, Va., area at different scales, 1:250,000, 1:50,000, and 1:24,000. A bar scale (graphic scale) is normally placed in the margin or legend of a map to aid linear measurements. The bar scale is a line of convenient length which is subdivided and labeled with the distances (generally in feet, miles, and kilometers) that its parts represent on the Earth.

The amount of detail that can be shown on a map varies with the scale. Unexpected problems sometimes occur when the map scale is changed photographically. Enlargements can make symbolization too big; reduction can make them too small. Changing scale using a pantograph allows changes in symbol size and detail. Some problems in map scale reduction can be solved by removing some of the separation guides. The map user must consider the effect that a change in scale has on detail.

On large-scale maps, such as standard 7.5-min topographic quadrangles, the scale is generally con

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FIGURE 20.-Stafford, Va., area at 1:250,000 scale. From the USGS Washington,
D.C.-Md.-Va., 1:250,000-scale topographic quadrangle map.

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FIGURE 21.-Stafford, Va., area at 1:50,000 scale. From the USGS Stafford County, Va., topographic map.

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FIGURE 22.-Stafford, Va., area at 1:24,000 scale. From the USGS Stafford, Va.,
7.5-min topographic quadrangle map.

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