ing wherever map detail prevented light from reaching the sheet so that a negative image of the original is produced.

The etching process is used when a finished positive copy is available, particularly if the reproduction contains a large quantity of numbers and symbols. Often contours are thus reproduced when a map is being revised. Generally, contours do not need as much revision as other map features so that contour guides frequently can be reused; the etching process is a quick and easy way to prepare copy. The disadvantage is that the lines sometimes print less crisply than desired. However, minor repair work can be done with a simple scribing needle in much less time than it would take to rescribe or reink the entire plate.

Solid-tint areas, which symbolize water, woodland, and urban areas on topographic maps or areas of equal distribution on thematic maps, usually are produced by peeling or stripping. An outline of the tinted area is compiled and contact-printed onto a

sheet coated with a special "peelcoat" material. The line image is etched into the peelcoat, and with the etched lines as a boundary, the tint areas are easily peeled away with a small knife, leaving open windows (fig. 17).

Sometimes lettering and symbols are prepared on transparent adhesive material called stickup or stripfilm (fig. 18), positioned on the appropriate manuscript, and burnished or rubbed on to adhere. Lettering can also be applied manually or with an electronic lettering machine.

If a map is to be multicolored, color-separation guides (not to be confused with feature-separation guides) must be made. The normal separation of colors is:

At least one color separation guide must be prepared for each color, containing only the data to be printed in that color. These guides can be scribed, film, or open-window negatives. For good register, they are prepared on a printed image reproduced from the compilation manuscripts. The important difference between compilation manuscripts and guides is that the manuscripts are normally drawn right-reading while guides are normally drawn wrong-reading because they are used for the contact exposure of photosensitive pressplates.

To make a color composite proof for checking, the color-separation guides are successively contact printed in their assigned colors on a white base. The editor easily identifies errors and indicates changes on the guides. In a complex multicolored map, such as a topographic map, corrections can sometimes cause new errors. Therefore, the editing-correctingediting cycle should be repeated once or twice before publication.

Final review is the last phase before map repro

duction. The manuscript and accompanying data are examined for accuracy, adequacy, completeness, and conformance to specifications. The reproduction materials are sent to the printing plant, and the related materials are sent for archival storage. Depending on agency or company policy, when all maps in an individual project are finished, the reviewer may prepare a report summarizing information pertaining to the production of the project, supplementing reports prepared for individual maps. The reports contain preparation data that cannot be shown on the maps and that do not duplicate any of the map content. The reports can be invaluable in answering questions about how the maps were prepared, especially in case of litigation.

MAP REPRODUCTION

Most maps are now printed in quantity on rotary offset presses from aluminum pressplates. For only limited distribution, manuscripts can also be repro

duced photomechanically. Other types of printingsuch as letterpress, gravure, and diazo-are described by Melcher and Larick (1966). The aluminum pressplates are prepared by contact photography using the color-separated reproducibles, one plate for each color. Multiple exposures of a plate are needed if all the features to be printed in one color have not been combined into a single reproducible, for example, drainage detail may be on a scribed drawing with water tint on a separate openwindow drawing. Various types of screens are used to produce tints and shades by blocking out specific percentages of the image. Therefore, it is possible to print several shades of a color from one pressplate. In offset printing the ink image is transferred from the pressplate to an intermediate rubber blanket and from the blanket to the paper, forming

one impression. For multicolor maps, a separate impression is needed for each color. Moreover, the impressions must be in exact register with each other to form an accurate map.

For the standard five-color topographic map (black, blue, brown, red, and green) the impressions can be obtained in five separate runs through a single press, which must be shut down, washed, replated, and reinked between runs. The same five impressions are more efficiently obtained on a fivecolor press (fig. 19), which consists of five press units operating in tandem, each unit printing a different color. Blank paper is fed into one end of the press, and completely printed maps emerge at the other end. A large volume of maps must be produced to justify spending the money needed to buy a fivecolor press.

MAPS

A map is a graphic representation, usually on a plane surface and at an established scale, of selected natural and manmade features on or below the surface of the Earth. Symbolization is used to enhance certain details according to the intended purpose of the map.

The number of types of maps that can be made for different uses is practically unlimited. Generally, however, maps can be classified in one or more of the following categories: planimetric, topographic, thematic, digital, line, or photographic.

PLANIMETRIC MAPS

Planimetric maps present the horizontal position of selected features but do not show relief in measurable form. Examples of planimetric maps are base, cadastral, line-route, and outline maps.

Base maps are used to plan or to compile data for the production of specialized maps.

Cadastral maps show the boundaries of subdivisions of land (usually with bearings and lengths and the areas of individual tracts) for describing and recording ownership. One type of cadastral map is the plat which often constitutes, or is an essential part of, a legal description of a parcel of land. The Bureau of Land Management is the major single producer of land plats.

Line-route maps are used by utility companies. They show the routes and type of construction of pipelines or wire circuits, plus the locations of facilities such as switchboards, valves, and telegraph stations.

An outline map presents only the information needed to provide a basis for the compilation of additional data. Outline maps often show only national and State boundaries and major drainage systems.

TOPOGRAPHIC MAPS

In addition to the features shown on planimetric maps, topographic maps portray the shape and elevation of the terrain, usually by contours, form lines, shading, color gradients, or hachures. Any map portraying relief by one of these conventions can be called a hypsometric map. A map on which the elevations are referred to a specific datum is called a hypsographic map. Standard topographic maps are in both categories.

In the United States the best known type of topographic map is the quadrangle series, which range in scale from 1:20,000 to 1:250,000. The quadrangle series are used in development, for selecting

industrial sites, planning highways, routing utility lines, selecting damsites, and locating communication facilities. They are also popular in recreation for hunting, fishing, and camping.

Other types of topographic maps include engineering, flood control, landscape, and bathymetric. Engineering maps are used for planning and cost estimating for projects. Flood control and storm evacuation maps are special-purpose topographic maps used to study areas subject to flooding. Landscape maps are used by architects to plan buildings that will conform to the topography of the site. Gardeners use landscape maps to maintain parks, playgrounds, and private estates. Bathymetric maps show water depths and underwater topography. Water depth ranges are generally portrayed by various colors or shades. Usually, uniform depth intervals are connected by solid lines called bottom contour lines. USGS and NOS produce bathymetric maps.

THEMATIC MAPS

Thematic maps are also called geographic, specialpurpose, or distribution maps. They emphasize a single topic such as geology, climatology, or crop distribution, and the entire map is devoted to presenting this distribution or concentration. Geographers use thematic maps to show the distribution of subjects such as population, languages, crop production, soil, climate, vegetation, land use, and industry. The distributions are shown by several methods, including dots, choropleths, or isopleths.

Dots are used to represent quantities such as 1,000 people or 500 acres of corn. The size and value of dots are selected so that the dots coalesce in areas of densest distribution. Sometimes dots of varying sizes are used for different quantities.

Choropleth maps are thematic maps in which sections determined by civil boundaries or other arbitrary division are colored, shaded, dotted, or hatched to make darker or lighter areas in proportion to the density of distribution. Because of the arbitrary selection of sections, choropleth maps can be somewhat misleading by showing an abrupt change between sections where the change is actually gradual.

Isopleth maps are used to show numerical values for continuous distributions (such as rainfall and temperature) rather than discrete variables. Isopleths are lines connecting places of equal value of distribution and have the same inherent virtues and deficiencies as contour lines. They provide exact information throughout their entirety but do not