Map scales and classifications

Map scale refers to the size of the representation on the map as compared to the size of the object on the ground. The scale generally used in architectural drawings, for example, is ¹/₄ inch to one foot, which means that ¹/₄ of an inch on the drawing equals one foot on the building being drawn. The scales of models of buildings, railroads, and other objects may be one inch to several feet. Maps cover more extensive areas, and it is usually convenient to express the scale by a representative fraction or proportion, as 1/63,360, 1:63,360, or “one-inch-to-one-mile.” The scale of a map is smaller than that of another map when its scale denominator is larger: thus, 1:1,000,000 is a smaller scale than 1:100,000. Most maps carry linear, or bar, scales in one or more margins or in the title blocks.

Nautical charts are constructed on widely different scales and can be generally classified as follows: ocean sailing charts are small-scale charts, 1:5,000,000 or smaller, used for planning long voyages or marking the daily progress of a ship. Sailing charts, used for offshore navigation, show a generalized shoreline, only offshore soundings, and are at a scale between 1:600,000 and 1:5,000,000. As an illustration of chart use, a 10-knot ship covers about 29 inches (74 centimetres) at 1:600,000 scale in a day.

General charts are used for coastwise navigation outside outlying reefs and shoals and are at a scale between 1:100,000 and 1:600,000. Coast charts are intended for use in leaving and entering port or navigating inside outlying reefs or shoals and are at a scale between 1:50,000 and 1:100,000. Harbour charts are for use in harbours and small waterways, with a scale usually larger than 1:50,000.

In rare instances reference may be made to the areal scale of a map, as opposed to the more common linear scale. In such cases the denominator of the fractional reference would be the square of the denominator of the linear scale.

The linear scale may vary within a single map, particularly if the scale is small. Variations in the scale of a map because of the sphericity of the surface it represents may, for practical purposes, be considered as nil. On maps of very large scale, such as 1:24,000, such distortions are negligible (considerably less than variations in the paper from fluctuations of humidity). Precise measurements for engineering purposes are usually restricted to maps of that scale or larger. As maps descend in scale, and distortions inherent to their projection of the spherical surface increase, less accurate measurements of distances may be expected.

Symbolization

Symbols are the graphic language of maps and charts that has evolved through generations of cartographers. The symbols doubtless had their origins as simple pictograms that gradually developed into the conventions now generally used.

Early cartographers recognized that common usages and conventions would minimize confusion and to some extent simplify compilation and engraving. Efforts in this direction were made over the years, but cartographers, being artists of a sort, preferred to vary their styles, and effective standardization was not achieved until comparatively recent times. National agencies in most countries established conventions with due regard to practices in other countries. International Map of the World agreements, NATO conventions, and the efforts of the United Nations and of international technical societies aid standardization.

Symbols may be broadly classed as planimetric or hypsographic or may be grouped according to the colours in which they are conventionally printed. Black is used for names and culture, or works of man; blue for water features, or hydrography; brown for relief, or hypsography; green for vegetation classifications; and red for road classes and special information. There are variations, however, particularly in special-purpose series, such as soil and geologic maps. Symbols will also vary, perforce, because of limitations of space in the smaller scales and the feasibility of drawing some features to true scale on large maps. Legends explain the less obvious symbols on many maps, while explanatory sheets or booklets are available for most standard series, providing general data as well as symbol information. When less familiar symbols are used on maps they are often labeled to prevent misunderstanding. The general located-object symbol, with label, is often used in preference to specific symbols for such objects as windmills and lookout towers for similar reasons.

Planimetric features (those shown in “plan,” such as streams, shorelines, and roads) are easier to portray than shapes of land and heights above sea level. Mountains were shown on early maps by sketchy lines simulating profile or perspective appearance as envisioned by the cartographer. Little effort was made at true depiction as this was beyond the scope of available information and existing capabilities. Form lines and hachures, among other devices, were also used in attempting to show the land's shape. Hachures are short lines laid down in a pattern to indicate direction of slope. When it became feasible to map rough terrain in more detail, hachuring developed into an artistic speciality. Some hachured maps are remarkable for their detail and fidelity, but much of their quality depends on the skill of draftsman or engraver. They are little used now, except where relief is incidental.

Contours are by far the most common and satisfactory means of showing relief. Contours are lines that connect points of equal elevation. The shorelines of lakes and of the sea are contours. Such lines were little used until the mid-19th century, mainly because surveys had not generally been made in sufficient detail for them to be employed successfully. Mean sea level is the datum to which elevations and contour intervals are generally referred. If mean sea level were to rise 20 feet (six metres) the new shoreline would be where the 20-foot contour line is now shown (assuming that all maps on which it is delineated are reasonably accurate).

The quality of contour maps, until recent times, depended largely on the sketching skill of the topographer. In earlier days funds available for topographic mapping were limited, and not much time could be spent in accurate placement. Later, the accurate location of more control points became feasible. An approximate scale of reliability is therefore indicated by the date of a topographic survey, taking into account the respective situations that existed in various countries. Modern surveys, being based on aerial photos and accurate plotting instruments, are generally better in detail and accuracy than earlier surveys. The personal skill of individual topographers, long a factor in map evaluations, has therefore been substantially eliminated.

Hill shading, or shaded relief, layer or altitude tinting, and special manipulations of contouring are other methods of indicating relief. Hill shading requires considerable artistry, as well as the ability to visualize shapes and interpret contours. For a satisfactory result, background contours are a necessary guide to the artist. Hypsographic tinting is relatively easy, particularly since photomechanical etching and other steps can be used to provide negatives for the respective elevation layers. Difficulty in the reproduction process is sometimes a deterrent to the use of treatments involving the manipulation of contours.

In the past, three-dimensional maps were laboriously constructed for studies in military tactics and for many other purposes. They were costly to produce, as contour layers had to be cut and assembled, filled with plaster and painted, after which streams, roads, etc., had to be drawn on the surface. Lettering then was applied, and models of large structures, such as buildings and bridges, were added. In view of the time and cost involved in such productions, they were sparingly used until recent years when better production methods and materials became available. During and after World War I a process was developed and improved whereby an aluminum sheet was “raised” by tapping along the contours copied on its surface. When the contours selected for tapping were completed, the sheet became, in effect, a mold for shaping plastic sheets to its convolutions. The map was printed on plastic sheets prior to the thermal process of shaping them to the mold. Sets of relief maps were soon produced in this manner for use in schools, military briefings, and many other activities.

During and after World War II the production of plastic relief maps was greatly expanded, while the processes and equipment were further improved and refined. Most significant among these developments was a pantograph-router, which cuts a model from plaster or other suitable material as the selected contours are followed by the operator on a topographic map. This eliminated the distortions inherent in shaping metal sheets by the tapping process. Selected topographic maps are now published in limited relief editions for military instruction, special displays, and general classroom instruction.

Most relief maps are exaggerated several fold in the vertical scale. The Earth is remarkably smooth, when viewed in actual scale, and many significant features would hardly be distinguishable on a map without some vertical exaggeration. Mt. Everest, for example, is actually only one-seventh of 1 percent of the Earth's radius in height, or only one-third of an inch (about eight millimetres) at a scale of 1:1,000,000. For this reason relief is usually shown at five, or even 10, times actual scale, depending upon the nature of the area represented. This exaggerated relief scale is always explained in the map legend.