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GEOLOGICAL SURVEY CIRCULAR 368

FEATURES SHOWN ON

TOPOGRAPHIC MAPS

GEOLOGICAL SURVEY

TOPOGRAPHIC INSTRUCTIONS

Chapter 1 B 3

FEATURES SHOWN ON TOPOGRAPHIC MAPS

Contents

Abstract ............... Principles of map representation

1. Definition of a n1ap • • a. Topographic map......... b. Planimetric map .....
2. Elements of map construction. a. Colors and classes of features.. b. Lines and symbols.. .... c. Letters and numbers.. •.. d. Map scale........... 3.. Problem of selecting features.
3. Selecting map features for a purpose
4. Other factors in selecting map

Page

features........ 7 a. Legibility...... 7 b. Cost of production..... 8 c. Obsolescence......... 8 d. Concept of a landmark 8 Map delineation of topographic relief 8

6. Methods and objectives 8 a. Measurement......... 9 b. Interpretation.... •... 9
7. Selection in mapping relief features. 9 a. Choosing a contour interval 9 b. Generalization........ 10
8. Topographic expression 10
9. Kinds of relief features 10 a. Erosional features... b. Residual features ... c. Depositional features..
10. Symbols and patterns used to show relief ........

a. Hachures.... 12

b. Area patterns.. 12

c. Relief shading.... 15 Map representation of water features. 15

11. Periodic changes in water features. 15 a. Classification of water features. 16 b. "Normal" water level ....... 16

Page

Map representation of water features--Continued

12. Kinds of water features and map

symbols.......... 16 a. Linear watercourses.. •... 16 b. Area water features....... 17 c. Glaciers and permanent snow- fields •....•. d. Springs and wells. e. Coastal features. • Manmade map features.. •

13. Coordinate systems, subdivisions, and boundaries... •... a. Coordinate systems • • • • b. Public-land subdivisions c. Boundaries..... •..
14. Travel and transportation routes. a. Roads and trails. •. b. Railroads .•..•... c. Other route features
15. Buildings and cities .. a. B11ildings ..•.. b. Building groups.. c. Towns and cities
16. Names, notes, and numbers • a. Map names. • • •. •. b. Geodetic control marks. c. Spot elevations •.... Information on map margins. •.

17. Identification, orientation, and

explanation... •. •.. a. Identification...... b. Responsibility, methods, and dates •. •. •. • ••• c. Scale, contour interval, and other data ..• Map references •. •.....

18. Special selected list.

19. Other reference maps

Chapter 1 B 3 1

FEATURES SHOWN ON TOPOGRAPHIC MAPS

ABSTRACT

Topographic maps represent features on the earth's surface by means of symbols and labels; separate colors distinguish the main classes of map features. The amount of information shown on a topographic map depends on the map scale, the purpose of the map, and the cost of obtain- ing the information. This chapter describes the three main categories of map information and the methods by which they are shown. Reference lists of published maps and a chart illustrating standard map symbols are included.

PRINCIPLES OF MAP REPRESENTATION

1. Definition of a map

A map is a graphical representation, at an established scale, of a part of the earth's sur- face, showing important natural and manmade features in their correct positions relative to a coordinate reference system and to each other.

a. Topographic map. --A topographic map, as distinguished from other kinds, portrays by some means the shape and elevation of the ter- rain. Geological Survey topographic maps usu- ally represent elevations and laNdforms--the shapes into which the earth's surface is sculp- tured by natural forces--by contour lines. Other features are shown by a variety of conventional signs, symbols, lines, and patterns, which are printed in appropriate colors and identified by names, labels, and numbers. The chart facing page 22 shows the standard symbols used on the topographic maps of the Survey.

b. Planimetric map. --A planimetric map is similar to a topographic map, except that it does not portray relief in a measurable form. The Survey publishes some planimetric maps for use until they can be replaced by topographic maps. (^1) The number 1 B 3 signifies Book 1, Part B, Chapter 3 of the Geological Survey looseleaf manual of Topographic Instructions. For a table of contents, see Chapter 1 A 2 (Circular 92).

2. Elements of map construction

A topographic quadrangle map is the prod- uct of both engineering and the graphic arts. Surveys made on the ground and from photographs are organized in a coherent form and reproduced graphically according to a plan. To be useful the map must present information legibly. The graphic accuracy must be consistent with the accuracy of the source surveys and with the pub- lication scale. The map must include the infor- mation essential to its purpose and must exclude nonessentials. The main -elements of quadrangle map construction have been formulated to attain these objectives.

a. Colors and classes of features. --The information shown on the quadrangle map is divided into three general classes, each printed in a different color. Information about the shape of the land surface--hypsographic or topographic information--is printe'd in brown. Water fea- tures--hydrographic information--are shown in blue. Cultural features--manmade objects·-- are shown in black. The system of division is not precise. Some manmade features--for ex- ample, levees and earth dams--are also topo- graphic features and are printed in brown, ·not black.

6 (Ch. 1 B 3) TOPOGRAPIDC INSTRUCTIONS

Besides the colors used for the three main classes of features, green is used to show wood- land-- timber, brush, vineyards, and orchards-- and red is used to show public-land subdivisions (cultural features) and the classification of the more important roads.

b. Lines and symbols. --Linear features are represented by lines of various weights and styles (solid, dashed, dotted, or some combina- tions). Structures, or individual features, are portrayed by a system of pictographs or symbols. The symbols originated as plan views of the ob- jects they represent, and they retain something of this character although they are now formal- ized. The building symbol, for example, is a solid or open square. The railroad symbol is a line with evenly spaced cross ties. The dam and levee symbols look approximately like dams or levees as seen from the air. (See fig. 1. )

D j^ (

• • ////I^ I\'.'\

II

BUILDINGS DAM

,,,,''.... "''"''',,,,,,, ~' ,,,, (^) ,, '•,

RAILROAD (^) LEVEE

Figure 1. --Pictographic map symbols.

c. Letters and numbers. --Because lines and symbols cannot represent map information completely, they must be supplemented by the names of places and objects. Notes must be added to explain some features that cannot be depicted clearly by symbols alone. In mapping

topographic features the information portrayed by contour lines must be supplemented by eleva- tion figures. Letters and numbers are essen- tial to map interpretation, but they tend to ob- scure other map information. Therefore, they must be selected and positioned carefully on the map so that interference with other detail is kept to a minimum.

d. Map scale. --Map scale is the relation- ship of the size of the map to the size of the ground area it represents. The relationship may be expressed as a linear equivalent, such as 1 inch equals 1 mile--meaning that 1 inch measured on the map represents 1 mile on the ground--or it may be expressed in many other ways. The scale of Survey maps is given in the form of a ratio or fraction, without dimensions; the numerator is the distance on the map, and the denominator is the corresponding distance on the ground. For example, 1:24,000 means that one unit of length on the map represents 24, similar units on the ground.

3. Problem of selecting features

The amount of information that can be de- picted legibly on a map is limited by the scale of the map. The effect of scale on map content is emphasized if the scale is considered as a ratio of areas rather than a ratio of distances. Figure 2 illustrates the actual paper areas rep- resenting 1 square mile at 2 standard scales for topographic mapping.

The major problem in map compilation is to make the best use of the available map space. The space cannot be crowded with lines and symbols beyond a definite limit without making the map unreadable, yet the amount of informa- tion that might be useful or desirable is almost unlimited. The cartographer must select the features that are the most valuable to the map user. The smaller the map scale, the more critical and difficult the problem of selection becomes.

4. Selecting map features for a purpose

Topographic maps are often made for a particular purpose. For example, a map made

8 (Ch. 1 B 3) TOPOGRAPHIC INSTRUCTIONS

Symbols larger than scale size take up extra map space; therefore, where small features are close together, all of them cannot be shown. Generally, the less important features are omit- ted in congested areas.

b. Cost of production. --The extent to which some kinds of map features are shown is deter- mined partly by the cost of compiling the informa- tion. Aerial photographs are the source of most map information, but features that cannot be identified on mapping photographs must be obtain- ed by field methods, a procedure that is relative- ly more expensive. As an example, not all sec- tion corners are mapped; they are too small to be seen on aerial photographs, and the cost of map- ping all of them by field surveys would be exces- sive.

c. Obsolescence. --Not only the original cost but also the cost of keeping the map up to date is considered in deciding what features to map. Generally, the more features depicted, the more quickly the map becomes out of date. Cul- tural features are especially subject to change. If the maps are to have a reasonably long useful life, the features portrayed must be restricted, to some extent, to relatively permanent.objects.

d. Concept of a landmark. --Many kinds of features are shown on some maps, although omitted from others, because of the landmark character of the features. In this sense, a land- mark is an object of sufficient interest in relation to its surroundings to make it outstanding. For example, buildings may be considered landmarks when they are used as schools or churches or when they have some other public function. They may be landmarks also because of their outstand- ing size, height, or design; or they may be land- marks because of their history, such as old forts or the birthplaces of famous men.

The same principle is applied to features other than buildings, but the adjacent area always is considered in relation to the object. Where map features are few, objects that would not be shown in more congested districts may be map- ped as landmarks.

MAP DELINEATION OF TOPOGRAPHIC

RELIEF

6. Methods and objectives

Contour lines are the principal means used to show the shape and elevation of the land sur- face. Other means are spot elevations and hachures and pattern symbols for special kinds of relief features that are not suited to contour- ing. Relief information is printed in brown on topographic maps.

Con tours are lines connecting points of equal elevation. They always are continuous lines, and, if the map is large enough, they re- turn to the points of beginning to form closed loops. A contour may be variously defined as--

An imaginary line on the ground, every point of which is at the same elevation above a specified datum surface (mean sea level for topographic maps of the Survey).

A level or grade line.

A line of constant elevation.

A coastline or shoreline of level water.

An assumed shoreline resulting from the assumed rising of a body of level water.

However it is defined, a contour is the line traced by the intersection of a level surface with the ground. A series of contours is traced by a series of level surfaces, a different contour for each elevation. Contours are illustrated in figure 3. Each contour line on the map repre- sents a definite ground elevation measured from mean sea level, and the contour interval is the difference in elevation between adjacent contours. The contour interval, together with the spacing of the contour lines on the .map, indicates the slope of the ground. On steep slopes the lines are more closely spaced than on gentle slopes.

FEATURES SHOWN ON TOPOGRAPHIC MAPS (^) (Ch. 1 B 3) 9

To make maps more readable, contours are classified and the classes distinguished by different weights and styles of lines. Index con- tours--every fourth or fifth contour, depending on the basic interval- -are accentuated by making the line wider than the other contours. Supple- mentary contours, used on the flatter areas of some maps, are shown as dashed or dotted lines. Elevation figures are shown on the contour lines at frequent intervals to facilitate their identifica- tion, as well as to assist in determining the values of adjacent contours.

The two main reasons for showing relief information on maps are to furnish coordinated data for engineering calculations or other scien- tific mensuration and to present a graphic picture of the ground surface. The two objectives are related but distinct, and sometimes they may be conflicting.

a. Measurement. --For engineers or scien- tists who are interested in exact measurement, topographic maps furnish dimensional information about elevations, areas, grades, and volumes. The approximate elevation of any point can be read directly or interpolated from contours. A series of elevations on a line determines the grade or profile of the line, and areas and volumes can be computed by combining line profiles in various ways. The relief information shown by contours is sufficient for calculating the storage capacity of a reservoir, the area of a watershed, or the volume of earth to be moved in a large road cut or fill.

b. Interpretation. --On the other hand, many persons who use maps are not concerned with exact ground elevations but are more interested in the general appearance and shape of the land. For them, contours are the graphic means of vis- ualizing the terrain and an aid in locating positions on the map.

7. Selection in mapping relief features

The amount of relief information that can be shown on a map depends largely upon the scale of the map and the contour interval used to portray the relief. If a great amount of relief detail is re- quired, the scale must be enlarged and the contour interval made smaller; but regardless of the scale

Figure 3. --Contours. and contour interval all information concerning the ground surface cannot be shown on maps. The mapmaker must always make a judicious selection of the features that it is desirable to portray.

a. Choosing a contour interval. 2 --A satis~ factory contour interval is one that shows the im- portant topographic features adequately, yet does not result in closely spaced contour lines that ar~ difficult to read. For a given scale and contour interval, the slope of the ground determines the spacing of contours on the map. Therefore, the most appropriate contour interval is selected ac- cording to the scale and the average ground slope in the quadrangle. Most quadrangles in continen- tal United States are mapped with intervals of 5, 10, 20, 40, or 80 feet. (^2) See Chapter 1 B 2, Contour intervals.

FEATURES SHOWN ON TOPOGRAPHIC MAPS (Ch.^ 1B3)^11

Figure 4. --Two maps of identical areas in southeast New York State. Each map covers 4 square miles. The upper one (1945) is at the scale of 1:24,000, with a contour interval of 10 feet; the lower one (1902) is at the scale of 1:62,500, with a contour interval of 20 feet,

12 (Ch. 1 B 3) (^) TOPOGRAPIDC INSTRUCTIONS

b. Residual features. --Features of this kind are the parts of the earth' s· crust that have resisted erosion effectively and have remained somewhat in their original state. Plateaus, benches, mesas, escarpments, cliffs, and ter- races are typical residual features. Important characteristics of this class of features shown by contours are the slope and the breaks or changes in the slope.

A slope may be uniform, concave, or con- vex. A uniform slope is indicated by even spac- ing of contours. On a concave slope, the contours are closely spaced near the top, gradually widen- ing toward the bottom. On a convex slope, the arrangement is reversed; the contours are close- ly spaced near the bottom, and the spacing in- creases toward the top. Land slopes are seldom of a single type and may be found in all possible combinations.

Rock escarpments, which usually border high flat plateaus, are characterized by closely spaced contours near the top, which bend sharply at the crest and meander across the plateau. The slope below the crest is usually concave, with the spacing between contours gradually increas- ing toward the base of the hill. Figure 5 illus- trates contoured slopes; the Juanita Arch, Colo., 7!-minute quadrangle is a good example of con- toured cliffs, escarpments, mesas, and other residual features.

c. Depositional features. --As the class name indicates, these features are deposits of soil, rock, and other material built up by such carrying agents as glaciers, streams, winds, and volcanoes.

Drumlins, illustrated in figure 6, are elliptical hills of unstratified glacial debris, larger and steeper at the north end and tapering toward the south. Their contoured shape is typ- ical and recognizable. Groups of drumlins occur in Wisconsin, New York, and Massachusetts.

Eskers are another form of glacial debris, deposited in long, narrow bands by streams be- neath glaciers. They vary in length from a few hundred yards to several miles; within an area they frequently occur in series.

Alluvial fans develop in valleys at the base of block mountains, being formed from rock and gravel washed out of canyon mouths. They may be almost any size from a few feet to several miles in diameter. Usually, they slope evenly from the center to the outer edge, and their contoured appearance is that of a fan--hence the name. Figure 7 illustrates a typical alluvial fan in Montana.

10. Symbols and patterns used to show relief

Although contours are the best method of showing most topographic features, symbols must be used to represent features that cannot be shown clearly or economically by contours. Hachures and patterns are frequently used to de- pict relief in areas having a predominance of distinctive ground features that are either too intricate or too small to be individually or ac- curately shown at the scale or contour interval selected for the map.

a. Hachures. --Hachures, short lines drawn in the direction of the ground slope, are used to show gravel or borrow pits too small to contour; piles of waste material, such as mine dumps (see fig. 8); small banks or escarpments that are prominent but not high enough to be shown by the selected contour interval; and sim- ilar relief features.

A slightly different form of hachures is used to depict small earth dams, small levees, spoil banks, and cuts and fills along roads or railroads. Con toured depressions are distin- guished from hills by short ticks at right angles to the contours, pointing toward the center of the depression.

b. Area patterns. --Intricate surface areas too irregular to contour except in a very general- ized manner, such as lava beds, sand dunes, and open strip mines, are shown by a variety of sym- bo1 patterns. The patterns are made up of dots, hachures, or form lines that indicate the typical appearance of the area. Where symbol patterns are used, no attempt is made to represent the topography in detail. The Medicine Lake, Calif., 15-minute quadrangle shows a volcanic area with many craters and lava beds. The lava beds are represented by a surface pattern and generalized contours.

14 (Ch. 1 B 3) (^) TOPOGRAPHIC INSTRUCTIONS

Figure 6. --Drumlins in northern New York. The elliptical hills a re oriented generally in the direction of glacial movement. Usually t hey are steeper at the north end. The map scale here is 1:24, 000; the con- tour interval is 10 feet. Part of the Sodus , N. Y., 7i - minute quad- rangle.

FEATURES SHOWN ON TOPOGRAPHIC MAPS (Ch.^ 1B3)^15

Figure 7. -- Cedar Creek alluvial fan in Montana. A deposit of sand and gravel washed out of Cedar Creek canyon in the mountains at the right. It forms a semicircular cone with a radius of almost 4 miles and with a height at the center of over 800 feet. The contoured slope is exception - ally uniform. The map scale here \S 1:62, 500; the contour interval is 40 feet. Part of the Ennis, Mont. , 15 - minute quadrangle.

c. Relief shading. - -The pictorial effect of some maps is emphasized by relief shading, a half-tone overprint that simulates the appear - ance of sunlight and shadows on the terrain and provides the illusion of solid, three - dimensional topography. Relief shading copy is prepared after the rest of the map has been produced in the conventional way. Of the maps listed in article 18, the Delaware, Mich., the Ironton, Mo. , the Waldron, Ark. , and the Ennis, Mont. ,

15- minute quadrangles are published in shaded- relief editions.

MAP REPRESENTATION OF WATER FEATURES

11. Periodic changes in water features

Unlike other natural features , most of which are reasonably stable, water features- -

FEATURES SHOWN ON TOPOGRAPHIC MAPS (Ch.^ 1B3)^17

shorelines are mapped to scale , and the spaces between the double lines are filled with blue tint (see fig. 9).

Solid lines are used for perennial water- courses, either single - or double -lined. Inter- mittent drains are shown by dash-dot lines (long dashes separated by three dots). Falls and ra pids in large streams are shown by hachures --i n small streams by crossbars ·. The Str asburg , Va., 15 - minute quadrangle shows the falls and rapids in the South Fork of the Shenendoah River by blue hachures.

b. Area water features. -- All perennial open-water features are outlined in blue and filled with blue tint or blue half tone. Intermit- tent lakes and ponds are shown with dashed out- lines and filled with a fine diagonal hatching. Marshes and swamps are shown with a character- istic pattern symbol; permanent stream channels passing through them are represented by solid lines. The Little Creek, Del., 7i - minute quad - rangle map shows a large swamp containing many permanent stream channels.

c. Glaciers and permanent snowfields. -- These are outlined with dashed lines and treated otherwise as relief features, except that the re- lief is shown in blue instead of brown. The re- lief and extent of these features may be indicated by regular contours, approximate contours (dash - ed lines), or by form lines.

d. Springs and wells. -- The landmark prin- ciple applies to the mapping of these water fea- tures. In arid regions where they are important, most of the springs and wells are shown; in humid areas, only the outstanding ones are mapped. A small circle with a tail pointing downst re am sym- bolizes a spring. The well symbol is a small circle.

e. Coastal features. -- Quadrangles that include seacoasts and tidal waters sometimes show depth curves, soundings, some obstructions to navigation, and other marine detail of general interest. These maps are not intended, however, to be used as navigation charts.

Depth curves are similar to contours, ex- cept that the interval is not necessarily constg.nt

Figu re 9. --Hydrogr aphic fe atu res. Map of part of the Pecos River in New Mexico, showing swamps, a sand bar, lakes, single-lined and double - lined streams, and intermittent and perennial streams. Scale 1:24, 000. Part of the Bottomless Lakes , N. Mex ., 7i-minute quadrangle.

and the reference datum is different. On the Atlantic Coast the datum is mean low wat er; on the Pacific Coast it is mean lower low water. The curves show the ave r age water depth below the datum, in feet; they ar e not shown below a depth of 600 feet. Data for depth curves and soundings are taken from official hydrographic charts.

Permanent fixed aids to navigation, such as lighthouses and beacons, a re mapped, but nonfixed aids, such as lightships and buoys, are not shown. Obstacles to navigation- -r ocks, reefs, wrecks, and similar objects--generally are mapped if they are visible at low tide.

18 (Ch. 1 B 3) TOPOGRAPHIC INSTRUCTIONS

The shoreline along coasts is normally shown at the mean high-water line. Where swamps and marshes border the coast, the shore- line is shown at the outer limits of vegetation. Tidal flats are outlined with a dotted line, filled with a pattern and labeled. The Petit Manan, Me., 7!-minute quadrangle is an example of coastal mapping, showing depth curves, reefs and rocks, with tidal flats portrayed by labeled surface pat- terns.

MANMADE MAP FEATURES

13. Coordinate systems, subdivisions, and boundaries

Except for lettering, the lines representing political boundaries, land subdivisions, and co- ordinate systems are the only items of map in- formation that are not physically evident on the ground. Boundary lines may be based on survey- ed ground monuments, or they may be only de- scribed by statute. The lines of coordinate sys- tems are mathematical abstractions.

a. Coordinate systems. --Quadrangle maps show two systems of reference coordinates, the universal geodetic coordinates in terms of lati- tude and longitude and State plane coordinates.

Parallels of latitude and meridians of longi- tude form the boundaries of standard quadrangles. Maps of continental United States at the scale of 1:62, 500 measure 15 minutes in latitude and 15 minutes in longitude, and maps at the scs.le of 1:24,000 measure 7!-minutes on each side. The coordinates of the boundary lines are printed in the margins at each corner, and ticks are placed at intervals along each edge and within the body of the map--at 5-minute intervals on 15-minute maps and at 2!-minute intervals on 7!-minute maps.

State plane-coordinate systems permit sur- veyors and others to use a simple reference grid and still base their work on the national net of geodetic control. Each State has an individual system, and most State systems have two or more zones. Within zones the coordinates of any point are the distances in feet north and east of an arbi- trarily chosen zero point.

Quadrangle maps show the State plane co- ordinates by labeled grid ticks along the border lines at 10, 000-foot intervals. If the mapped area lies in more than one zone, eacp zone is shown with a distinctive style of tick. The grid can be laid out on the map by drawing straight lines between corresponding ticks on opposite edges.

b. Public-land subdivisions:--The sur- veyed lines that divide over two-thirds of the United States into mile-square sections form a cadastral grid system. The subdivision lines are property boundaries or the references for boundaries. Frequently they form in part the boundaries of political subdivisions, and in many areas the routes of public roads follow the sub- division lines.

The subdivision lines and corners are shown on quadrangle maps to the extent that their positions can be determined from evidence recovered on the ground. "Found" subdivision corners are shown by he2.vy red crosses; lighter weight crosses are used for "indicated'' corners. Indicated corners are those marked in some manner, but not with markers set by the original surveyors.

Subdivision lines accurately located are represented by solid red lines; dashed red lines are used where the location is uncertain. On maps published before about 1940, subdivision lines are shown by black dotted lines; on those published between about 1940 and about 1948, black solid lines were used. On modern maps dotted red lines sometimes are used to represent subdivision lines of an unofficial nature.

Map represBntation of public-land subdivi- sions is not intended to be authoritative or offi- cial; it is presented as useful information, as accurately and completely as it can be at reason- able cost. In areas where a reasonable amount of field evidence cannot be found, the section lines are sometimes omitted.

4 See Chapter 3 A 4, Mapping of public--land subdivisions.