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8 (Ch. 1 B 3)TOPOGRAPHIC INSTRUCTIONSSymbols larger than scale size take upextra map space; therefore, where small featuresare close together, all of them cannot be shown.Generally, the less important features are omitted in congested areas.b. Cost of production. --The extent to whichsome kinds of map features are shown is determined partly by the cost of compiling the information. Aerial photographs are the source of mostmap information, but features that cannot beidentified on mapping photographs must be obtained by field methods, a procedure that is relatively more expensive. As an example, not all section corners are mapped; they are too small to beseen on aerial photographs, and the cost of mapping all of them by field surveys would be excessive.c. Obsolescence. --Not only the originalcost but also the cost of keeping the map up todate is considered in deciding what features tomap. Generally, the more features depicted, themore quickly the map becomes out of date. Cultural features are especially subject to change.If the maps are to have a reasonably long usefullife, the features portrayed must be restricted,to some extent, to relatively permanent.objects.MAP DELINEATION OF TOPOGRAPHICRELIEF6. Methods and objectivesContour lines are the principal means usedto show the shape and elevation of the land surface. Other means are spot elevations andhachures and pattern symbols for special kindsof relief features that are not suited to contouring. Relief information is printed in brown ontopographic maps.Contours are lines connecting points ofequal elevation. They always are continuouslines, and, if the map is large enough, they return to the points of beginning to form closedloops. A contour may be variously defined as-An imaginary line on the ground, every pointof which is at the same elevation abovea specified datum surface (mean sealevel for topographic maps of the Survey).A level or grade line.A line of constant elevation.A coastline or shoreline of level water.d. Concept of a landmark. --Many kinds offeatures are shown on some maps, althoughomitted from others, because of the landmarkcharacter of the features. In this sense, a landmark is an object of sufficient interest in relationto its surroundings to make it outstanding. Forexample, buildings may be considered landmarkswhen they are used as schools or churches orwhen they have some other public function. Theymay be landmarks also because of their outstanding size, height, or design; or they may be landmarks because of their history, such as old fortsor the birthplaces of famous men.The same principle is applied to featuresother than buildings, but the adjacent area alwaysis considered in relation to the object. Wheremap features are few, objects that would not beshown in more congested districts may be mapped as landmarks.An assumed shoreline resulting from theassumed rising of a body of level water.However it is defined, a contour is theline traced by the intersection of a level surfacewith the ground. A series of contours is tracedby a series of level surfaces, a different contourfor each elevation. Contours are illustrated infigure 3. Each contour line on the map represents a definite ground elevation measured frommean sea level, and the contour interval is thedifference in elevation between adjacent contours.The contour interval, together with the spacingof the contour lines on the .map, indicates theslope of the ground. On steep slopes the linesare more closely spaced than on gentle slopes.

FEATURES SHOWN ON TOPOGRAPHIC MAPS(Ch. 1 B 3) 9To make maps more readable, contoursare classified and the classes distinguished bydifferent weights and styles of lines. Index contours--every fourth or fifth contour, dependingon the basic interval- -are accentuated by makingthe line wider than the other contours. Supplementary contours, used on the flatter areas ofsome maps, are shown as dashed or dotted lines.Elevation figures are shown on the contour linesat frequent intervals to facilitate their identification, as well as to assist in determining the valuesof adjacent contours.The two main reasons for showing reliefinformation on maps are to furnish coordinateddata for engineering calculations or other scientific mensuration and to present a graphic pictureof the ground surface. The two objectives arerelated but distinct, and sometimes they may beconflicting.a. Measurement. --For engineers or scientists who are interested in exact measurement,topographic maps furnish dimensional informationabout elevations, areas, grades, and volumes.The approximate elevation of any point can beread directly or interpolated from contours. Aseries of elevations on a line determines the gradeor profile of the line, and areas and volumes canbe computed by combining line profiles in variousways. The relief information shown by contoursis sufficient for calculating the storage capacityof a reservoir, the area of a watershed, or thevolume of earth to be moved in a large road cutor fill.b. Interpretation. --On the other hand, manypersons who use maps are not concerned withexact ground elevations but are more interestedin the general appearance and shape of the land.For them, contours are the graphic means of visualizing the terrain and an aid in locating positionson the map.7. Selection in mapping relief featuresThe amount of relief information that can beshown on a map depends largely upon the scale ofthe map and the contour interval used to portraythe relief. If a great amount of relief detail is required, the scale must be enlarged and the contourinterval made smaller; but regardless of the scaleFigure 3. --Contours.and contour interval all information concerningthe ground surface cannot be shown on maps.The mapmaker must always make a judiciousselection of the features that it is desirable toportray.a. Choosing a contour interval. 2 --A satis factory contour interval is one that shows the important topographic features adequately, yet doesnot result in closely spaced contour lines that ar difficult to read. For a given scale and contourinterval, the slope of the ground determines thespacing of contours on the map. Therefore, themost appropriate contour interval is selected according to the scale and the average ground slopein the quadrangle. Most quadrangles in continental United States are mapped with intervals of 5,10, 20, 40, or 80 feet.2See Chapter 1 B 2, Contour intervals.

10 (Ch. 1 B 31TOPOGRAPHIC INSTRUCTIONSVvhere slopes vary considerably within aquadrangle, the interval chosen may not giveenough information in flatter areas because thecontours are too far apart. Where this occurs,supplementary contours--at one-half or onefourth of the basic interval--are added in the flatareas. If the interval based on average slopecauses too much congestion of contours in thesteeper areas, intermediate contours may bedropped for short distances to avoid coalescencein printing. This treatment in drafting or scribing is called ''feathering.''The contour interval used determines notonly the amount of relief information that can beshown but also the allowable tolerances in thevertical accuracy of the map. The two are directly related. 3b. Generalization. --Small irregularitiesof the ground surface are omitted from the mapby drawing the contours as smooth lines throughthese areas. The technique of ignoring the verysmall features and drawing the contour lines sothat the larger features are emphasized is calledgeneralization.contours could be drawn to meet accuracy specifications and yet have such poor topographic expression that the map might give a misleadingpicture of the landscape. Good topographic expression makes the map easier to interpret-that is, the relief features are easily identified.For example, the contours depicting asmooth, rolling hill, without abrupt breaks orsharp ridges, are drawn as a series of gentlycurved lines, with no acute angles. On the otherhand, the contours portraying a rugged, kniferidged hill, with many cliffs, are jagged andbent sharply to indicate these characteristics.The amount of generalization and the accuracymay be the same for both features; the differenceis in the detailed delineation of the contours.The contrast in contour treatment for twodifferent kinds of terrain can be seen by comparing the smooth hills of New York in theTiconderoga quadrangle with the rough, brokenmountains of Wyoming in the Maverick Springsquadrangle. Both are ?!-minute maps at thescale of 1:24, 000, with 20-foot contour intervals.9. Kinds of relief featuresGeneralization is used to some extent incontouring at any map scale, because it is obviously impossible, even at the largest scale,to show every irregularity of the ground surface.The amount of detail omitted varies inverselywith the map scale; some of the relief detailmapped at the scale of 1:24,000 may be omittedat the scale of 1:62, 500. Figure 4 illustratesan area contoured at the scales of 1:24,000 and1:62, 500; the greater amount of detail shown atthe larger scale is plainly evident.8. Topographic expressionThe drawing of contour lines on a map isboth an art and a science. It is a science in thesense that the lines must be positioned withincertain limitations of accuracy; it is an art inthat within this limitation the mapmaker has alimited choice in the way the lines are drawn.When the contour lines are drawn so that theyrepresent the typical characteristics of theground surface, the map is said to have goodtopographic expression. On the other hand, the3mapping.See Chapter 1 B 4, Accuracy specifications for topographicTo illustrate the contoured appearance ofdifferent kinds of topographic features, examplesfrom three general classes have been selected.a. Erosional features. --These featuresare formed by the erosive action of wind, waves,glaciers, and streams. Features formed bystream courses are the most common and include valleys, gullies, washes, and gorges.The typical contour characteristic of streamcourses is the shape of the contour reentrant,the part of the contour. that curves or bendstoward and away from the drainage channel. Ina series of contours outlining a hill, reentrantsshow the courses of streams too small to beshown on the map with blue lines. The spacingbetween the reentrants along a stream showsthe grade or slope of the watercourse. Theshape of the reentrants sometimes suggests thesteepness of the banks of the drain, or the kindof soil or rock formation through which thewater flows. In flat areas the width of a streamchannel often must be exaggerated to avoidrunning the contour lines together.

FEATURES SHOWN ON TOPOGRAPHIC MAPS(Ch. 1B3) 11Figure 4. --Two maps of identical areas in southeast New York State. Each mapcovers 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 of1:62,500, with a contour interval of 20 feet,

12 (Ch. 1 B 3)TOPOGRAPIDC INSTRUCTIONSb. Residual features. --Features of thiskind are the parts of the earth' s· crust that haveresisted erosion effectively and have remainedsomewhat in their original state. Plateaus,benches, mesas, escarpments, cliffs, and terraces are typical residual features. Importantcharacteristics of this class of features shownby contours are the slope and the breaks orchanges in the slope.Alluvial fans develop in valleys at the baseof block mountains, being formed from rock andgravel washed out of canyon mouths. They maybe almost any size from a few feet to severalmiles in diameter. Usually, they slope evenlyfrom the center to the outer edge, and theircontoured appearance is that of a fan--hencethe name. Figure 7 illustrates a typical alluvialfan in Montana.A slope may be uniform, concave, or convex. A uniform slope is indicated by even spacing of contours. On a concave slope, the contoursare closely spaced near the top, gradually widening toward the bottom. On a convex slope, thearrangement is reversed; the contours are closely spaced near the bottom, and the spacing increases toward the top. Land slopes are seldomof a single type and may be found in all possiblecombinations.10. Symbols and patterns used to show reliefRock escarpments, which usually borderhigh flat plateaus, are characterized by closelyspaced contours near the top, which bend sharplyat the crest and meander across the plateau. Theslope below the crest is usually concave, withthe spacing between contours gradually increasing toward the base of the hill. Figure 5 illustrates contoured slopes; the Juanita Arch, Colo.,7!-minute quadrangle is a good example of contoured cliffs, escarpments, mesas, and otherresidual features.c. Depositional features. --As the classname indicates, these features are deposits ofsoil, rock, and other material built up by suchcarrying agents as glaciers, streams, winds,and volcanoes.Drumlins, illustrated in figure 6, areelliptical hills of unstratified glacial debris,larger and steeper at the north end and taperingtoward the south. Their contoured shape is typical and recognizable. Groups of drumlins occurin Wisconsin, New York, and Massachusetts.Eskers are another form of glacial debris,deposited in long, narrow bands by streams beneath glaciers. They vary in length from a fewhundred yards to several miles; within an areathey frequently occur in series.Although contours are the best method ofshowing most topographic features, symbolsmust be used to represent features that cannotbe shown clearly or economically by contours.Hachures and patterns are frequently used to depict relief in areas having a predominance ofdistinctive ground features that are either toointricate or too small to be individually or accurately shown at the scale or contour intervalselected for the map.a. Hachures. --Hachures, short linesdrawn in the direction of the ground slope, areused to show gravel or borrow pits too small tocontour; piles of waste material, such as minedumps (see fig. 8); small banks or escarpmentsthat are prominent but not high enough to beshown by the selected contour interval; and similar relief features.A slightly different form of hachures isused to depict small earth dams, small levees,spoil banks, and cuts and fills along roads orrailroads. Contoured depressions are distinguished from hills by short ticks at right anglesto the contours, pointing toward the center ofthe depression.b. Area patterns. --Intricate surface areastoo irregular to contour except in a very generalized manner, such as lava beds, sand dunes, andopen strip mines, are shown by a variety of symbo1 patterns. The patterns are made up of dots,hachures, or form lines that indicate the typicalappearance of the area. Where symbol patternsare used, no attempt is made to represent thetopography in detail. The Medicine Lake, Calif.,15-minute quadrangle shows a volcanic area withmany craters and lava beds. The lava beds arerepresented by a surface pattern and generalizedcontours.

FEATURES SHOWN ON TOPOGRAPHIC MAPS(Ch. 1 B 3) 13Figure 5. --Contou.red slopes . A flat - topped mesa in Wyoming mapped at the scaleof 1:24, OCO, with a 20 -foot contour interval. The steep mesa rim is portrayed by closely spaced contours, which come together in places wherethere are vertical cliffs. Toward the foot of the hill the contour spacingis wider, indicating a generally concave slope. The outlined squares arepublic - land subdivisions 1 mile, more or less, between corners . Part ofthe North Butte , Wyo. , 7!- minute quadrangle.

14 (Ch. 1 B 3)TOPOGRAPHIC INSTRUCTIONSFigure 6. --Drumlins in northern New York. The elliptical hills are orientedgenerally in the direction of glacial movement. Usually they aresteeper at the north end. The map scale here is 1:24, 000; the contour interval is 10 feet. Part of the Sodus , N. Y., 7i - minute quadrangle.

FEATURES SHOWN ON TOPOGRAPHIC MAPS(Ch. 1B3) 15Figure 7. --Cedar Creek alluvial fan in Montana. A deposit of sand and gravelwashed out of Cedar Creek canyon in the mountains at the right. Itforms a semicircular cone with a radius of almost 4 miles and with aheight at the center of over 800 feet . The contoured slope is exceptionally uniform. The map scale here \S 1:62, 500; the contour interval is40 feet . Part of the Ennis, Mont. , 15 - minute quadrangle.c . Relief shading. - -The pictorial effect ofsome maps is emphasized by relief shading, ahalf-tone overprint that simulates the appear ance of sunlight and shadows on the terrain andprovides the illusion of solid, three - dimensionaltopography. Relief shading copy is preparedafter the rest of the map has been produced inthe conventional way. Of the maps listed inarticle 18, the Delaware, Mich., the Ironton,Mo . , the Waldron, Ark. , and the Ennis, Mont. ,15- minute quadrangles are published in shadedrelief editions .MAP REPRESENTATION OF WATERFEATURES11. Periodic changes in water featuresUnlike other natural features , most ofwhich are reasonably stable, water features- -

16 (Ch. 1 B 3)TOPOGRAPHIC INSTRUCTIONSFigure 8. --Depression contours and hachures. Iron mines in the Mesabi districtof Minnesota mapped at the scale of 1:24,000, with a 10-foot contour interval. Hachures show the piles of ore and waste material. Depressioncontours, with short right-angle ticks, show the open-pit mines. Manyof the worked-out miries contain water. Part of the Virginia, Minn. ,7 -minute quadrangle.shown in blue on the map--change from time totime. The levels of rivers, lakes, and reservoirs fluctuate during the year, according to theamount of rainfall and runoff. Smaller streamsand springs may be entirely dry for a part of theyear . Daily or even hourly variations in streamflow and water level occur in areas where melt ing snowfields or glaciers feed the streams orwhere streams are controlled for irrigation, orindustrial purposes .water usually undergo seasonal changes. Asfar as practical, the level shown on the maprepresents the so -called "normal" stage, defined as the one prevailing during the greaterpart of the year. Usually it corresponds to theline of permanent vegetation on the banks orshores. Strictly s

by symbols larger than·the true scale size of the features. Roads, for example, are shown 90 feet wide on 1:62, 500-scale maps despite the fact that most roads are not actually this wide. Buildings and other structures also are shown by minimum-size symbols that may be larger than the actual scale size of the buildings. On