Topographic Map
Interpretation

INTRODUCTION

This manual is written to fill a specific need - that for instruction in the interpretation and quantitative analysis of topographic maps. There are two methods of interpreting topographic maps - recognition and cognitive study. In this manual it is assumed that you, the user, have already been introduced to recognition through an introductory Earth Science course in which you used a standard laboratory manual (for example: "Exercises in Physical Geology", W.K. Hamblin and J.D. Howard, Burgess, 225 p.)

You are thus equipped to RECOGNIZE excellent examples of common geomorphic features. You might wish to strengthen your ability to recognize such features through the purchase of a supplementary text such as:

• "100 Topographic Maps", R. DeBruin (1970), Hubbard Scientific Co., Northbrook, IL, 128 pp
• "The Interpretation of Topographic Maps", R. D. Salisbury and W. W. Atwood (1908), U.S.G.S. Professional Paper 60, 84 pp
• "Atlas of Landforms", United States Military Academy (1984), John Wiley and Sons, NY, 165 pp.

Nevertheless, you certainly will have problems INTERPRETING most topographic maps. The difficulty arises from two separate circumstances - the fact that few landforms possess the classical symmetry of the examples, due to both nonuniformity of process and later modification by other processes, and the fact that most landscapes are complexes of many landforms, of many ages, of many processes. Understanding topographic maps is like enjoying Chinese food - you must first enjoy the basics, but most of the pleasure is in the subtleties! To enjoy the subtleties of topographic maps, you must learn to take them apart in your mind, to look for clues to origin, to see small but significant anomalies which are evidence for major episodes of landform genesis.

The method you will use to perceive the clues to interpreting topographic maps is anomaly analysis. Anomalies are defined as occurrences which deviate from the normal. This presents a problem: What does a "normal" topographic map look like? There is, of course, no such thing. Each topographic map is "right" for the area it describes, and the world is composed of such a variety of environments that no single map can be held up as representative. For the sake of argument, however, I have designated an "average" topographic map (Williams Mountain, VA). William Morris Davis would have loved this map, because it depicts an area in which precipitation exceeds evapotranspiration and in which there has been relatively little recent tectonic activity.

The "average" topographic map has a number of characteristics which will be significant when we look for anomalies. Many of them are listed below. Compare the list to the map to see what they look like. (Soon you will be trying to see exceptions to them!)  
 
Williams Mountain, WV 

a) The area is homogeneous. That is, all parts look similar to all other parts. Implication: It is an area of uniform geology, uniform process, and uniform age. (NOTE: The triumvirate of STRUCTURE, PROCESS, and STAGE is considered to be the cornerstone of Geomorphology.)

b) The area is cut by perennial streams (solid blue lines on the original) which form a dendritic pattern (a consistent downstream direction, but random variation in tributary orientation and size) which is neither coarse (few kilometers of channel per square kilometer), nor fine (a dense, lacy network of channels). Implication: It is an area in which precipitation exceeds evaporation and transpiration and it is underlain by rocks which are neither remarkably resistent to nor prone to erosion.

c) The slopes in the area are sinusoidal (gentle at the crest and the foot and steepest in between). Implication: Fluvial (stream) process is dominant over all others, and over rock structure and lithology.

d) There is relief (elevation difference) of several hundred meters across the map. Implication: The area has not been dominated by erosion ("old age"), nor by constructional processes ("youth").

You might note that nothing has yet been said concerning slope angle nor vegetation. Slope angle cannot be "seen" on topo maps because of variation of both vertical and horizontal scales. (It can, however, and will be, measured.) Vegetation is potentially significant in interpreting subtleties of topography. It is not, however, present on all maps, thus is not always available as evidence. On our "average" map, the vegetation cover is uniform, with breaks only in the river valleys due to excess water.

Finally, humanity's geomorphic activities have not been significant in this quadrangle. This is not the case for many areas, where mining, housing construction, road construction, water management, and even waste disposal have changed the face of the land. Those areas are still in the minority, and our "normal" quadrangle shows little effect of human action.

The designation of a "normal" quadrangle implies, and even requires, comparison. Such comparison can be begun even without another map.

You have posed a series of working hypotheses. Your task in map interpretation will be to pose such hypotheses, then test them by quantification of map information. Good thinking!

As stated above, it is assumed that the user of this manual has had an introductory Earth Science course, and has learned the basics of topographic map interpretation. The next part of this chapter serves as a review, first of the information presented around the margins of the United States Geological Survey topographic maps, and second of the common symbols used on topographic maps.

MARGINAL INFORMATION

On a topographic map, the mapped area only covers between 50% and 75% of the paper. The remainder is the margins. The side margins vary on the standard-sized paper (22" x 27") with latitude, the top and bottom margins are generally constant. Although there is often surplus paper (usually trimmed off by weight-conscious hikers!), the information in the margins is as important as the map itself because it gives us a frame of reference in which the map can be examined. What good would a map be if you didn't know what area was being described?

In the following discussion, the information will be discussed from the top to the bottom, with randomly distributed and circumferential information at the end. NOT ALL INFORMATION IS FOUND ON ALL MAPS!

	The top left corner of all USGS topographic maps carries the imprint of the authority responsible for the mapping of the United States - The Geological Survey, a branch of the Department of the Interior. Much more information on maps, mapping, and the Survey's mission can be found in the USGS publication "Maps for America", by M. M. Thompson (1979).
	At top center is the agency responsible for the selection (and often, funding) of the quadrangle for its most recent mapping. The selection is based on national priorities (and funding!), which results in rapidly changing cities being remapped many times prior to the completion of mapping in rural areas.
	In the upper right corner is the complete quadrangle name (by which it may be purchased, and by which it is filed in most map libraries). The state is also given, as may be the county. The state name given first is that in which a majority of the map lies, which means that the neighboring quadrangle may be filed with those of another state. Beneath the state name is the map series, including the area covered (ex: 7.5', 15', 1ox2o) and the type of map (ex: topographic, orthophotomap). These data are important in ordering maps, as the same name is often used at different scales. If not specified otherwise, the most detailed topographic maps are usually sent. The names and types are listed on state indices which are updated often and are available at most retail outlets and map libraries. Beneath the map series may be found the name of the next smaller-scale map (larger area) of which this one is a part.

The bottom of the map contains the most significant information in the entire margin. Some of this is significant only to specialists, but most is important to you.

	In the lower left corner is the credit legend, a complex of information including the agencies and individuals responsible for and supporting mapping, editing, publishing, surveying ("control"), or supplying additional information. You should follow the same pattern in your own work, giving credit where credit is due. Additional information includes map projections used and warnings of incomplete or approximate data.
	At the bottom left center is the SINGLE MOST IMPORTANT information for the field use of topographic maps - the magnetic declination. The star indicates true north: the direction of the North (rotational) Pole (and also the North Star). "MN" indicates the direction of the North Magnetic Pole, presently located in the eastern Canadian Arctic. This is the direction a compass needle would point if you are at the center of the map in question. It changes slightly from year to year (secular variation), but this change is not significant to most purposes. Both for field geology and orienteering the compass declination is critical, and must be compensated for. There is another arrow on most maps, labelled "GN" (Great North? Go North? Grid North!). The "grid" referred to here is the Universal Transverse Mercator (UTM) grid, which is a neat way to locate yourself on a topographic map. It will be discussed below.
	At bottom center is the SINGLE MOST IMPORTANT piece of information for quantitative use of topographic maps - the scales. The significance of the scale is shown by the variety of ways in which it is given. First is the representative fraction (RF)(for example: 1:24,000, 1:25,000, 1:62,500, 1:63,360). The RF is often given as a fraction (for example: 1/24,000). Note that 1/24,000, although a mighty small number, is larger than 1/100,000, thus maps which show things as large (although the map covers a small area) are termed large-scale maps. The bar scales beneath the RF show several alternative units for the measurement of distance. These are excellent for rapid approximation. Beneath the bar scales is the vertical "scale" - the contour interval. The contours are the brown lines, representing the same elevation at any point. The contour interval is the vertical distance between these lines. It is usually constant across a map, but may vary if part of the area is very flat and part is steep. (If so, there will be a small map in the lower margin showing the distribution of contour intervals within the map.) The contour interval commonly varies between maps, causing one of the MAJOR PROBLEMS in the qualitative interpretation of slope, thus landforms. Watch out for this one! Also listed at bottom center is information regarding underwater contours, interval, and datum. The datum is the point from which measurement begins, which may be high water or arbitrarily fixed for lakes; and mean sea level, low water, or arbitrarily fixed for sea level. Tide range (if appropriate) is also listed. This may be particularly significant in the interpretation of coastal features, particularly in areas of significant tidal flux. Below the datum information is a disclosure concerning map accuracy. If it is missing, the map may not conform to standards, which are: Vertical accuracy to within +1/2 contour interval, and horizontal accuracy of well-defined points of +.02 in. Remember these numbers for the evaluation of error in the quantification of topographic data. It is also important to realize that a map is not a picture of the topography, but an interpreter's rendition. The errors should, however, be minimal. Also found at bottom center is information on the purchase of topographic maps. The addresses given are thus of national suppliers, who carry all of the maps; they are also available locally in sporting goods stores.
	To the right of the scale is the SINGLE MOST IMPORTANT piece of information for qualitative map interpretation - the quadrangle location. Each quadrangle is shown as a black square superimposed on a state map (see top right corner).  This information allows you to place the mapped area in a broader geographic context. For example: closed depressions on a Florida quadrangle would be unlikely to indicate glaciation, but would be consistent with a karst origin (solution of limestone.) As your geographic and geologic knowledge increases, this regional context will become more and more important to you.
	In the bottom right corner is a key to roads on the map - not important to interpretation, but to field use. You might note how most roads are controlled by the landforms they cross! Below the road key is the name, state, and smaller-scale quadrangle (again!) and below that the latitude and longitude of the SE corner of the map and the series. For example: N4430-W8252.5/7.5 indicates that the map is a 7 1/2 minute quadrangle with its SE corner at 44o30' north latitude, 87o52'30" west longitude. Beneath this information is the date of the map - one of the most significant pieces of information available. The date is significant because it implies accuracy as of that date, according to standards of that time. Although maps compiled prior to World War II can be of great use in the Earth Sciences, they lack the consistent accuracy of those generated later, using aerial photography. Underneath may be the words "photoinspected" (in red) or "photorevised" (in purple) with a later date. This implies that new airphotos have been examined, and significant changes made (in purple), but that the map as a whole retains the accuracy of the original. Beneath the dates is the Army Map Service (AMS) designation for the map. (Remember: "There is a right way, a wrong way, and the Army way"!)

Additional information is distributed around the entire map margin. This is in two forms, text and numbers, both of which are emphasized by color.

The written information includes the names, scales (if different from the map considered), and Army Map Service designation for adjoining quadrangles (in black). These names are usually adjacent to the corners of the map and the centers of the map sides.  One corollary of Murphy's Law is that the more important a geographic feature, the more likely it is to lie on a map boundary (or worse - on a corner!) Reference to the maps used in the exercises will emphasize how often this is true.

In red are the distances by road to the nearest towns, which are most useful for fieldwork, when you are hungry!

The marginal numbers indicate 4 ways of locating a point on the surface of the earth. They will be described briefly here, and their use will be discussed in the methods chapters. The location of a point can be described using a grid system - one in which the surface of the earth is viewed as covered by an ordered series of lines. Any point can be uniquely defined with reference to those lines. The four grids, which can be either spherical or square, serve different purposes - each has its own advantages and drawbacks.

The spherical grid is, of course, latitude and longitude. The complete coordinates are given at each corner of the map, and partial coordinates (in minutes and seconds) at intervals of 1/3 of a map side. (NOTE: The 1/3 intersections are also marked within the map, dividing the map into ninths for approximate location.) Unfortunately latitude and longitude, although precise and universal, are cumbersome for exact locations because of the convergence of meridians (lines of longitude) toward the poles. A unit of latitude varies in length with distance from the equator. (Note that the sides and top/bottom of different maps vary in length, despite common lengths in degrees, and that even the top and bottom of the same map sheet are not the same length!)

The other grids are designed for a flat earth (thus are obviously inexact on a global scale). They are most suitable for small areas, such as a state. Two such grids are marked on most maps, the UTM (in black lettering with blue tics) and a state grid (in black lettering with black tics). The UTM grid is in kilometers and the state grids (usually) in feet. Because they are essentially square, and on a decimal scale, they are considerably easier to use than the spherical, base-sixty scale of latitude and longitude. At grid intersections, however, major difficulties may arise (see below).

The last grid (in red lettering) is the township and range system. These designations, based on the 1785 Congressional plan for survey of public lands, indicate parcels of land approximately 6 miles square, subdivided into 1 square mile parcels (sections). Because of incomplete and inaccurate surveys, this system is not well-suited to defining points. It is, however, the most common way of defining areas, and is found in most legal documents concerning real estate. We will use this system often for rapid designation of points of interest. Note also that the horizontal scale can be approximated from the one-mile grid, but watch out for survey error! A surprising number of maps show sections which are not square.

The marginal information described above makes a map useful: it would not be usable without it. You must realize, however, that the marginal information may vary between maps of different ages, scales, and organizations. Be prepared to improvise, if necessary. Now to proceed to the map itself!

MAP SYMBOLS

Common symbols found on topographic maps are shown (minus the red and green plates) on page 10 of the "100 Topographic Maps" book. Note that some variation may occur, particularly on older maps. Familiarity with this material will be assumed in later exercises.  

PHYSIOGRAPHY

US Physiography diagram - right-click to open in new window to compare with text below.

As stated above, the most critical information for qualitative interpretation of topographic maps is the location map, because it allows for the use of regional context. But that sounds like a circular argument - how can you develop a regional context without first looking at the map in question? The answer lies in Physiography - Physical Geography. The brief introduction here will allow you to begin to develop an awareness of regional trends in landforms and processes; each map will further improve that awareness. A map of the United States with areas of similar climate, lithology and structure, and age (Physiographic regions) outlined is provided on pages 114-115 of the "100 Topographic Maps" book. The names and subdivisions are given on pages 111-113 of that book, along with VERY brief descriptions. Slightly more detailed dexcriptions of each province follow: you will build your own awareness of physiography as the class progresses.

NORTHEASTERN REGION - This region is dominated by the effects of Pleistocene glaciation. In some areas it is the only obvious feature, in others the resistent bedrock has been molded by the ice.

Within that region the Appalachian Mountains are a series of subdued, generally linear, ice-molded ridges and valleys. In contrast, the Adirondacks and the Canadian Shield are cored by resistent basement rock, with jointing being the major control on glacial erosion. The St Lawrence River valley, glacially scoured in less resistent rocks, was covered by the ocean following deglaciation. It thus is characterized by low relief on sediments and by streamlined bedrock. The Great Lakes region also represents basins glacially scoured into weak sedimentary bedrock. Here the basins are so large that they in turn affected ice flow, funnelling the ice towards the south. The Glaciated Lowlands represent the area of deposition of material eroded from the Canadian Shield and Great Lakes, with ice-marginal landforms predominant. The entire region is characterized by a cool moist climate, thus streams have begun to modify the glacial landscape. The relatively short time since deglaciation, however, means that the drainage system is not fully integrated (there are many lakes, waterfalls, etc.)

SOUTHEASTERN REGION - This area is distinctive because of domination by the effects of water.

The Atlantic Coastal Plain is composed of nearly flat-lying recent (<60 million years) sediments and weak sedimentary rocks, with the effects of Quaternary sea level change evident over much of the area. Inland, the Piedmont area is underlain by resistent basement rocks. Jointing has controlled the orientation of stream erosion in this area. Still further inland the Appalachian Mountains are linear fold- and fault-controlled ridges and valleys of sedimentary rock - again the orientation of drainage is structurally controlled. The Appalachian Plateaus to the northwest are the result of the dissection of nearly flat-lying sedimentary bedrock by the many large rivers. There is high relief here, but few mountains. Still further west a prong of interior lowlands, but unglaciated, extends across Tennessee. The relief is more gentle here than in the plateau country to the east. All of this region is characterized by a warm moist climate in which streams, wave action, and the solution of carbonate rocks are dominant processes.

SOUTHCENTRAL REGION - This area, like the Southeastern Region, is characterized by a warm moist climate. The relief is, however, generally much lower.

Like the Atlantic Coastal Plain, the Gulf Coastal Plain is underlain by young weak sediments and sedimentary rocks and overprinted by the effects of sea level change. Unlike the Atlantic region, however, the Gulf region is characterized by the presence of many large rivers (including the Mississippi), some with high sand loads from the semiarid Great Plains and Rocky Mountains. These rivers have affected both the coastal plain and the coast itself by deposition of that load. The Ouachita Mountains and Ozark Plateaus are similar to the Appalachian Mountains and related plateaus which lie across the Mississippi Embayment from them.

WESTERN INTERIOR - The Western Interior is composed of the Great Plains, Rocky Mountains, Colorado Plateau, and Basin and Range. The variety of climate, lithology, and structure within this region is extreme! Nevertheless, some consistency is present.

The Great Plains are (of course!) generally flat, and they range from semiarid to arid. Structural control on topography is minimal (except in the Black Hills), thus surface processes such as glaciation (north), eolian (north-central), stream dissection (south-central), and solution of carbonate (south) dominate.

The Rocky Mountains include several physiographic regions, each with its own characteristics. The Southern Rockies consist largely of broad folded mountains, although volcanic activity has occurred locally. The cores of the ranges are generally metamorphic, and glaciation has strongly overprinted the original fluvial dissection. The Central Rockies include larger basins in which eolian processes may dominate, but still consist mainly of folded mountain ranges. Volcanism has been significant in the northern part of the region, and glaciation reached the margins of most major ranges. The Northern Rockies are dominantly fault controlled, with both normal (south) and thrust (north) faults controlling the topography. Resistent sedimentary rock packages have been repeated by this faulting. Glaciation of the ranges often coalesced to send ice onto the plains and the intermontane basins.

The Colorado Plateau is generally well-known. Flat-lying rocks in which jointing controls erosion are interrupted by faults and broad folds. Fluvial dissection of sedimentary rocks is the most common process, although volcanism is locally important in the southern portion of the Plateau.

The Basin and Range Province too is generally recognizable. The fault-block ranges have generally been dissected by streams, but the lack of well-developed regional drainage due to low rainfall leads to the deposition of debris in the basins. Locally eolian, volcanic, and glacial features occur.

NORTHWESTERN - The Northwestern region includes areas of high precipitation near the coast and low precipitation east of the Cascades. It is dominantly volcanic in origin, although deformed rocks are found along the coast.

The Snake River Plain and Columbia Plateau are both composed of relatively recent basaltic lava flows, in some cases loess- covered and usually dissected by the major rivers and their tributaries. The Cascade Range, in contrast, is largely andesitic and has been dissected by both streams and glaciers. Continental glaciation affected the area as well, in the Puget Sound lowland. The Olympic and Coast Ranges are composed of relatively weak rocks which have been highly dissected by fluvial and mass movement (Coast Ranges) and glacial (Olympic Mountains) processes.

SOUTHWESTERN - The Southwestern region likewise includes both wet and dry areas. The Coast Ranges of California are similar to their namesakes to the north - weak rocks highly eroded by mass movement and streams. Fault control of dissection may be evident. Northern California and southern Oregon include the south end of the Cascades volcanic province. The Peninsular Ranges of southern California are largely granitic rocks, little deformed but stream dissected. The Transverse Ranges to the north are composed of softer rock which was highly deformed prior to dissection. The two most significant areas are the Sierra Nevada, a granitic fault block with evidence of glaciation at high elevations, and the Central Valley, an arid sediment trap with mostly fluvial landforms.

WARNING! DO NOT ATTEMPT TO FORCE ANY INTERPRETATION INTO THE OVERSIMPLIFIED DESCRIPTIONS ABOVE! THOSE DESCRIPTIONS ARE AIDS, NOT ANSWERS. VIRTUALLY ANY LANDFORM CAN BE FOUND IN VIRTUALLY ANY PART OF THE COUNTRY!