Topographic Maps. 3. Scale indicates how much of the Earth's surface is portrayed on the map. Scale can be presented in one of three ways.

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1 TOPOGRAPHIC MAPS Introduction Maps are used to show the spatial distribution of features in a region. The features can range from political boundaries, roads, or cities to population density, annual precipitation, or mineral resources. One of the most useful types of map is a topographic map which is used to show the shape of the land surface. Topographic maps will be used in many of the labs in this course and the ability to read them is an essential part of this course. This skill will also prove valuable if you ever plan to head off into the woods, mountains, or desert. Purpose This exercise is intended to illustrate the types of information presented on maps and to develop your skill in constructing and interpreting topographic maps. Method You will first be introduced to the basic information included on maps. By constructing topographic maps and profiles, you will learn how to quickly and accurately interpret a topographic map in terms of the shape of the land surface. Introduction A map is a representation of a portion of the Earth's surface drawn to some scale, usually on a flat two-dimensional surface. In special cases maps are produced on a spherical surface, such as a globe of the Earth, or landscape features are modeled in three dimensions on raised relief maps. Maps ordinarily represent land areas whereas representations of portions of the oceans or large lakes are called charts. Maps are included in some of the earliest historical records from the Middle East, and even people without written language have used a type of map, such as those used by early Polynesians to show the location of islands and ocean currents in the southwest Pacific. Some of you may know that Christopher Columbus was a map-maker as well as a navigator, and that it was maps made by him and other early explorers that enabled European people to settle the New World. Over the half-millennium since Columbus' time, maps have become diverse and highly specialized. Most students are more or less familiar with road maps, but few students have seen a topographic map, a structure map, or a gravity map. Maps are created for specific purposes. A geologist, for instance, is unlikely to consult a road map in his search for an oil reservoir, but will commonly consult several other maps before advising his company where to locate exploration wells that cost from thousands to millions of dollars to drill. Typical maps a petroleum geologist might consult include a gravity map, a magnetic map, a map of known oil reserves, and a variety of special geologic maps showing the distribution of rock units and their structural arrangement. What's on a map? For maximum usefulness, a map must include certain essential types of information. These include the following. 1. Location indicates what particular portion of the Earth's surface is portrayed on the map. It is commonly included in the title. In addition, many maps include a coordinate system for locating places plotted on the map.

2 2. Direction indicates the orientation of the map. It is commonly shown by a north arrow. By convention, north is toward the top of the map unless a different orientation is indicated. 3. Scale indicates how much of the Earth's surface is portrayed on the map. Scale can be presented in one of three ways km a. graphic scale b. verbal scale 1 cm = 10 km c. ratio scale 1:24,000 - one unit on the map = 24,000 units on the earth's surface (e.g., one cm on the map = 24,000 cm, or 24 m, on the earth s surface) 4. Date indicates when the map was made and thus how current the information is. Features shown on maps can change through time. 5. Legend explains the symbols on the map. 6. Source indicates what individual, group, or agency produced the map. Topographic maps show the three-dimensional geometry of the Earth s surface in twodimensions (a map). Topographic features, such as hills and valleys, are shown through the use of contour lines, which are lines of equal elevation. With a little practice, you will be able to interpret the pattern of contour lines in terms of topography and will be able to use this readily available and useful type of map. Knowing how to read a topo map may come in handy for hiking, deciding whether to buy a piece of property, or assessing the flood potential of an area. To build your skill in interpreting topographic maps, we will start by looking at a series of related maps to show you how the shape of the earth s surface can be represented with contour lines. You will learn how to draw contour lines yourself and construct a topographic profile. Finally, you will look at a topographic map and test your skill at visualizing the topography. Terminology Topography shape of the land surface Elevation distance above or below sea level which has an elevation of 0 feet or meters Contour line line of equal elevation Relief difference between the highest and lowest elevations in an area Contour interval difference in elevation between adjacent contour lines Slope change in elevation over a certain distance Topographic profile a cross section of the topography along a specified line 2

3 Contour lines and topography As mentioned earlier, a contour line is a line of equal elevation. It can be visualized by imagining a series of horizontal planes intersecting with the land surface. Where each plane intersects the land surface would be a contour line corresponding to the elevation of that plane (Fig. 1). If you were to walk along a contour line, you would neither go up or downhill. Higher elevations would always be on one side and lower on the other Topographic map Ocean 200 Imaginary horizontal surfaces Figure 1. Creation of contour lines by intersecting different imaginary horizontal surfaces with the topography. The intersection is a contour line and is projected onto the topographic map above. By looking at the shape, spacing, and location of contour lines, you can accurately interpret the shape of the land surface. Qualitative interpretation of topographic maps based on geometry of contour lines The most important skill in working with topographic maps is to be able to visualize the shape of the land surface by looking at the pattern of contour lines. With just a few simple rules, you can make an accurate description of the topography. The table below illustrates the important rules for interpreting contour lines.

4 Hill closed contours with highest elevation in center Depression closed contours with lowest elevation in center. Will have hachures (short lines) on some contour lines Valley V-shaped contours with lower elevations in middle of V. Contour lines V upstream 80 Ridge V-shaped contours with higher elevations in middle of V 80 Uniform slope parallel contours. The more closely spaced the contours, the steeper the slope Contouring a map from spot elevations Another valuable way to gain an understanding of topographic maps is to draw your own contour lines from a series of spot elevations. These elevations may have come from a surveyor or from an automated process using aerial photographs. The process of contouring is used in a wide range of applications, not just creating a topographic map. For example, you might want to contour a map of population density to show where the population centers are located. 1. Start by locating the greatest elevation(s) and marking with an X. 2. Find an even number elevation corresponding to one of the contour lines near your X and connect it with other locations with the same elevation. 3. When drawing a contour line between two elevations, estimate its position between the two points by using proportions. For example, a 580 contour line would be closer to an elevation of 584 than Continue with remaining contour lines making sure that the contour lines follow the rules below

5 Rules for contour lines Contour lines never cross. Contour lines never merge except for a vertical cliff. Contour lines V when crossing a stream. The direction of the V points upstream. Contour lines never end: they form closed loops (they may not close on your map, but will on adjacent maps). Try contouring the simple map below. Start by drawing the m contour, then draw a contour line every 50 m. How would you describe the topography? Topographic profiles A topographic profile is a cross section of the topography (Fig. 3). They are useful for showing the nature of the terrain along a straight or curved line. For example, constructing a profile along the route of your planned hike will show you what sort of relief you will face on your hike. A profile along a river shows where you may encounter rapids.

6 Profile Line 160 Figure 3. Construction of a topographic profile. Constructing a profile is simple and requires only a pencil and ruler. 1. Draw the profile line on the contour map. 2. Lay a piece of paper along the line and mark where each contour line crosses the profile line and indicate the value of the contour line. 3. Transfer these markings to a line drawn on another piece of paper. 4. Measure up a scaled distance corresponding to the elevation of each marked point and put a point. 5. Connect all your points z 640 y v above 620 m, less than 640 m elevation w 640 m elevation x w v x above 640 m, less than 660 m elevation y back down, on slope within depression to 640 m elevation z x y z wbelow 640m, more than 620 m elevation v Lab Exercises 1. Contour the map below and construct a topographic profile along A-A. The numbers are the elevation above sea level of the locations shown by dots. The dashed lines represent streams. Using a contour interval of feet, draw contour lines connecting all points of equal elevation above sea level in even 's of feet. Use a pencil only. Sketch lightly so you can make adjustments easily. When sketching is complete, darken the contour lines and label each line with numerals to indicate its elevation.

7 A OCEAN A' Contour interval = feet 0 0' 2000' 3000' A 600' Construct a topographic profile along line A-A' using the graph below. A' 400' 200' 0' Interpret the topographic map of Paw Paw, MD/WV. 2. a) What is the contour interval on this map? feet b) What is the highest elevation along Purslane Mountain? feet c) What is the relief on Purslane Mountain? feet d) What is the lowest elevation on the map? feet

8 e) Which direction does the Potomac River flow? f) What is the length of the longest straight stretch of the Potomac River? feet g) Estimate the gradient (slope) of the Potomac River throughout the area. feet/mile h) Which way is Town Hill elongated? i) Estimate what proportion of the map is in Maryland and West Virginia j) Maryland West Virginia k) How would you describe the topography in the map area? Sketch (i.e., general shape to approximate scale) a topographic profile from the southeast corner to the midpoint on the western map boundary. Label the ends of your profile with compass directions and mark major topographic features (e.g., Town Hill, Potomac River, etc.). Pre-Lab Questions 1. In the most general sense, what does a map show? 2. What does a topographic map show? 3. What is a topographic contour line?

9 4. A topographic map has contour lines that are very closely spaced. What does that tell you about the topography? 5. Sketch a contour line crossing the river below. The direction of flow is indicated by the arrow.