Lab #4 Topographic Maps and Aerial Photographs Purpose To familiarize you with using topographic maps. Visualizing the shape of landforms from topographic maps is an essential skill in geology. Proficiency at topographic map use is essential in geologic mapping and using topographic maps to interpret geologic processes. Logistics This lab consists of doing exercises in this handout as well as Exercise 7 in the lab book, Exercises in Physical Geology, to be found in the classroom. Topographic Maps A topographic map, or topo for short, is a two-dimensional (flat) representation of a three-dimensional land surface. It shows the three-dimensional information (relief or height variation) by using contour lines to represent elevations of hills and valleys. Topographic maps also show accurate representations of water bodies, vegetation, roads, buildings, and political boundaries, all at an accurate scale. Topographic maps are a valuable tool in geological and engineering studies. They also are used by hikers, hunters, campers, and anyone who needs to know the three-dimensional aspect of land surfaces. Most U.S. topo maps are published by the US Geological Survey. They cover rectangular sections of the Earth s surface called quadrangles, or quads for short. Latitude and Longitude. Lines of latitude and longitude are used to locate positions on a topo map and to locate the boundaries for the topo map. Lines of latitude are imaginary lines that run parallel to the equator. They are labeled by their angular distance north or south of the equator as shown below. Lines of longitude run from pole to pole and are labeled by their angular distance east or west of Greenwich, England. The north-south line that runs through Greenwich, England is known as the Prime Meridian. See fig 7.4 in the lab book as well. The angular unit of degrees are traditionally broken into units called minutes. There are 60 minutes in a degree. In other words, a minute is 1/60 th of a degree. Topo maps are traditionally named for how many minutes of land they cover. One popular
series now published by the USGS is the 71/2 minute series. They are commonly referred to as 71/2 minute quads. These maps cover 71/2 minutes of latitude and 71/2 minutes of longitude. Look at the Crater Lake quad. 1. What is the latitude of the northern boundary of the map? What is the latitude of the southern boundary? 2. What is the longitude of the western boundary of the map? What is the longitude of the eastern boundary? 3. Why do the longitude markers increase to the west? 4. How many minutes of longitude does this map cover? How many minutes of latitude? I Scale -- Measuring distance The ratio of distance on the map to actual distance on the earth s surface is the scale. Two kinds of scales are typically provided on map legends: The first is a fractional scale such as 1:100,000 where the first number on the map is equal to the second number on the Earth using any scale of measurement.. As an example, 1:100,000 means that 1 inch, foot, millimeter, or finger on the map is equal to 100,000 inches, feet, millimeters, or fingers on the surface of the Earth. The second is a bar scale. The bar scale shows graphically how far some measure of distance is on the map. They can be used like a ruler. A third type of scale, not provided on map legends, is a verbal scale. On the Crater Lake map, the verbal scale is 1 inch = 1 mile. 1. What is the fractional scale of the Crater Lake quad? 2. On the Crater Lake quad, how far is it in miles from the top of Mt Thielsen to the Peak on Wizard Island? How far is it in kilometers? 3. How far is it from Mt. Bailey to Mt Thielsen?
4. Contour Lines. Contour lines represent elevations and landforms on a flat piece of paper. Contour lines are imaginary, horizontal lines that connect points on the earth of equal elevation. See the demonstration in class and the diagram below. The vertical distance between contour lines is called the contour interval. On the diagram below, the contour interval is 50 feet. You need to be able to look at contour lines on a map and visualize the topography. See also pages 80-89 in the lab book. Use the stereoglasses to look at the images on the following page. Look at all of them. You can also use the original on page 85 of the lab book. But don t write in the lab book! Only write on this handout to answer the following questions. 1. On the top diagram, where is a hill? Where is a closed depression? Where is a steep slope? Where is the highest point on the map. Mark these things on this handout, not the lab book. 2. On the Canyons, where are v-shaped contours pointing upstream? This is the classic shape for contours along streams on maps. Where is relatively flat terrain? Where is steep terrain? Mark these things on this handout, not the book.
More detailed contour rules. Don t try to memorize this; rather think through the situations where contours are repeated and where they do not repeat.
The diagram below shows elevations. The diagram on the right a possible 500 foot contour drawn through these elevations. Follow this example by drawing the exact same contour on the left-hand diagram. Then add the other contours. Use a 5-foot contour interval. So draw a 485, 490, 495, 505, 510, and 515 foot contour lines. On the following diagram, label the contour lines. Use a 40 meter contour interval. Remember to follow the rules shown on the previous page.
Draw a topographic profile of the map shown below. Look on pages 88-89 of the lab book for directions. The vertical scale is given for you. Be sure to note that the contour interval is 20 feet. Now back to the Crater Lake map. 1. What is the contour interval of the Crater Lake map? 2. What is the area of the Crater Lake map in square miles? 3. Two inches on the Crater Lake map represents how many feet on the ground? How many miles? How many kilometers? 4. What kind of volcano is the mountain on which Crater Lake lies?