Lab F - Introduction to Geographic Tools - Contour Lines and Profiles

LAB F - INTRODUCTION TO GEOGRAPHIC TOOLS - CONTOUR LINES AND PROFILES Introduction Topographic maps are distinguished by their ability to show elevation relationships with contour lines. The brown lines on a topographic map are contour lines, lines representing an imaginary line on the ground along which all points are at the same elevation. Contour lines also show the shape of features on the terrain. Given a series of surveyed data points and their elevations, one can construct a contour map of any known surface. In this exercise the student will become familiar with contour lines and their characteristics. Students will also use an enlarged section of the Folsom, New Mexico Quadrangle to “interpret the language of the contours”, to determine elevations, types of slope and features (landforms) on the map. Lastly, students will construct a contour profile, or cross section diagram, of a selected map section Materials needed: pencil, Buffalo Head enlarged section + 2 topographic maps (on reserve) Part I. Characteristics of Contour Lines Before beginning, review the following rules or characteristics concerning contour lines. 1. Closely spaced contour lines indicate a steep slope (Figure F-1). 2. Widely spaced contour lines indicate a gentle slope (Figure F-2). 3. Evenly spaced contour lines indicate a uniform slope (Figure F-2), while irregular spacing indicates an irregular slope (Figure F-1). 4. Contour lines that are widely spaced at the top of the slope and closely spaced at the bottom of the slope depict a convex slope. Contour lines that are closely spaced at the top of the slope and widely spaced at the bottom depict a concave slope. 5. Contour lines may lie very close together, may even appear to coalesce. Such lines indicate a very steep slope or cliff; however, contour lines never cross, merge, touch, branch or fork. 6. Completely closed contour lines indicate tops of hills, mountains, peaks etc. These contours would indicate higher elevation than the surrounding area in all directions (Figure F-3). 7. Bent contour lines indicate a rise in elevation (spur or ridge), or a drop in elevation (valley) An example of a valley is shown at (Figure F-4). 8. Lines depicting a valley bend up-valley, toward the higher elevation (Figure F-4). 9. Hachure marks within contour lines indicate a significant, closed depression (Figure F-5). 10. Brown is the color used by cartographers for most contour lines. Light brown is used for intermediate contour lines. Every 4th or 5th contour line (depending on the contour interval) is an index contour line, is a darker shade of brown and labeled with the appropriate elevation (Figures F-4, F-5 or F-6). Index contour lines speed the process of interpretation. 11. The vertical distance between adjacent contour lines is known as the contour interval. This contour interval is given in the marginal data of the map sheet. Unless otherwise marked, points that lie between contour lines assume a value of one half of the contour interval of the map. F-1 Part I. Elevations, Slopes and Features (Landforms) Shown With Contour Lines Figure F-1: Steep, Irregular Slope Figure F-2: Gentle, Uniform Slope Figure F-3: Peak Figure F-4: Valley Figure F-5: Closed Depression Figure F-6: Saddle F-2 Part II. Interpreting a Contour Map Use the Buffalo Head (enlarged section) and the Folsom, New Mexico Quadrangle map sheet to answer the following questions. Review the Characteristics of Contour Lines and sample feature (landform) diagrams on pages F-1 and F-2 if you have difficulty. Be careful, some questions ask for the elevation and some ask for the type of feature. 1. What is the contour interval of this map section? __________ 2. Elevation at Point A? __________ 5. Feature type at Point E? __________ 3. Elevation at Point C? __________ 6. Feature type at Point D? __________ 4. Elevation at Point B? 7. Feature type at Point F? __________ __________ 8. What is the highest elevation on the Buffalo Head enlarged map? __________ 9. With an arrow, diagram the direction of the flow of water in the intermittent stream near Point G. 10. Find the Buffalo Head section(s) on the Folsom map sheet. Describe its location in USPLS terms. Sections__________, Township__________, Range__________ Use the Glendale Quadrangle map sheet to answer the following questions. 11. What is the contour interval of the Glendale map sheet? __________ 12. Why such a small contour interval? ___________________________________________ ________________________________________________________________________ 13. What is the highest elevation on the Glendale map sheet? __________ 14. What is the lowest elevation within the closed depression in Section 8, Township 2N, Range 2E? _____________________________ 15. Look at the overall pattern of the contour lines on the Glendale map sheet. When it rains, what is the general direction of flow of water? __________________________________ F-3 Part III. Constructing a Contour Profile The identification of landforms from an examination of the contour lines is adequate for many purposes, but when precision is demanded, a contour profile (Fig F-7) is usually necessary. Constructing a contour profile employs the same graphing skills learned in the first lab exercise (Line Graph) and provides additional information about complex landforms, slopes, trafficability and intervisibility between points on the terrain. Fig F-7: Contour Profile F-4 Based on the demonstration of the procedure for constructing a contour profile, construct a profile of the area between Points H and I on the Buffalo Head map below. Use the graph paper on page F-7. The X-axis (horizontal axis) represents the true length of the line connecting points H and I. F-5 Here is the map needed to answer the questions on your quiz for this week. Do not use this map to answer questions within the lab exercise. F-6 Graph paper for Buffalo Head Contour Profile F-7