Geol115 – F22 Lab 1: Topographic Maps Pre-Lab Reading A map represents a projection -- a two-dimensional model of part or all of our 3-D spheroid Earth. We use them all the time, especially electronic ones (on smartphones, GPS, Google maps). We see them on the news and weather reports. Maps are so ubiquitous that most people use them without appreciating how incredibly powerful they can be. It's amazing to think that a simple, 2D surface can convey multiple dimensions of information. We will use maps in every class during our study of geological hazards. Locations on a map are determined using a coordinate system. The most common coordinate system is latitude and longitude, which are measured in degrees and sub-divisions of degrees. The reference point of latitude is the Equator (0°), which then increases in magnitude as a point moves toward the North and South poles which are 90° N and 90° S, respectively. Longitude is reported as degrees East or West from a reference line that runs around the Earth from pole to pole. This line is the Prime Meridian, designated to run through Greenwich in the UK (Figure 1.1). Many specialized maps use a coordinate system from a specified reference or origin point. This coordinate system may be used only for that map. Most applications for which we need maps focus on a small portion of the globe. Thus, not all maps represent the Earth at the same size. 1 Geol115 – F22 Figure 1.1. Latitude and longitude (source: Wikipedia) Map Scale On a map, scale can be expressed in two ways: fractional scales and graphical scales. A fractional scale indicates how much the mapped portion of Earth’s surface is reduced from its actual size. For example, a map with a scale of 1 to 24,000 or 1:24000 indicates that 1 unit on the map represents 24,000 of those same units on the surface of the Earth. Note that the type of unit is not specified -- it could be inches or meters. One inch across the map would cover 24,000 actual, on-the-ground inches; one centimeter across the map would represent 24,000 real centimeters. A graphic scale or bar scale, is a bar that is divided into segments that shows the relationship between distance on the map and the actual distance on Earth (Figure 1.2) 2 Geol115 – F22 Figure 1.2. Typical graphic or bar scale of a map (Source USGS) It is also important to understand the orientation of a map. All maps are oriented based on some reference point. North is at the top of maps such as on GPS displays, road maps, and Google maps by convention. Professional maps will have an arrow pointing to true North, or the North Pole. Since compasses point at the magnetic pole (which wanders around the true pole) there may also be a second arrow pointing towards the magnetic pole. In the southern hemisphere, true and 3 Geol115 – F22 magnetic South poles are often indicated instead. Be aware that special purpose maps may display a different orientation. Contours What if you wanted to show special information on a map, such as population density, location of forests, swamps, soil properties, etc.? This can be done using symbols, colors or patterns. There are many useful applications where displaying discrete numerical values would be more useful, and yet a map cluttered with numbers would be difficult to read. The way around this problem is to enclose areas of the same values within lines, a process called contouring. The information shown by a contour could be a water contaminant concentration, or an earthquake's perceived intensity. The most common use of contours is to display topography, or the shape of the land. On topographic maps areas of the same elevation are linked by a contour line. Contour lines represent discrete intervals in elevation (ex: every 10 feet), known as the contour interval. Not every contour line will be labeled; only certain index contours are. Index lines are thicker or heavier lines and are spaced every fifth or tenth line (Figure 3). Figure 3. Mount Adams, Washington (left); a topographic map of Mt. Adams (right). Rules For Topographic Contouring There are several rules to use interpreting topographic contour lines: 4 Geol115 – F22 The rule of V's: When contours make sharp-pointed Vs, the map usually indicates a stream valley, where the stream channel, the lowest point, represents the bottom of the V. An important rule of thumb: the "V" on the topographic map always points upstream. The rule of O's: closed loops are normally uphill on the inside, and downhill on the outside, with the innermost loop representing the highest area (Figure 3). Spacing of contours: The more closely spaced the contours, the steeper the slope or gradient. Little space between contour lines represents a great change in elevation over a short distance. If lines appear to meet, this indicates a vertical cliff. Contour lines should never cross because each line represents a distinct elevation (i.e., the same point can't be two elevations at once). Topographic Profiles A useful way to understand what topographic maps represent, and to understand important nuances of the landscape, is to construct a topographic profile. A topographic profile is a cross-sectional view along a line drawn through a portion of a topographic map. If you could slice through a portion of the earth and then look at it from the side, the top line traced by that slice would be a topographic profile. Because the vertical scale (or elevation) of topographic contours is usually different than the horizontal scale (or distance) of a map, topographic profiles often display vertical exaggeration. Vertical exaggeration should be reported in the legend of a topographic profile to give the reader an idea of how vertically stretched or shrunk the profile is. Vertical exaggeration is calculated according to the formula: Vertical Exaggeration = Horizontal Scale/Vertical Scale Introduction to Google Earth Pro Google Earth is a free and trusted software that you will use in nearly all labs and quizzes in GEOL115. It provides a powerful platform 5 Geol115 – F22 through which we can access, explore, and manipulate freely available geologic data. There’s an old saying in geology that “the best geologist is the one who has seen the most rocks.” With Google Earth, we have the opportunity to visualize interesting geologic settings -- and how aspects of those settings might affect human populations -- without leaving Newark. 1. Download Google Earth Pro for your PC: https://www.google.com/earth/desktop/. Install the program. UD computer clusters may already have Google Earth Pro installed. If not, ask the cluster administrator to assist you in downloading and installing the program. 2. Watch these two tutorials to learn the basics of how to confidently manipulate the Google Earth environment: Google Earth Basics 2016 Tutorial (running time 15:45): https://www.youtube.com/watch?v=klK27l3unng&t= Google Earth Advanced Tools 2016 Tutorial (running time 11:32): https://www.youtube.com/watch?v=fa7c4SVzo0I&t= Keep Google Earth open in a window while you watch the videos so you can practice skills as you learn them. You will need to be able to use the software confidently in lab. A Google Earth keyboard/mouse shortcuts PDF file is available in the “Files” menu of our course Canvas site. Pre-Lab 1 Terms contour: on a map, an outline that connects or bounds points of similar value. contour interval: the difference in elevation represented by each contour line on a topographic map. distortion: in cartography, incorrect distances between points that result from inherent issues involved in projecting the curved Earth onto a flat map. projection: in cartography, the representation on a plane surface of any part of the surface of the Earth. scale: the ratio of the distance on a map to the actual distance on the Earth's surface. topography: the arrangement of natural and artificial physical features of an area, especially as pertains to their elevation and shape. 6 Geol115 – F22 topographic map: a map that uses contour lines, and sometimes also labels or shading, to convey elevation information. vertical exaggeration: a scale used in cross-section perspectives to emphasize vertical features that might be too small to identify relative to the horizontal scale. The vertical exaggeration is given by the formula VE = Horizontal Scale/Vertical Scale. See also Appendix C, pages 511-517 in Keller and DeVecchio. 7
