GLY 4400, Spring 2016 Lab #2 TOPOGRAPHIC MAPS Objectives: 1- To develop an understanding of map reading with an emphasis on topographic maps. 2- To become familiar with the concepts of scale, location (latitude and longitude), elevation and contour lines. 3- To identify the direction and steepness of land slope and visualize the form of the land Introduction: are two-dimensional representations of three-dimensional surface morphologies. They convey relief, gradients, altitudes, drainage, and show the position of natural and man-made features of the landscape. are excellent navigational aids and a prerequisite for making a geologic map. Both are needed to interpret the geology of a region. show three-dimensional information (relief or height variation) by using contour lines to represent elevations of hills and valleys. This vertical information is the distinguishing feature of a topographic map. A topographic map differs from the more familiar planimetric map (such as a highway map), which does not show relief. Elements of a Topographic map: The main elements of a Topographic map are: Title: establishes the map subject Date: when the map was published Source: who published the map. North Arrow: establishes directional orientation of the map Legend: defines symbols used on the map. Without a legend, a map is virtually meaningless. Relief: refers to the difference in elevation between two points. Total relief is the difference between the highest and lowest points in an area (or on a map), while local relief refers to the difference in elevation between two nearby points (e.g. a hilltop and nearby valley) Scale: denotes the relationship between actual ground distances and distances on the map. The concept of scale is very important in understanding a map. If you have a scale that reads 1:24,000, this is stating that 1 unit on the map is equivalent to 24,000 units on the ground (or in reality). The term unit can be referred here as any distance unit (e.g. meters, feet, inches, kilometers ). Therefore, in our case, 1cm on the map represents 24,000 cm on reality; or 1 inch on the map represents 24,000 inches on reality. Scales can also be expressed as a fraction (e.g. 1/24,000), or as a bar graph (as used in Figure 1). 1
Other common map scales are 1:25,000; 1:50,000; 1:62,500; 1:100,000; 1:125,000; or 1:250,000. As we increase the fraction, we increase the scale: the larger the fraction, the larger the scale. For instance, between one map with a scale of 1/24,000 and another map with a scale of 1/62,500; the first map has the larger fraction and therefore the larger scale. In this map you would have greater detail, but less land area; whereas in the 1/62,500 map you would have more land represented but less detail. Therefore: 1:24,000 = larger scale, greater detail, less area depicted 1:62,500 = smaller scale, less detail, greater area depicted Latitude-Longitude: One of the most important functions of a map is to provide the location of a feature, point, or area, so the location can be found again. The most effective way to do this is by dividing the earth into a series of grid lines and quadrangles. A quadrangle is a section of Earth s surface that is bounded by lines of latitude at the top (north) and bottom (south) and by lines of longitude on the left (west) and right (east). Latitude and longitude are both measured in degrees ( ). Latitude is measured from 0 at the equator to 90 N (at the North Pole) or 90 S (at the South Pole). Longitude is measured in degrees east or west of the prime meridian, a line that runs from the North Pole to the South Pole through Greenwich (England). For finer measurements each degree can be divided into 60 equal subdivisions or minutes ( ), and minutes into 60 equal subdivisions or seconds ( ). Because longitude lines form the left and right boundaries of a topographic map, north is always at the top of the quadrangle grid, which is called grid north (GN) and usually corresponds to the direction of the true north on the actual Earth. But compasses are attracted to the magnetic North Pole (MN), which do not coincide with the geographic North Pole. show at the bottom the declination (difference in degrees) between the compass North (MN) and the true North (usually a star symbol). Since the magnetic pole migrates slowly, the declination value is exact only for the year that was measured. Contour lines: are miniature models of Earth s three-dimensional surface, printed on two-dimensional pieces of paper. Measurements of length and width are easily reduced from the real world to the map. But the third dimension, height (or elevation), is shown using contour lines. Each contour line connects all points of the map that have the same elevation above the sea level. It acts as an imaginary boundary, separating areas above that elevation from areas below it. Figure 1 shows the conceptual process of construction of a topographic map The contour interval given on a topographic map indicates the elevation difference between each contour line. On standard topographic maps, every fourth of fifth line is called index contour, and is printed darker and labeled with its elevation. Remember, elevation is the height above sea level, while relief is the difference in elevation between two points. Figure 2 shows the rules followed by contour lines on topographic maps. 2
Figure 1: Topographic map construction. A contour line is drawn where a horizontal plane (such as A, B, or C) intersects the land surface. Where seal level (plane A) intersects the land, it forms the 0-ft contour line. Plane B is 50 ft above sea level, so its intersection with the land is the 50-ft contour line. D is the resulting topographic map. Figure 2: Rules for contour lines on topographic maps. 3
Topographic profiles and vertical exaggeration A topographic map provides an aerial view of an area, depicting features and relief by means of its symbols and contour lines. Occasionally a cross section of the topography is very useful. A topographic profile is a cross section that shows the elevation and slopes along a given line. Figure 3: Topographic profile construction and vertical exaggeration. (Steps 1 and 2), topographic profile constructed along line A-A (Step 3), and calculation of vertical exaggeration (Step 4) are shown. Vertical exaggeration gives you the number of times the vertical scale is exaggerated relative to the horizontal scale. See example on Figure 3, Step 4. Note that 4
you get the horizontal scale from the map scale, and that you determine vertical scale by choosing the desired elevation per distance unit on the paper. Gradient: The gradient tells you the slope of the land between two points. It is calculated by dividing the relief by the path distance: Gradient = Relief / Path distance Path distance is the distance measured along the path traveled (NOT necessarily the straight line distance!). For example, if you were planning a canoe trip, you may want to know the gradient between your starting point upstream and your ending point downstream. If your starting point was 50 feet above sea level, and you ending elevation was 20 feet above sea level, the relief would be: 50-20 = 30 feet. Although the straight line distance between the two points might be only 2 miles, with the curvature of the river, the actual path distance is greater, let s say 3 miles. In this case, the gradient will be: Gradient = (starting point ending point) / path distance = ( 50 feet - 20 feet ) / 3 miles = 10 feet / mile APPENDIX I CONVERSION BETWEEN UNITS: In general, to convert units, we need to multiply the quantity we want to convert by its conversion factor. The conversion factor basically tells us how to convert one unit into another. Converting one unit to another involves only multiplying and dividing. To do this properly, we need to make sure we can cancel equal units as shown in the example: Example 1: How many meters are 12 feet? Example 2: How many hours are 12,500 seconds? SOME IMPORTANT CONVERSIONS: APPENDIX II 1 mile = 5,280 feet 1 kilometer = 0.62 miles 1 mile = 63,360 inches 1 kilometer = 1,000 meters 12 inches = 1 foot 1 degree ( )= 60 minutes ( ) = 3,600 seconds ( ) 1 foot = 0.3048 meters 1 mile = 1.6 kilometers 5