Unformatted text preview: Lab 7: Topographic Maps and Profiles Maps • Direct statement of scale •6 inches on the map represents one mile on the ground • A ratio: 1:1,000,000 •one unit on the map represents one million units on the ground • EX: 1 mm in the map represents 1,000,000 mm or 1 km on the ground), or; •As a bar scale, •The bar represents a distance of 10 km Maps are the basic form of data representation in geology but they must represent three-dimensional objects in two-dimensional forms. Shaded relief map of Arizona. Colors indicate elevation above sea level, as shown on the scale to the right of the map. Map Projection Map Selecting the map projection: how positions on the surface of a planet are to be represented on the map The surface of a planet is basically spherical and therefore some scheme must be used to depict a spherical surface on a flat map. Small areas= flat surface, but as the area increases in size, the greater becomes the difference between the planetary surface and the map surface. Three different map Three projections for the United States. The Mercator projection keeps azimuths (directions) accurate but distorts distances; the Lambert Conformal Conic projection keeps distances roughly correct, but distorts azimuth; an Uncorrect, Projected latitude and Longitude projection Projected in which latitude and longitude are assumed to keep a constant scale across the map, distorting both distances and azimuths. Source: http://www.colorado.edu/geography/gcraft/notes/mapproj/gif/threepro.gif http://www.colorado.edu/geography/gcraft/notes/mapproj/gif/threepro.gif . Latitude and Longitude Lines Latitude Longitude Longitude (northsouth lines) line space changes from the equator to the poles as they converge at the poles. Assume a planet to be perfectly spherical: the spacing of lines of latitude (the eastwest lines) is uniform. If R is the radius of the planet then 1o of latitude is given by:
– Can’t use! – As there are 90o of latitude N of the equator and 90o of latitude S, the spacing of the lines of latitude is half the circumference of the sphere divided by 180o (2 x 90o) per degree of latitude. pi (= 3.14159) For Earth, with a radius of 6378.15 km, 1o of latitude is approximately 111.3 km This information is generally known as topography, and is represented on maps by topographic contours. Topographic contours are the map representation of lines that join points of equal elevation on the surface. Contour lines generally represent equally spaced elevations, such as 0, 20, 40, 60, 80, 100 m, etc. Using contours, a complex threedimensional surface may be represented on a twodimensional map. Surfacebased surveying, which is impracticable for most planetary surfaces. Elevations on a planet's surface may be measured by bouncing radar waves off the surface from an orbiting spacecraft (a radar altimeter), or by using stereo pairs of images, the same principle that is used in 3D images. Contours Contours Topographic Map: This image shows a portion of an actual topographic map. The heavy brown lines are 100' contour intervals, the fine grey lines are 20' contour intervals. The course of intermittent streams are shown by the blue dashdot lines. A dirt road is shown by the double dashes. The elevation of any point on the map can be determined by looking at the contour lines. For example, point D is at 5560' (between the 5600' and 5700' contours on the third grey line away from 5600'), B is at 5800', I is at 5710' (half way between the gray line and the 5700' line). Topographic Profiles Topographic A common method of analysis of topographic maps is to construct topographic profiles across the maps to give a direct visual representation of the topography. This process is most simple achieved by placing a strip of paper along the desired profile on the map, marking the positions where the contour lines intersect the top of the paper (mark also their values), then transferring the contour positions to a crosssectional profile, as shown below. The strip of paper is placed at the bottom of the crosssection, as shown, and each contour intersection is traced vertically upward to the appropriate elevation on the profile. The points generated by the contour elevations are then joined with a smooth line to generate the profile. The profile should not cross any elevations where a contour line is not indicated but may vary between contour lines where no intersections are marked. The horizontal scale of the profile by this method is automatically made the same as the map scale. Worked Example – Earth: SP Crater
SP Crater is the depression in the top of the hill and the volume of this crater is clearly much smaller than the volume of the hill, strongly suggesting that the crater is a volcanic crater at the top of a volcanic construction. SP Crater is the second youngest cinder cone in the San Francisco volcanic field with an age of about 17,000 years. EastWest cross section of SP Crater along the line AB. Upper section is drawn to true scale (vertical scale = horizontal scale). Lower section is drawn with a 2x vertical exaggeration (vertical scale = 2 x horizontal scale). The paper strip used to transfer the contour crossing points from the contour plot (originally at the same scale as the cross sections) to the cross sections is shown below the cross sections. Lab Slides Lab Lab Slides Lab Devil’s Tower Lab Slides Lab Model of topographic map of a portion of the Venus surface. The scale bar shows 30 km. The contour interval is 20 m and contours between 340 m and 480 m on the inside of the feature are not numbered, but are evenly spaced at 20 m. The top of the map is geographic north. ...
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- Spring '09
- Cartography, Map projection, contour lines, SP Crater, Longitude Lines Latitude