Lab 7_Topo - Lab 7: Topographic Maps and Profiles Maps are...

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Lab 7: Topographic Maps and Profiles Maps are the basic form of data representation in geology but they must represent three- dimensional objects in two-dimensional forms. Map Scale First let us consider map scale . Scale is commonly given in one of three forms: A direct statement of scale, such as 6 inches to a mile (6 inches on the map represents one mile on the ground); A ratio , such as 1:1,000,000 (one unit on the map represents one million units on the ground, or 1 mm in the map represents 1,000,000 mm or 1 km on the ground), or; As a bar scale , such as shown below. The length of scale on the map represents the distance indicated on the ground; in this example, the bar represents a distance of 10 km. Another form of scale may be given by latitude and longitude lines or tick marks on the map. The spacing of lines of longitude (the north-south lines) changes from the equator to the poles as they converge at the poles. These lines are therefore inconvenient for determining scale. If we assume a planet to be perfectly spherical, however, the spacing of lines of latitude (the east-west lines) is uniform. As there are 90 o of latitude north of the equator and 90 o of latitude south of the equator, the spacing of the lines of latitude is half the circumference of the sphere divided by 180 o (2 x 90 o ) per degree of latitude. If R is the radius of the planet then 1 o of latitude is given by: where, π is mathematical pi (= 3.14159). For Earth, with a radius of 6378.15 km, 1 o of latitude is approximately 111.3 km. The advantage of using a bar scale or a latitude/longitude scale on a map is that if the map is enlarged or reduced, the scale remains correct as it is enlarged or reduced with the map. A direct statement of scale or a ratio scale becomes invalid if the map is enlarged or reduced. Map Data A wide variety of information may be displayed on a map, including cultural features (buildings, roads, administrative boundaries), biological features (vegetation or animal ranges), or geological information (rock type and geological structures). Unless modified by human activities, the shape of the land surface is a direct result of the interactions among the surface geological process, such as weathering, erosion, deposition and impacts, subsurface geological process, such as faulting, folding and volcanism, and the underlying mechanical structure of the near-surface rocks and their resistance to the surface processes. Thus, perhaps the most basic geological information is the shape of the land surface. This information is generally known as topography, and is represented on maps by topographic

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This note was uploaded on 01/01/2011 for the course GLG 190 taught by Professor C.v. during the Spring '09 term at University of Arizona- Tucson.

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Lab 7_Topo - Lab 7: Topographic Maps and Profiles Maps are...

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