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1 Lab 09-6 ONLINE LESSON

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2 If viewing this lesson in Powerpoint Use down or up arrows to navigate

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3 Do take notes as we peruse through this lesson…

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4 Contours

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5 Contours refer to lines on a topographic map that represent height (elevation) above sea level…

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6 A topographic map is a 2D representation of a 3D object on the ground…

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7 A topographic map provides a view from a vantage point above the earth…contour lines allow a viewer to see elevation on a 2D depiction of the earth…

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8 Take a look at LPC on the Livermore Quadrangle and see a whole bunch of brown lines…these are contour lines…they depict elevation …

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9 The value in the blue circle represents an elevation of 500 feet above sea level…the red circle represents an elevation of 420 feet above sea level…

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10 Here is an area just north of LPC…notice that contour lines are in two shades…bold thick brown and light thin brown…

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11 The bold thick brown lines are called index lines…the thinner brown lines are contour lines…index lines are also contour lines…

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12 A map cannot include every elevation…the map would be too crowded… instead intermittent reference points called index lines are provided…

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13 The index lines are at 100 feet intervals…the thinner contour lines are at 20 feet intervals…

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14 The contour interval is displayed on the bottom of the Livermore Quadrangle…

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15 This means that the interval between all contour lines is 20 feet…

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16 The interval between index lines is 100 feet

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17 The elevation in the red ellipse is at the 600 feet elevation…the elevation is the same all along this line…

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18 The elevation in the red ellipse is at the 700 feet elevation…the elevation is the same all along this line…

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19 This elevation here is at the 800 feet elevation…the elevation is the same all along this line…but wait…there is no 800 index line number

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20 A map is too small to include every bit of elevation information…so we have to deduce that the next line is at the 800 feet elevation…

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21 If we start at 600 feet…

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22 Then 700 feet

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23 We can deduce that the next index line is at 800 feet…besides… this is towards the top of a hill…how do we know this

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24 Hilltops are a series of closed concentric elevations… RULE # 11 of contour lines…

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25 Another hint that this is a hilltop is the orientation of contour lines…in the red ellipse are a series of contour lines that form “V” shapes…

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26 This is the rule of “Vs”…”Vs” as in more than one “V”…“Vs” point towards a higher elevation… RULE # 13 of contour lines

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27 “Vs’ indicate a sloping valley or a downhill flowing stream…

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28 The contour lines in the red ellipse are close together…the contour lines in the blue ellipse are widely spaced…

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29 Closely spaced lines represent a steep feature…widely separated lines indicate a flat terrain… RULE # 10

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30 A feature that looks like this is called a depression…little tick marks that point toward the center of a series of closed concentric elevations…

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31 A top of a hill…

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32 A gradient is the measure of steepness of a particular terrain… To determine gradient measure the horizontal distance from one point to another…

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33 From the 828 elevation to the 600 feet elevation…the horizontal distance is 1,300 feet… Use the scale at the bottom of the map for reference…

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34 Since elevation is in feet, use the mile scale…1 mile equals 6.7 cm…

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35 The horizontal distance is 1,300 feet…the change in elevation is 228 feet… The change in elevation is calculated by subtracting 600 ft from 828 ft…

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36 To determine the gradient…divide the vertical value by the horizontal value…. 228 ft 1300 ft The gradient is 0.17 then multiply this by 5,280 feet.

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37 This provides a value of 926 feet…which means that for every horizontal mile (5,280 feet), there is 926 feet of vertical elevation…

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39 END L09-6

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