1 Lab 09-6 ONLINE LESSON. 2 If viewing this lesson in Powerpoint Use down or up arrows to navigate.

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Lab 09-6 ONLINE

LESSON

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If viewing this lesson in Powerpoint

Use down or up arrows to navigate

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Do take notes as we peruse through this

lesson…

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Contours

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Contours refer to lines on a topographic

map that represent height (elevation) above sea level…

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

ground…

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

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The contour interval is displayed on the bottom of the

Livermore Quadrangle…

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This means that the interval between all contour lines is 20

feet…

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The interval between index lines is 100 feet

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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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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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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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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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If we start at 600 feet…

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Then 700 feet

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

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

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

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

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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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A top of a hill…

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

6.7 cm…

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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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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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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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END

L09-6