Lab 09-6 ONLINE LESSON
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Contours
Contours refer to lines on a topographic map that represent height (elevation) above sea level…
A topographic map is a 2D representation of a 3D object on the ground…
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…
Take a look at LPC on the Livermore Quadrangle and see a whole bunch of brown lines…these are contour lines…they depict elevation …
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…
Here is an area just north of LPC…notice that contour lines are in two shades…bold thick brown and light thin brown…
The bold thick brown lines are called index lines…the thinner brown lines are contour lines…index lines are also contour lines…
A map cannot include every elevation…the map would be too crowded… instead intermittent reference points called index lines are provided…
The index lines are at 100 feet intervals…the thinner contour lines are at 20 feet intervals…
The contour interval is displayed on the bottom of the Livermore Quadrangle…
This means that the interval between all contour lines is 20 feet…
The interval between index lines is 100 feet
The elevation in the red ellipse is at the 600 feet elevation…the elevation is the same all along this line…
The elevation in the red ellipse is at the 700 feet elevation…the elevation is the same all along this line…
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
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…
If we start at 600 feet…
Then 700 feet
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
Hilltops are a series of closed concentric elevations… RULE # 11 of contour lines…
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…
This is the rule of “Vs”…”Vs” as in more than one “V”…“Vs” point towards a higher elevation… RULE # 13 of contour lines
“Vs’ indicate a sloping valley or a downhill flowing stream…
The contour lines in the red ellipse are close together…the contour lines in the blue ellipse are widely spaced…
Closely spaced lines represent a steep feature…widely separated lines indicate a flat terrain… RULE # 10
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…
A top of a hill…
A gradient is the measure of steepness of a particular terrain… To determine gradient measure the horizontal distance from one point to another…
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…
Since elevation is in feet, use the mile scale…1 mile equals 6.7 cm…
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…
To determine the gradient…divide the vertical value by 228 …. 228 ft 1300 ft And the horizontal value by 228.
This provides a ratio 1 foot of vertical relief to 5 This provides a ratio 1 foot of vertical relief to 5.7 feet of horizontal distance…which means that for every 1 foot up, there are 5.7 feet horizontally…
END L09-6