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 earthcontour 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 linesthese are contour linesthey depict elevation

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

Here is an area just north of LPCnotice that contour lines are in two shadesbold thick brown and light thin brown

The bold thick brown lines are called index linesthe thinner brown lines are contour linesindex lines are also contour lines

A map cannot include every elevationthe map would be too crowded instead intermittent reference points called index lines are provided

The index lines are at 100 feet intervalsthe 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 elevationthe elevation is the same all along this line

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

This elevation here is at the 800 feet elevationthe elevation is the same all along this linebut waitthere is no 800 index line number

A map is too small to include every bit of elevation informationso 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 feetbesidesthis is towards the top of a hillhow 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 linesin the red ellipse are a series of contour lines that form V shapes

This is the rule of VsVs as in more than one VVs 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 togetherthe contour lines in the blue ellipse are widely spaced

Closely spaced lines represent a steep featurewidely separated lines indicate a flat terrain RULE # 10

A feature that looks like this is called a depressionlittle 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 terrainTo determine gradient measure the horizontal distance from one point to another

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

Since elevation is in feet, use the mile scale1 mile equals 6.7 cm

The horizontal distance is 1,300 feetthe change in elevation is 228 feetThe change in elevation is calculated by subtracting 600 ft from 828 ft

To determine the gradientdivide the vertical value by the horizontal value.228 ft1300 ftThe gradient is0.17 then multiply this by 5,280 feet.

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

ENDL09-6