earth science mapping; interactive topographic maps

Topographic Maps

Internet InteractiveMr. Corner

Below is a Topographic Map

Contour line

Contour Interval

Contour Lines and Contour Intervals

• Contour Line: lines on topographic maps that are of equal elevation.

• Contour Interval: difference in elevation between two contour lines.

Aerial (top) View and Profile (side) View

Aerial (top) View Profile (side) view)

Click on either picture to see how a topographic map is made to make the contour line.

Completed Contour Map with Labeled Contour Lines

Calculating the Contour Interval• Find the difference

between 2 index contours 2500-1250=1250 or 1250-0=1250

• Then Divide by 5 1250/5=250

Spot Elevations: elevations not exactly on a contour line

• Point A falls between 0’ and 250’contours

• So its elevation is between 1’ and 249’

• (it’s not 0’ or 250’ because those are exactly on the line!)

• You can estimate that since A is drawn 1/2 way between contour lines, its elevation is halfway, or 125’

Any place inside a contour line is HIGHER than the contour line...

Anyplace outside the contour line is LOWER than the contour line

Contour lines are NEVER left out, so on this map there is no elevation 600’ or less, OR 900’ or higher!

Hills and Valleys• Hills are contour lines

in a series of circles where the elevations get higher

• Valleys are a series of circles where the elevations get lower OR

Where contour lines repeat

Lower contours not shown on the map would appear if we made the

map larger...

Can You Determine The Elevation? Click Picture.

• What is the contour interval?______

• What is the Elevation of points.

A ___________B ___________C ___________D ___________E ___________F ___________G ___________H ___________

.G

Can You Label the Contours? Click on

the Picture.

• Hints: You know the contour interval is 40’

• Every contour line is a multiple of the contour interval (0,40,80, etc)

• You know the elevation 323’. Which 2 multiples of 40 does it fall between?

A______________ B______________

C______________D______________E______________

All contour lines eventually make a circle. Sometimes only a part of the

circle shows on your map

So, is the elevation of contour line D 600’ or 700’?

Sometimes you just can’t tell

Sometimes there is a hint. Water always flows downhill. There is a river flowing thru Woodland

Valley. If D was a hill (700’)then the river flows east, because water can’t flow uphill

The Rule of V’s

• Water always flows from higher to lower elevation• contour lines make

a “V” shape when they cross rivers

The point of the V always points upstream--toward the higher

elevation

This river is flowing southwest

The V points upstream-and the river flows the other way!

The “V” lets us know the elevation of contour line D…..

The “V” points west--upstream, to higher elevation- so the river flows east, to lower elevation, so D must be 600’

A Quickie Quiz. Click Picture.

• In which geographic direction is the river flowing?_________

• What is the elevation of contour lines

A_____________ B_____________ C_____________

Relief• Relief: is the difference in elevation between

the highest and lowest points in a region

No, we’re not done interpreting maps yet...

Steepness: the closer together the contour lines, the steeper the slope

Depressed? Got yourself into a hole?

The volcano, with a crater in the middle, rises above a flat plain.

The highest elevation is at the rim of the crater. The lowest point of the volcano

is in the middle of the crater.

C.I.=200’

At the top of the volcano, the contours go crazy...

…remember, everything inside the contour is supposed to be higher than the contour!

But don’t get depressed...

Get depression contours!

Hachure marks

A depression contour

shows a temporary change in direction-a “pothole”.

-Everything inside a depression contour is LOWER than the contour

So, the topographic map of the volcano should

look like this:

Rule: the first depression contour repeats the elevation of the contour line before it

The volcano’s topo map

Click

A 3-D Look at Depression Contours

C-D is a “regular” valleyG-H is a “depression”: a temporary change in direction

Interpreting Depression ContoursClick on Picture

• Contour A _____• Contour B _____• Contour C _____• Contour D _____• Contour E _____• Contour F _____• Contour G _____• Point q ________• Point r _________• Point w _________• Point y __________

Contour Spacing and Steepness• Steep Slope: when contour lines are close together.• Less Steep Slope (gentle/gradual): when contour lines

are farther apart.• Contours never touch or cross each other!

Steep and Gentle Slopes

Profile (side) view of an island

Aerial view of island showing contour linesCloser together on the steepest side

Gradient• Gradient tells you how steep your route is!• It is the difference in elevation (ED) divided by the horizontal distance (HD):

GRADIENT = ED/HD• Take a look at this example: • The vertical distance (here, the difference in elevation between points 'A' and

'B') = 10,000 feet • The horizontal distance from 'A' to 'B' = 5 miles • Gradient = vertical distance (elevation difference (ED)) divided by horizontal

distance (HD) • The GRADIENT is 2000 feet per mile (10,000 feet divided by 5 miles)

Other Gradient Examples

Gradient Quiz InstructionsClick Picture to Take Quiz

• GRADIENT QUIZ INSTRUCTIONSIn the menu bar under "Select Line", click on "Scale".

• Click on the "Measure Line" button. A ruler will appear on the map. • In the text box next to the question "One inch on map is equal to how many

miles?", type in your answer and click on the red check mark. • Look in the bottom right 'response text area' for a response to your input. • If your answer is not correct, follow the hint and try again. • If the response is "Correct!", go to the menu bar again and click on "Line A-C".

Then, click on "Measure Line." • Next, input an answer to indicate the "Distance on Map" and click on the red check

mark. • If your answer is not correct, follow the hint and try again. • If the response is "Correct!", input an answer to indicate "Actual Distance" and click

on the red check mark. • If your answer is not correct, follow the hint and try again. • If the response is "Correct!", input an answer to indicate "Elevation Difference" and

click on the red check mark. • If your answer is not correct, follow the hint and try again. • If the response is "Correct!", input an answer to indicate "Gradient" and click on

the red check mark. • If your answer is not correct, follow the hint and try again. • If the response is "Correct!", input an answer to indicate which of the illustrated

lines has a slope similar to the line whose gradient you have calculated. Then, click on the red check mark.

• If your answer is not correct, follow the hint and try again. • If your answer is correct, go to the menu bar and select the next line to work with. • For curved lines, a straightened line and a ruler will appear in the box under the

map • © 2002

Wayne G. Powell and David J. Leveson

Click on Either Picture to Practice Labeling Contours

NOW...

We’re Done!

References

• http://academic.brooklyn.cuny.edu/geology/leveson/core/linksa/maptop.html