Introduction to Topographic Maps + Contour Lines

1. Topographic Maps 2-dimensional model of the Earth’s surface representing the 3-D real world Topographic maps are also known as contour maps. Show elevation above sea level using contour lines.

Topographic Maps A. Contour Line: line on a map that connects points of EQUAL elevation. shows elevation and shape of the land Relief - Difference between high and low elevations

Topographic Maps B. Contour Interval – Difference in elevation between each line. MUST be separated by same interval. Contour interval = 20 feet 520 540 560 580

Topographic Maps C. Index Contour – Usually every 5th line is printed darker and has an elevation printed on it.

Let’s take a walk up a hill!

We’re now at an elevation of 100 feet. Draw + label on your map – 100’ Contour lines 100 ft

Now we’re at 200 feet. Contour lines 200 ft 100 ft Draw + label on your map – 200’ Contour lines 200 ft 100 ft

We’ve made it to 300 feet! Contour lines 300 ft 200 ft 100 ft Draw + label on your map – 300’ 300 ft 200 ft 100 ft

We’re on the peak, but what’s our elevation? 300 ft Contour lines 200 ft 100 ft

Add contour lines for every 50 feet and see if that helps. Any ideas? Add contour lines for every 50 feet and see if that helps. 300 ft 200 ft 100 ft

We know that we are above 350 ft, but less than 400 ft.

Let’s now look at the same hill, but the way we might see it from an airplane.

Each color change represents a 50-foot change in elevation.

Now, let’s try the same hike! Our elevation is 0 feet.

Now, we are at the peak, elevation between 350’ – 400’.

Imagine the sea level rising steadily Imagine the sea level rising steadily. Those would be our contour lines = lines of same elevation.

500’ upward What is contour interval? “Real World” 500’ Do the contour lines bend upward or downward at a stream? Contour Map upward

2. Rules for Contours Contour lines never cross

Rules for Contours 2. Contours form closed loops (even if not shown of the map.

Rules for Contours 3. Contours bend upstream (uphill) when crossing a stream.

Rules for Contours The maximum possible elevation for a hill is 1 less than what the next contour “should” be. --OR-- The highest possible elevation of the hill is just below the value of  the next line that is not shown. 90 80 70 60 50

399 239

5. Closely Spaced Contours Closely spaced contours = steep gradient.

6. Widely Spaced Contours Widely spaced contours = gentle, gradual gradient.

A A B B

Add gradient to your map

7. Depressions Contour lines which show a depression, crater, or sinkhole on a map. Shown by dashed lines (hachure marks) on the inside of a contour line. Draw a hachure mark The elevation of the first depression contour is the same as the lowest regular contour near it.

Rules for Contours d. The lowest possible elevation for a depression is 1 more than what the next contour “should” be. -- OR -- The lowest possible elevation of a depression is just above the value of  the next line that is not shown. 90 90 51 80 70 60 50

100 100 50

8. Benchmarks a location whose exact elevation is known and is noted on a brass or aluminum plate. bench marks are shown on maps by an X with the letters BM written next to them.

Map Scales Indicates the distance on the map compared to distance in the real world 3 types: 1. Graphical - a line divided into equal parts and marked in units of length.

Map Scales 2. Fractional – Expressing distance as a ratio. Example 1:63,360 This means: One unit on the map equals 63,360 units in the real world. (Fyi…there are 63,360 inches in a mile)

Map Scales 3. Verbal– expressing distance as a statement. Example: Saying aloud, 1 cm on the map = 1 km in the real world.

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4. Gradient The steepness of a slope between any 2 points on a map. Change in elevation Distance

Gradient A trail is four miles long as measured by the scale on a map. The beginning of the trail is at the 1,060 ft contour line and the end of the trail is at the 960 ft contour line. Calculate the gradient of the trail. Gradient = 1060 ft – 960 ft 4 miles = 25.0 ft/mi

What is gradient between points A and B? Change in elevation Contour Interval = 20 ft Distance B 2250 ft / 6 miles A = 375 ft/mile 6 miles 2 4 8 10