introduction to topographic maps. topographic maps two dimensional model of the earth’s surface...

Introduction to Introduction to Topographic Topographic

MapsMaps

Topographic MapsTopographic Maps

• Two dimensional model of the Earth’s surface (represents 3-D world)

• Topographic maps are also known as contour maps.

• Show elevation above sea level using contour lines.

Contour Map

“Real World”


• Contour LineContour Line – - line on a map that connects points of EQUAL elevation.- show elevation and shape of the land

• Relief – Difference between high and low elevations


• Contour IntervalContour Interval –

difference in elevation between each line. MUST be equal spacing.

Contour interval =

20 feet520

540

560

580


• Index Contour –

Usually every 5th line is printed darker and has an elevation printed on it.

Rules for Contours

1. Contour lines never cross

Rules for Contours

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

Rules for Contours3. Contours bend upstream (uphill) when

crossing a stream.

PSC 121 Prince George's Community College

Tracking Flowing Water

• How does water always flow?

DOWNHILL

• How can we tell what direction (N,S,NW,ESE) is downhill?


Tracking Flowing WaterYou can look for

elevation changes since water will flow from higher to lower elevations.

Here the stream flows toward the northwest.

2100 ft2000 ft


Tracking Flowing Water

You can look at the contour lines as they cross the stream. They will always point upstream.

Downstream Upstream

Rules for Contours

4. The maximum possible elevation for a hill is “1” less than what the next contour “should” be.

The highest possible elevation of the hill is just below the value of the next line that is not shown

5060

70

80

90

239

399

179

Closely Spaced ContoursClosely Spaced Contours

• Steeper Slope (Gradient) – contour lines are closer together.

Wide Spaced ContoursWide Spaced Contours

• Gradual/Gentle Slope (Gradient) – contour lines are farther apart.


Steep or flat?

You can tell whether a region on a map is steep terrain (hill/mountain) or relatively flat (plain) by looking at contour lines.

HOW?

We need to calculate a terrain gradient. This is the change in elevation over the change in distance along the ground.

Elevation (ft)Gradient =

Ground Distance (mi)


Steep or flat?

0.5 mile distanceElevation change2170-2150 = 20 feetGradient 20ft/0.5 mi = 40 ft/mi Flat

Elevation change1380-780 = 600 feetGradient 600ft/0.5 mi = 1200 ft/mi Steep


Contours and Profiles

You can generate a profile of a landform from the contour lines on a topographic map.

Simple contour map

Click on imageto see contoursdevelop

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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

• The elevation of the first depression contour is the same as the lowest regular contour near it.

Rules for Contours

The lowest possible elevation for a depression is “1” more than what the next contour “should” be.

The lowest possible elevation of a depression is just above the value of the next line that is not shown

50

90 90

80

7060

51

100100

50

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 ScalesMap Scales

• Indicates the distance on the map compared to distance in the real world

• Graphical - by a line divided into equal parts and marked in units of length.

Map ScalesMap Scales

• Numerically – usually by writing a fraction to show what part of the true distances map distances really are.

• 1:63,360

• One inch on the map equals 63,360 Inches in the real world.

(There are 63,360 inches in a mile)

GradientGradient

• 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 miles25.0 ft/mi


Contour Lines

Elevation is indicated by contour lines. A contour line is a line of equal elevations. All points on a contour line have the same elevation.

800 feet

700 feet

http://raider.muc.edu/~

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20Maps/

contour.htm


Contour Lines

The elevation difference from one line to another is called the contour interval. On the map portion below the contour interval is 20 ft.

20 ft elevation increase

20 ft elevation increase

20 ft elevation decreasehttp://raider.m

uc.edu/~m

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contour.htm


Contour Lines

The contour interval is usually printed on a topographic map.

However, you can always calculate the intervalHowever, you can always calculate the interval

from the lines with values.from the lines with values.

Review Questions

• How are contour lines drawn on maps?• How does a contour map show whether a

slope is gentle or steep?• What is a benchmark?• How is a benchmark shown on a map?• How is distance measured on a contour

map?

introduction to topographic maps. topographic maps two dimensional model of the earth’s surface...

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