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CHAPTER 9

Contour Lines

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Civil Drafting Technology, 7thedition

Madsen, Shumaker, and Madsen

2010 Pearson Higher Education,

Upper Saddle River, NJ 07458. All Rights Reserved.2

Contour Lines

Used to represent differences in elevation.

Connect points of equal elevation.

Show the general lay of the land. Describe geographical features.

Used in topographic maps.

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Contour Interval

The difference in elevation represented by each

contour lineon a topographicmap.

The difference in altitude (elevation) represented by

the space between two contour lines on a map

ELEV. 250

ELEV. 200

CONTOUR INTERVAL

50 FEET

SLOPE
http://dictionary.reference.com/browse/linehttp://dictionary.reference.com/browse/topographichttp://dictionary.reference.com/browse/topographichttp://dictionary.reference.com/browse/line

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Slopes

The steepness of a slope can be determined by

the spacing of contour lines.

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Slopes

Uniform gentle slope Contour lines are

approximately the same

distance apart. Greater distance between

contour lines.

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Slopes

Uniform steep slope Contour lines are

approximately the same

distance apart. Less distance between

contour lines.

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Slopes

Convex slope Steeper towards the

bottom.

Greater distance between

contour lines at the top of

the slope than at the

bottom.

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Cliffs

Vertical slope Contour lines merge.

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Streams and Ridges

Single stream Streams form V shaped contour lines. The bottom of the V

points upstream.

Interesting streams Intersecting streams form M shaped contour lines. The tops of

the M point upstream.

Hills and ridges

Hills and ridges form U shaped contour lines. The bottom of theU points downhill.

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Relief Features

Obvious geographic features.

Dome-shaped hill Peak of a hill or mountain forms circular contour lines

of ever-decreasing diameter. Figure 9-8a

Saddle

A low spot between two side by side peaks. Figure 9-8b

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Relief Features

Depression Human made holes or natural sinks. Short lines point into the depression.

Figure 9-8c Overhang:

Lower elevation contour lines run under higher

elevation contour lines. Figure 9-8d

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Types of Contour Lines Index Contours

Every fifth contour line. Thick. Broken at specific intervals and labeled with the contour line elevation.

Intermediate Contours

Four contour lines between the index contours. Define the contour interval (distance between parallel lines in reference to

elevation). Thin. Typically not labeled with the contour line elevation.

Supplementary Contours

Sometimes used to describe additional detail, especially on flat features. Usually dashed, and shown as half the contour interval.

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Types of Contour Lines

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Refer to Figure 9-9 to answer the following: What is the distance in elevation between the index lines?

50 What is the distance in elevation between the intermediate

lines? 10

What is the distance in elevation between the supplementarylines? 5

What is the contour interval? 10

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Elev. Of Figure 9-9INDEX

ELEV. 500

ELEV. 475

ELEV. 490

ELEV. 470

ELEV. 480

ELEV. 460

ELEV. 450

SUPPLEMENTARY

INTERMEDIATE

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Contour Line Values

Values represent elevations above mean sea

level.

Mean sea level The average elevation of the ocean between high

and low tides.

0 feet elevation.

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Contour Line Values

Contour interval values depend on: The size of the survey.

The purpose of the map.

The desired detail.

The difference in elevation on the plot.

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Ways of Constructing Contour Lines

Traditional and manual techniques.

Interpolation.

Using specialized CADD software. Ground survey required to accurately indentify

topography.

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Control Point Survey

A common method of establishing elevations foruse in contour mapping.

Process:

1. Survey field notes are plotted on the map and labeled.2. A contour interval is chosen based on the land area,topography, and elevation differential. The contourinterval defines the elevation of each contour line.

3. Contour lines are drawn.

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Control Point Survey

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Interpolation

Interpolate contour line points in order to find

where a specific contour line elevation crosses

the horizontal and vertical grid: Estimation

Mathematical interpolation

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Interpolation by Estimation

Study Figure 9-12 and follow the information

presented in the Interpolating Contour Lines

by Estimation section of Chapter 9 to better

understand how to estimate elevation pointsquadrangle maps.

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Interpolation

Mathematical interpolation (page 240):

Slope = Rise (difference in elevation of known points)

Run (distance between points of known elevation)

See Figure 9-14.

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Grid Survey

1. A plot of land is divided into a grid. Vertical grid lines are identified with letters A, B, C, D Horizontal grid lines (stations), are identified in hundreds of feet

plus tens of feet: 0+00 = 0 foot station 0+20 = 20 feet from 0+00 station 1+50 = 150 feet from 0+00 station 9+00 = 900 feet from 0+00 station

Distance between horizontal and vertical grid lines is equal(creates squares). The size of the squares is determined by the

surveyor based on the land area, topography, and elevationdifferential.

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Grid Survey

2. The elevation of each grid intersection ismeasured and labeled in field notes. Table 9-1

3. Draw the grid. Figure 9-16a

4. Label the elevation at each grid intersection usingthe field note measurements. Figure 9-16b

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Grid Survey

5. Identify the contour interval. The contour

interval will define the elevation of each

contour line.

6. Interpolate contour line points in order to find

where a specific contour line elevation crosses

the horizontal and vertical grid.

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Grid Survey

7. Connect points of equal elevation using a

curved line (spline).

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2010 Pearson Higher Education,Upper Saddle River, NJ 07458. All Rights Reserved.30

Radial Survey

Used to locate property corners, structures, naturalfeatures, and elevation points.

Often used when the instrument cannot be located at aknown point.

Radiation locating control points from a centrallocation called a transit station. Angular (azimuths) and distance measurements, known as

side shots, are taken between the transit station and control

points.

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Radial Survey

Multiple transit stations may be required to

survey a large property. Baseline the line between two stations.

Transit lines the lines between three or more

stations.

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Radial Survey

Drawing contour lines:1. Label the elevation at each side shot using field note

measurements.

2. Identify the contour interval. The contour interval will

define the elevation of each contour line.3. Interpolate contour line points in order to find where a

specific contour line elevation between existingpoints.

4. Connect points of equal elevation using a curved line(spline).

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Radial Survey

Interpolate contour line points in order to find where aspecific contour line elevation between existing points: Calculate the difference in elevation between two points. Identify the distance between the two points.

Step 1 Step 2 = percent of slope. Subtract the lower elevation from the nearest contour. Step 4 Step 3 = distance from point of lower elevation to

the nearest contour.

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Radial Survey

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Plotting Contour Lines with CADD

Manual methods. Specialized CADD systems:

Surveyed elevations are imported into the CADD

software, transferring data from the surveyinstrument (such as an electronic total station) to anoffice computer.

Survey points are connected into a triangulation

network of surfaces, which can then be used togenerate contours.

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Civil Drafting Technology 7th edition 2010 Pearson Higher Education

Chapter 9 Activities

Chapter 9 Test all but 9-14

Chapter 9 Problems p9-2 and p9-4

Use the CAD files from the student website to

draw the contour lines on each survey.

In P9-2 use estimation.

In P9-4 use calculated interpolation.

Civil Pp09

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