Basics of Contours

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    Lecture on

    Basics Characteristics of Contour

    Lines

    by

    RAAJ RamsankaranBITS Pilani

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    Contour lines

    Contour lines are a method of depicting the 3-dimensional

    character of the terrain on a 2-dimensional map.

    Understanding contours is a very useful navigation skill

    because you can identify the lay of the land and landscape

    features as they appear on the ground.

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    View of contours in 3D and 2D

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    Steep slopes - contours are closely spaced

    Gentle slopes - contours are less closely spaced

    Valleys - contours form a V-shape pointing up the hill - these

    V's are always an indication of a drainage path which could

    also be a stream or river.

    Ridges - contours form a V-shape pointing down the hill

    Summits - contours forming circles

    Depressions - are indicated by circular contour with lines

    radiating to the center

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    Why do you need to know the

    terrain features on a map?

    In order to locate a point on a map

    In order to navigate from one point to the next

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    Types of Terrain Features

    Major-Five

    Minor-Three

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    Identification of Terrain Features

    using Hand

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    Five Major Terrain Features

    Hill

    Ridge

    Valley

    Saddle

    Depression

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    Hill

    A hill is a point or small area of high ground. When you are on a hilltop, theground slopes down in all directions. The height and slope of a hill are

    represented on a map by the number of concentric contour rings and the

    distance between the rings.

    Major

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    Ridge

    A ridge is a line of high

    ground with height variations

    along its crest. A ridge is not

    simply a line of hills. All of the

    points of the ridge crest arehigher than the ground on

    both sides of the ridge. You

    can identify a ridge on a map

    by looking for contour lines

    forming a U or V. The closedend of the U or V points away

    from high ground.

    Major

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    Valley

    A valley is a reasonably

    level ground bordered on thesides by higher ground. It

    generally has room to

    maneuver within it. A valley

    may or may not contain a

    stream course running

    through it. Contour lines

    indicating a valley are U-

    shaped and are usually a

    dotted or broken line.

    Arrows that point

    upstream are valleys

    MajorMajor

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    What terrain feature is represented by contour

    lines that tend to parallel a stream before crossing

    and form a U?

    VALLEYVALLEY

    What terrain feature is represented by contourWhat terrain feature is represented by contourlines forming concentric circles?lines forming concentric circles?

    HILLHILL

    What terrain feature is a line of high ground withWhat terrain feature is a line of high ground with

    height variations along its crest?height variations along its crest?

    RIDGERIDGE

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    Saddle

    A saddle is a dip or low

    point along the crest of a

    ridge. A saddle is not

    necessarily the lower ground

    between two hilltops. It may

    also be a break along anotherwise level ridge crest. A

    saddle is normally represented

    as an hourglass shape on a

    map. The hourglass is made

    by contour lines narrowing

    between two groups of

    concentric circles that indicate

    hilltops or points of raised

    ground.

    MajorMajor

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    Depression

    A depression is a low point

    or hole in the ground that is

    also surrounded on all sides

    by higher ground. A

    depression generally has noor little room to maneuver

    within it. The map symbol

    that represents a depression

    is closed contour lines that

    have tick marks. The tickmarks point toward the low

    ground or down slope

    MajorMajor

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    HillHill DepressionDepression RidgeRidge

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    ValleyValley SaddleSaddle

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    Two Minor Terrain Features

    Draw

    Spur

    Cliff

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    Draw

    A draw is similar to a valley except itA draw is similar to a valley except it

    usually has a less developed course.usually has a less developed course.

    There is generally no level groundThere is generally no level ground

    and, therefore, little or no room toand, therefore, little or no room to

    maneuver. The ground slopes upwardmaneuver. The ground slopes upward

    on each side and toward the head ofon each side and toward the head of

    the draw. Draws are caused by flashthe draw. Draws are caused by flash

    floods and can be found on flat terrainfloods and can be found on flat terrain

    but are more often found along thebut are more often found along the

    sides of ridges. Contour linessides of ridges. Contour lines

    indicating a draw are shaped like a Vindicating a draw are shaped like a Vwith the point of the V pointingwith the point of the V pointing

    toward the head of the draw. Thattoward the head of the draw. That

    means the point is pointing toward themeans the point is pointing toward the

    high ground.high ground.MinorMinor

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    Spur

    A spur is usually a short,A spur is usually a short,

    continuously sloping line ofcontinuously sloping line of

    higher ground, normallyhigher ground, normally

    jutting out from the side ofjutting out from the side ofa ridge. A spur is oftena ridge. A spur is often

    formed by two very parallelformed by two very parallel

    streams cutting gullies orstreams cutting gullies or

    draws down the side of adraws down the side of a

    ridge. You can spot a spurridge. You can spot a spur

    on a map when the contouron a map when the contour

    lines form a U or V shapelines form a U or V shape

    that points away from thethat points away from the

    higher ground.higher ground.

    MinorMinor

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    CliffA cliff is a vertical or nearA cliff is a vertical or near--vertical slope. A cliff may be shownvertical slope. A cliff may be shown

    on a map by contour lines that are very close together. A cliff ison a map by contour lines that are very close together. A cliff isalso shown as a ticked contour line with the tick marksalso shown as a ticked contour line with the tick marks

    pointing towards the lower ground.pointing towards the lower ground.

    MinorMinor

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    What terrain feature is a vertical or near vertical

    slope?

    CLIFF

    What terrain feature is represented by contour lines

    are V-shaped with the point of the V toward the headof the high ground?

    DRAW

    What terrain feature is represented by short,

    continuously sloping line of higher ground jutting out

    the side of a ridge?

    SPUR

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    CliffCliff DrawDraw SpurSpur

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    Contour Lines and Valleys

    ContoursContours

    Contours wrapped in 3D viewContours wrapped in 3D view

    Source: Stephen J. Reynolds and Carla McAuliffe, Arizona State University

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    Methods of Contouring

    Direct Methods

    Indirect Methods

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    Direct Methods

    In the direct method, the contour to be plotted is actually

    traced on the ground. Points which happen to fall on adesired contour are only surveyed, plotted and finally

    joined to obtain the particular contour. This method is

    slow and tedious and thus used for large scale maps, small

    contour interval and at high degree of precision.

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    Indirect Methods

    In this method, the spot levels of selected guide points are

    taken with a level and their levels are computed. Thehorizontal positions of these points are measured or

    computed and the points are plotted on the plan. The

    contours are then drawn by a process called interpolation of

    contours from the levels of the guide points. The following

    are the indirect methods are commonly used for locating

    contours.

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    Different Indirect Methods

    SquaresSquares oror GridGrid methodmethod

    Cross section method

    Tacheometric method

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    Sources of Information

    http://www.outdoored.com/Articles/Article.aspx?ArticleID=111

    http://www.armystudyguide.com/content/army_board_study_guide_topics/land_navigation_map_reading/identify-major-minor-

    terr.shtml