Contours
CE 221 ‐ Higher Surveying
CE 221 Higher Surveying
Types of contours
Types of contours
•
•
•
•
•
Index contours
Index
contours
Intermediate contours
S
Supplemental contours
l
l
Depression contours
Approximate contours
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Index contours
Index contours
• Shown
Shown as heavy lines, usually twice the gauge as heavy lines usually twice the gauge
of the standard contours
• Normally shown every fifth (5
Normally shown every fifth (5th) contour
) contour
• Carry the number or elevation designation
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Index contours
Index contours
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Intermediate contours
Intermediate contours
• Lines
Lines of lighter weight between index contours
of lighter weight between index contours
• Usually not labeled, except when the terrain I relatively flat
relatively flat
• Conform to the contour interval of the map
• In parts of the map where intermediate contours I
t f th
h
i t
di t
t
are closely spaced as to nearly unite or merge into a single line it is customary not to portray
into a single line, it is customary not to portray the lines for short distances for readability (the technique is called feathering)
technique is called feathering)
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Supplemental contours
Supplemental contours
• Used
Used when the terrain is so flat and the when the terrain is so flat and the
intermediate contours are too far apart
• Drawn as dashed lines or lines of dots
Drawn as dashed lines or lines of dots
• Usually drawn at one‐half the specified contour interval of the map
i
l f h
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Depression contours
Depression contours
• Used
Used to show low spots around which to show low spots around which
contours close
• Index or intermediate contours are used to Index or intermediate contours are used to
which ticks drawn perpendicular to the contour lines on the downhill side
contour lines on the downhill side
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Depression contours
Depression contours
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Approximate contours
Approximate contours
• Used
Used when contours cannot be drawn when contours cannot be drawn
accurately possibly due inaccessibility of the area
• Based on the educated guess of the maker of the map
the map
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The contour interval
The contour interval
• The constant vertical distance between two adjacent contour lines
• Small intervals are appropriate for areas with light slopes and with small features such as hills
light slopes and with small features such as hills
• Large intervals are used for areas with steep slopes
• Commonly fixed at multiples of 0.5, 1, 2, 5, 10, 20, 50 and 100 meters
• Interrelated with the map scale; generally, the smaller the scale the larger the contour interval
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Selection of contour interval
Selection of contour interval
• Relative costs
Relative costs
– The cost of the map will be higher as the contour interval is reduced
interval is reduced
• Purpose of the map
• Nature of the terrain
N t
f th t
i
• Scale of the map
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Characteristics of contours
Characteristics of contours
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Characteristics of contours
Characteristics of contours
• All
All points on any one points on any one
contour have the same elevation
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Characteristics of contours
Characteristics of contours
• Every
Every contour closes on contour closes on
itself, either within or beyond the limits of the map. The closure may occur within the mapped area, but often d
b t ft
happens outside the area and thus will not
area and thus will not appear on the map sheet.
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Characteristics of contours
Characteristics of contours
• A
A contour which closes contour which closes
within the limits of the map indicates either a summit or depression
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Characteristics of contours
Characteristics of contours
• Contours
Contours on the ground on the ground
cannot cross one another except where an overhanging cliff, a vertical ledge or wall is represented on the t d
th
map.
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Characteristics of contours
Characteristics of contours
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Characteristics of contours
Characteristics of contours
• Contours
Contours are evenly are evenly
spaced on a uniform slope
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Characteristics of contours
Characteristics of contours
• Contours
Contours are straight are straight
and parallel to each other on a plane surface.
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Characteristics of contours
Characteristics of contours
• Irregular
Irregular contours contours
signify rough, rugged terrain
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Characteristics of contours
Characteristics of contours
• The
The horizontal distance horizontal distance
between adjacent contours indicates the steepness of the slope of the ground; the closer the contours, the l
th
t
th
steeper the slope
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Characteristics of contours
Characteristics of contours
• Contours
Contours cross curbs cross curbs
and a crowned sloping street in typical U‐
shaped curves
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Characteristics of contours
Characteristics of contours
• Contours
Contours cross streams cross streams
in a V pattern pointing upward
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