I. Contour Maps
Dr.
SaMeH Saadeldin Ahmed
Assistant professor of Mining and Environmental Engineering
Email: dr_sameh_mining04@yahoo.com
A short course for
Egyptian Cement Company (ECC)
April, 2008
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Outlines
1.
2.
3.
4.
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Introduction
Definition
General Contouring Rules
Contouring Procedure
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1. Introduction
The purpose of any map is to present
information in an interpretable form.
Topographic maps, structure maps, and
isopach maps, are quantitative. Each is a
graphical integration of a specific type
of information presented in a readily
understandable form.
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Although contour maps cannot replace
the written word in communicating
complicated ideas, they illustrate
graphically
the
three-dimensional
concept better than any other means yet
devised.
Topographic and structure maps are the
most widely used contour maps, but there
are many other special applications of
contouring.
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2. Definition
Contours are lines drawn on a map to
connect points of equal value compared
to some chosen reference.
Information being contoured may be
expressed directly as a numerical values,
or may be expressed as a ratio to or
percentage of the reference.
A contour map represents lines of intersection
between a horizontal plane and a surface which
are projected onto a horizontal plane.
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3. General Contouring Rules
• There are about 5 rules controlling the
contouring process. However, before
talking about them ..
• Let us discuss the different ways to conduct
a contour map in the field..
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Rule 1
• Contour
another
lines cannot cross one
Contour lines represent given values, and
since each line represents a different
value these lines cannot cross.
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Example 1
• The 100-foot line represents only values
of 100 feet. The 200-foot line represents
only the values of 200 feet. Therefore,
there can be no point on either line which
represents both the 100 value and the 200
value, and since there is no point
common to both lines they cannot cross.
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Rule 2
Contour lines may not merge with contours
of different values or of the same values
When a vertical plane is projected upon a map
the contours appear to be merging.
In space, however, those lines would not be in
contact but would be one above the other.
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Example 2
• A cliff 400 feet high may have a 100-foot
contour at the base and a 400- foot
contour vertically above it. On a map, the
100-, 200-, 300-, and 400- foot contours
appear to converge, because they occupy
the same horizontal position upon a
plane surface.
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Rule 3
Contours must always close or end at
the edge of the map
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Example 3
• One way visualize a circular hill as being
slowly inundated by the sea, with a contour
line representing each new sea level. If this
process were viewed from the air the hill
would be appear as a circular island of smaller
and smaller dimensions. The contour lines on a
map illustrating the rising shore-line would be
a series of concentric circles of smaller and
smaller diameter. Since not all hills slope
equally in all directions the contours may be
contoured or elongated, but they will always
be closed.
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• Special case a : Small hills within the
confines of the map will always be
shown by a series of closed contours.
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• Special case b : The area covered by a
map may be so small that only a part of
the hill is shown. The contours will end
at the edge of the map, and the direction
of slope may be all in one direction .
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• Special case c
The map may cover the crest of
the hill, with slope in two
directions away from a central
area. Slope away from the
crest will be represented by a
series of repeated contours,
which are of lower and lower
values.
Although
these
contours end at the edge of the
map, in space they continue
around the ends of the hill
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Contours outlining the “V” of the valley
wall continue around the land mass into
which the valley is being cut
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Rule 4
Contours of the same value must be
repeated to indicate a reversal of
direction of slope
Where the contour lines represent the
intersection of a horizontal plane with a
physical surface, the plane will intersect all
sides of a hill or both walls of a valley.
Therefore, there must be contour lines on all
sides of the hill or valley to represent the line
of intersection
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Example 4
• Regardless of the data being contoured,
it is obvious that high values surrounded
by low values are segregated within a
closed contour. If, however, high values
are flanked by low values on two sides
only, the separation is shown by a
repetition contours on each side of the
high values.
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Rule 5
The contour interval, or unit upon which the
map is drawn, should be a function of:
1. the scale of the map,
2. the amount of variation between the
values being contoured,
( 3. the amount of detail which is desirable for
the special purpose of the map.
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Example 5
• If the map scale is small or the relief is
great, the contour interval is necessarily
large. If the scale is large and the
variations are small, the contour interval
should be small, in order to illustrate the
features that do appear.
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4. Contouring Procedure
Contour line are quantitative illustrations.
The rate at which the values changes is
indicated by the spacing between
contours.
Lines close together indicate a rapid rate of
change for a given contour interval;
Lines far apart indicate a slow rate of change.
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Example A
•
•
•
•
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Points 0 indicate well locations.
Map distance between points is 4 inches.
Map scale is 1 inch to 1 mile.
I inch on horizontal scale is distance covered for 100
feet of vertical rise.
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The distance between the 100-foot and
500-foot elevation must be broken into
four equal units if the contour interval is
100 feet.
If a 50-foot interval is chosen, there must
be eight equal units.
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Example B
• Points 0 indicate well locations.
• Map distance between points is 2.5 inches.
• Map scale is 1 inch to 1 mile.
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The vertical distance between the two well
locations is 119 feet. This distance can be
divided evenly only if the contour interval is 1
foot.
Since the interval is too small for most maps, it
is better to use large interval, such as 25
feet, and to determine the approximate
number of divisions necessary.
By dividing the 119 into 25 foot intervals, there
will be four equal units, with 19/25 ths
remaining.
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The
distance
can
be
divided
mathematically, to determine the exact
proportionate horizontal distance which
represents 25 feet,
But exact computation are too time
consuming usually for the amount of
data to be contoured.
It is better to learn to make the divisions
by eye, checking at intervals by exact
computation until the eye is trained.
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Mathematical determination of the map
distance between point 0 (106 feet) and
contour point of 125 feet is:
d = 0.4 inches
Check the calculation
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Example C
Procedure for contouring:
1. Draw light lines, which can be erased,
connecting the highest point to all
surrounding points.
2. Determine the position of each contour
value along these lines. (see B.)
3. Draw lines connecting of equal value,
according to the general rules for contouring.
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Note: the 200- and 300 foot contour lines
curve into a valley between the two
high areas.
Although no 100-foot datum is available
to make the 100-foot contour line follow
the low, logical contouring suggests
that the 100-foot line parallels the other
contour lines, unless data were
available to indicate otherwise.
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Thanks for your attention
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