527
A P P E N D I X
B
A map is a representation on a flat surface of all or a part of
Earth’ s surface drawn to a specific scale. Maps are often the
most effective means for showing the locations of both natural
and human structures, their sizes, and their relationships to one
another. Like photographs, maps readily display information
that would be impractical to express in words.
While most maps show only the two horizontal dimensions, geologists, as well as other map users, often require that
the third dimension—elevation—be shown on maps. Maps
that show the shape of the land are called topographic maps.
Although various techniques may be used to depict elevations,
the most accurate method involves the use of contour lines.
Contour Lines
A contour line is a line on a map representing a corresponding
imaginary line on the ground that has the same elevation above
sea level along its entire length. While many map symbols are
pictographs, resembling the objects they represent, a contour
line is an abstraction that has no counterpart in nature. It is,
however, an accurate and effective device for representing the
third dimension on paper.
Some useful facts and rules concerning contour lines
are listed as follows. This information should be studied in
conjunction with Figure B.1.
1. Contour lines bend upstream or upvalley. The contours
form V’ s that point upstream, and in the upstream
direction the successive contours represent higher
elevations. For example, if you were standing on a stream
bank and wished to get to the point at the same elevation
directly opposite you on the other bank without stepping
up or down, you would need to walk upstream along the
contour at that elevation to where it crosses the stream
bed, cross the stream, and then walk back downstream
along the same contour.
2. Contours near the upper parts of hills form closures. The
top of a hill is higher than the highest closed contour.
3. Hollows (depressions) without outlets are shown by
closed, hatched contours. Hatched contours are contours
with short lines on the inside pointing downslope.
4. Contours are widely spaced on gentle slopes.
5. Contours are closely spaced on steep slopes.
A P P E N D I X B
Topographic Maps
6. Evenly spaced contours indicate a uniform slope.
7. Contours usually do not cross or intersect each other,
except in the rare case of an overhanging cliff.
8. All contours eventually close, either on a map or beyond
its margins.
9. A single high contour never occurs between two lower
ones, and vice versa. In other words, a change in slope
direction is always determined by the repetition of the
same elevation either as two different contours of the
same value or as the same contour crossed twice.
10. Spot elevations between contours are given at many
places, such as road intersections, hill summits, and lake
surfaces. Spot elevations differ from control elevation
stations, such as benchmarks, in not being permanently
established by permanent markers.
Relief
Relief refers to the difference in elevation between any two
points. Maximum relief refers to the difference in elevation
between the highest and lowest points in the area being
considered. Relief determines the contour interval, which is
the difference in elevation between succeeding contour lines
that is used on topographic maps. Where relief is low, a small
contour interval, such as 10 or 20 feet, may be used. In flat
areas, such as wide river valleys or broad, flat uplands, a
contour interval of 5 feet is often used. In rugged mountainous
terrain, where relief is many hundreds of feet, contour intervals
as large as 50 or 100 feet are used.
Scale
Map scale expresses the relationship between distance or area
on the map to the true distance or area on Earth’ s surface.
This is generally expressed as a ratio or fraction, such as
1:24,000 or 1/24,000. The numerator, usually 1, represents
map distance, and the denominator, a large number, represents ground distance. Thus, 1:24,000 means that a distance
of 1 unit on the map represents a distance of 24,000 such
units on the surface of Earth. It does not matter what the
units are.
A P P E N D I X
B
A map is a representation on a flat surface of all or a part of
Earth’ s surface drawn to a specific scale. Maps are often the
most effective means for showing the locations of both natural
and human structures, their sizes, and their relationships to one
another. Like photographs, maps readily display information
that would be impractical to express in words.
While most maps show only the two horizontal dimensions, geologists, as well as other map users, often require that
the third dimension—elevation—be shown on maps. Maps
that show the shape of the land are called topographic maps.
Although various techniques may be used to depict elevations,
the most accurate method involves the use of contour lines.
Contour Lines
A contour line is a line on a map representing a corresponding
imaginary line on the ground that has the same elevation above
sea level along its entire length. While many map symbols are
pictographs, resembling the objects they represent, a contour
line is an abstraction that has no counterpart in nature. It is,
however, an accurate and effective device for representing the
third dimension on paper.
Some useful facts and rules concerning contour lines
are listed as follows. This information should be studied in
conjunction with Figure B.1.
1. Contour lines bend upstream or upvalley. The contours
form V’ s that point upstream, and in the upstream
direction the successive contours represent higher
elevations. For example, if you were standing on a stream
bank and wished to get to the point at the same elevation
directly opposite you on the other bank without stepping
up or down, you would need to walk upstream along the
contour at that elevation to where it crosses the stream
bed, cross the stream, and then walk back downstream
along the same contour.
2. Contours near the upper parts of hills form closures. The
top of a hill is higher than the highest closed contour.
3. Hollows (depressions) without outlets are shown by
closed, hatched contours. Hatched contours are contours
with short lines on the inside pointing downslope.
4. Contours are widely spaced on gentle slopes.
5. Contours are closely spaced on steep slopes.
A P P E N D I X B
Topographic Maps
6. Evenly spaced contours indicate a uniform slope.
7. Contours usually do not cross or intersect each other,
except in the rare case of an overhanging cliff.
8. All contours eventually close, either on a map or beyond
its margins.
9. A single high contour never occurs between two lower
ones, and vice versa. In other words, a change in slope
direction is always determined by the repetition of the
same elevation either as two different contours of the
same value or as the same contour crossed twice.
10. Spot elevations between contours are given at many
places, such as road intersections, hill summits, and lake
surfaces. Spot elevations differ from control elevation
stations, such as benchmarks, in not being permanently
established by permanent markers.
Relief
Relief refers to the difference in elevation between any two
points. Maximum relief refers to the difference in elevation
between the highest and lowest points in the area being
considered. Relief determines the contour interval, which is
the difference in elevation between succeeding contour lines
that is used on topographic maps. Where relief is low, a small
contour interval, such as 10 or 20 feet, may be used. In flat
areas, such as wide river valleys or broad, flat uplands, a
contour interval of 5 feet is often used. In rugged mountainous
terrain, where relief is many hundreds of feet, contour intervals
as large as 50 or 100 feet are used.
Scale
Map scale expresses the relationship between distance or area
on the map to the true distance or area on Earth’ s surface.
This is generally expressed as a ratio or fraction, such as
1:24,000 or 1/24,000. The numerator, usually 1, represents
map distance, and the denominator, a large number, represents ground distance. Thus, 1:24,000 means that a distance
of 1 unit on the map represents a distance of 24,000 such
units on the surface of Earth. It does not matter what the
units are.