BIOL 2406 – Fall 2011
Topographic Maps
A topographic map is a two dimensional representation of a
three dimensional land surface. Elevation or relief is shown
through the use of contour lines that are continuous points
of the same elevation or vertical distance above a reference
point such as sea level. There are also colors and symbols
that represent other features on the land, such as water,
vegetation, roads, boundaries, urban areas and structures.
quadrangle
The US Geological Survey produces a series of topographic
maps that depict sections of the Earth's surface. These maps
systematically divide the United States into precise rectangles
based on a latitude/longitude grid system (Figure 1). These
maps are commonly referred to as quadrangles or “quads”.
Figure 1. Latitude/longitude grid
system showing quadrangle.
Latitude and Longitude
The most common way to locate points on the
surface of the Earth is by standard,
geographic coordinates called latitude and
longitude (Figure 2). These coordinate
values are measured in degrees, and
represent angular distances calculated from
the center of the Earth. Latitude values
indicate the angular distance between the
Equator and points north or south of it on the
surface of the Earth. A line connecting all the
points with the same latitude value is called a
line of latitude. All lines of latitude are parallel
to the Equator, and are referred to as
parallels. Parallels are equally spaced. The
Equator is given the value of 0 degrees
Figure 2. Geographic coordinate system.
latitude. There are 90 degrees of latitude
going north from the Equator and 90 degrees to the south of the Equator. The North Pole is at 90 degrees
N and the South Pole is at 90 degrees S. When the directional designators are omitted, northern latitudes
are given positive values and southern latitudes are given negative values.
Lines of longitude, called meridians, run perpendicular to lines of latitude, and all meet at the poles.
Lines of longitude are not parallel; the closer they are to the poles, the shorter the distance between
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them. There is no obvious 0-degree point for longitude, as there is for latitude. By international
agreement, the meridian line through Greenwich, England, is currently given the value of 0 degrees of
longitude; this meridian is referred to as the Prime Meridian. Longitude values indicate the angular
distance between the Prime Meridian and points east or west of it on the surface of the Earth. There are
180 degrees of longitude to the east of the Prime Meridian; when the directional designator is omitted
these longitudes are given as positive values. There are also 180 degrees of longitude to the west of the
Prime Meridian; when the directional designator is omitted these longitudes are given as negative values.
The 180-degree longitude line is opposite the Prime Meridian on the globe, and is the same going either
east or west. The 180 degree meridian is also known as the International Date Line.
Counting and Converting Minutes and Seconds
Degrees alone are not accurate enough to find a precise location. At best, one degree of latitude and
longitude would define an area of 70 square miles. To overcome this problem, degrees are divided into
minutes and minutes are divided into seconds:
1 degree (1°) = 60 minutes (60’)
1 minute (1’) = 60 seconds (60”)
When counting seconds, if the count reaches 60, add 1 to the minutes count and start counting seconds
again from 0. When counting minutes, if the count reaches 60, add 1 to the degrees count and start
counting minutes again from 0.
Latitude and longitude can be expressed as degrees minutes seconds (dddo mm’ ss.s”), degrees decimal
minutes (dddo mm.mmm’), or decimal degrees (ddd.ddddo). It is essential to know how to make
conversions between these. To convert from degrees minutes seconds to decimal degrees first divide the
seconds by 60 to get the degrees decimal minutes. Next divide the decimal minutes by 60 to get the
decimal degrees. For example, to convert 48° 20' 30" into decimal degrees:
30" ÷ 60 = 0.5' yielding 48° 20.5'
20.5’ ÷ 60 = 0.34167° yielding 48.34167°
To convert from decimal degrees to degrees minutes seconds multiply the decimal degrees by 60 to get
degrees decimal minutes then multiply the decimal minutes by 60 to get degrees minutes seconds.
For example, to convert 48.34167° into degrees minutes seconds:
0.34167° x 60 = 48° 20.5002”
0.5002” x 60 = 48° 20' 30"
Contour Lines
Contour lines connect points on the map that have the same elevation above sea level. As a result of
this every contour line must eventually close on itself to form an irregular circle. Contour lines do not
overlap or cross one other, or else you have two elevations in the same exact place. However there are
exceptions to this rule. If there is a vertical cliff contour lines can merge to form a single line. On an
overhanging cliff or a cave the contour line representing the lower elevation is dashed. Where contour
lines are spread out, you have a gentle slope. Where contour lines are close together, you have a steep
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slope. Individual contour lines are usually brown in color. Heavy brown lines are referred to as index
contours because they often have elevations printed on them. The distance (or elevation) between
contours is referred to as the contour interval and is often specified on most topographic maps in feet or
meters. Moving from one contour line to another always indicates a change in elevation. To determine if it
is a positive (uphill) or negative (downhill) change you must look at the index contours on either side. On
a hill with a consistent slope, there are always four intermediate contours for every index contour. If
there are more than four intermediate contours it means that there has been a change of slope and one
or more contour line has been duplicated. This is most common when going over the top of a hill or
across a valley. At certain points on a topographic map, individual elevation points apart from the contour
lines are measured. These more precise measures are called bench marks. On the ground these points
are marked by a brass plaque. On the map they are marked by an X with the elevation labeled beside it.
Sometimes the bench mark has the initials BM in front of the measurement.
Rules of Contour Lines
•
Contours do not cross or intersect each other, except in the rare case of an overhanging cliff.
•
All contours eventually close, either on a map or beyond its margins.
•
Contours are widely spaced on gentle slopes.
•
Contours are closely spaced on steep slopes.
•
Evenly spaced contours indicate a uniform slope.
•
The inside of a closed contour line is the high side. A series of closed contours represent a hill.
The top of a hill is higher than the highest closed contour.
•
Hollows (depressions) without outlets are shown by closed, hatched contours. Hatched contours
are contours with short lines on the inside pointing downslope. The bottom of the hollow is lower
than the lowest closed contour.
•
Where a contour line crosses a stream or valley, the contour bends to form a "V" that points
upstream.
•
In the upstream direction the successive contours represent higher elevations.
•
Elevation increases as you move away from a stream.
•
In crossing a valley, the last contour met before reaching the stream is the first one encountered
on the other side of the stream.
•
A single higher elevation contour never occurs between two lower ones, and vice versa. 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.
•
Contour lines never run into a body of water.
Reading the Margins
Information about the map is found in the margins (Figure 3). For example, the names of the eight
adjoining quadrangles are in the corners and sides of the map.
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Agency or Author Who Created Map
In the upper left corner is the agency or author who created the map. This same information can also be
found in the bottom left corner.
Map Title
The title of the map is in the right upper corner. This corner section provides the name of quadrangle,
state (and sometimes the county) where the quadrangle is located, and map series. Quadrangles are
often named after a prominent city, town, or natural feature that is within the quadrangle.
adjoining
quadrangle
map author
longitude
tic mark
adjoining
quadrangle
longitude
tic mark
map title
adjoining
quadrangle
longitude &
latitude reference
coordinates
latitude
tic marks
map scale
date created &
projection used
contour
interval
legend
quadrangle
location
revision
date
Figure 3. Plano quadrangle showing margin information.
Latitude and Longitude
Latitude and longitude lines are indicated with fine black tick marks. These graticules are intersections of
latitude and longitude lines and are marked on the map edge, and appear as black cross-marks (+) at
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four points in the interior of the map. Topographic maps do not show the latitude/longitude lines – just the
tick marks. The numbers next to the tick marks indicate degrees (°), minutes (') and seconds ("). On
1:24,000 scale maps, latitude and longitude tick marks are indicated every 2.5 minutes. Longitude tick
marks are on the top and bottom edges of the map and latitude tick marks are on the right and left edges.
Note that the degrees may be left off (as an abbreviation) and you may only see the minute and/or
second designations. Reference coordinates for latitude and longitude (degrees, minutes, and seconds)
are black and located on the four corners of the map.
Legend
In the right lower corner there is a legend that contains symbols for the roads. Slightly to the left of this
area there is the quadrangle location. The location is pinpointed on a map of the state. Also in this corner
is the revision date, which is when the map was last updated. If the map is old, it may not be accurate.
Refer to the map production information in the bottom left corner for additional information on map dates.
Contour Interval
Contour lines can be drawn for any elevation, but to simplify things only lines for certain elevations are
drawn on a topographic map. These elevations are chosen to be evenly spaced vertically. The contour
interval is the difference in elevation between two adjacent contour lines. A typical interval is 10 feet. This
means that every time you go from one contour line to another the elevation would change 10 feet. Index
lines would therefore occur every 50 feet.
Figure 4. Typical 7.5 minute topographic map scale.
Map Scale
The scale of the map is found at the bottom center. Map scale represents the relationship between
distance on the map and the corresponding horizontal distance on the ground. Scale is represented in
two different ways on a topographical map (Figure 4). The first is a fractional or ratio scale. A typical map
scale is 1:24,000. This means is that one inch on the map represents 24,000 inches on the ground.
These maps are commonly known as 7.5-minute quadrangle maps; each map covers 7.5 minutes of
latitude and 7.5 minutes of longitude, which is approximately 8 miles (north/south) and 6 miles
(east/west).The USGS produces maps using the 1:24,000 scale, but also produces maps using 1:62,500,
1:100,000, and 1:250,000 scale. The 1:24,000 scale provides larger and clearer details than the
1:250,000, but it does not cover as large an area (Figure 5). Below the ratio scale is a bar or graphic
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scale representing distance in miles, feet and meters. The bar is divided into segments, each of which
represents an equal distance on the map. The left side of the bar is usually divided into smaller units of
equal size to make it easier to measure shorter distances. The bar scale is useful when a user does not
have a ruler or measuring scale; the distance between two points can be transferred to a piece of paper
and then compared to the bar scale.
1:250,000
1:100,000
1:24,000
Figure 5. The effect of map scale
Estimating Slope
Slope is the degree of inclination or steepness and it is usually expressed in percent. A one percent slope
indicates a rise or drop of one unit over a distance of 100 horizontal units. For example, a one percent
slope rise would indicate a one foot rise over a 100 foot horizontal distance. Slope along with
soil texture (sand, loam, clay) and ground cover determines how fast water will drain from an area. Water
drains quickly from steep slopes; however erosion may be a problem. Flat surfaces may result in
saturated soils. To calculate slope using a topographic map, you will need to determine the following:
•
Vertical Distance (also referred to as Rise) – This is the difference in elevation between two
points; it is calculated by subtracting the elevation of one point from the elevation of the other
point.
•
Horizontal Distance (also referred to as Run) – This is the distance from one point to the other
and is calculated by measuring distance with a ruler and applying the map scale. For example, if
the map scale is 1:24,000 and the distance between the two points when measured with a ruler is
½ inch, the horizontal distance would be 12,000 inches or 1,000 feet.
Slope can then be calculated using the slope formula:
Vertical Distance
X 100 = %Slope
Horizontal Distance
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Estimating Distances
First, refer to the scale at the bottom of the map. On a 7.5 minute
topographic quad 1 inch equals 2,000 feet. Straight line distances
can easily be measured using a standard ruler or engineer’s 20
scale ruler that has 20 graduations per inch. The side of the ruler
used to measure latitude previously is an example of such a scale.
Measure the distance in decimal inches and then multiply by 2000
to convert it to feet. Use the conversion factors in the adjacent table
to change units. Since most roads and trails are not straight,
measuring instruments are helpful. A map wheel (Figure 6) is such
a device. It has a counter which measures distance as the wheel
moves along the route. Some measuring devices are electronic and
must be set to the map’s scale before they are used.
Figure 6. Mechanical measuring wheel.
Steps for using a mechanical map wheel:
•
Set the counter to zero by adjusting the wheel. Notice that moving the wheel causes the arms to
rotate within the display.
•
Place the edge of the wheel at one end of the distance to be measured.
•
Orient the map wheel so that moving it causes the arms to rotate clockwise.
•
Slowly move the device along the route. Stay as close to the path as possible. Zigzagging adds
distance.
•
Repeat the process 3 times and average the values.
•
Convert results into desired units.
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Common Topographic Map Features
A hill is an area of high ground. From a hilltop, the
ground slopes down in all directions. A hill is shown
on a map by contour lines forming concentric circles.
The inside of the smallest closed circle is the hilltop.
Hill
A saddle is a dip or low point between two areas of
higher ground. A saddle is not necessarily the lower
ground between two hilltops; it may simply be a dip
or break along a level ridge crest. If you are in a
saddle, there is high ground in two opposite
directions and lower ground in the other two
directions. A saddle is normally represented as an
hourglass.
Saddle
A valley is a stretched-out groove in the land, usually
formed by streams or rivers. A valley begins with
high ground on three sides, and usually has a course
of running water through it. If standing in a valley,
the three directions offer high ground, while the
fourth direction offers low ground. Depending on its
size and where a person is standing, it may not be
obvious that there is high ground in the third
direction, but water flows from higher to lower
ground. Contour lines forming a valley are either Ushaped or V-shaped. To determine the direction the
water flows, look at the contour lines. The closed end
of the contour line (U or V) always points upstream
or toward high ground.
Valley
A ridge is a sloping line of high ground. If you are
standing on the centerline of a ridge, you will
normally have low ground in three directions and
high ground in one direction with varying degrees of
slope. If you cross a ridge at right angles, you will
climb steeply to the crest and then descend steeply to
the base. When you move along the path of the ridge,
depending on the geographic location, there may be
either an almost unnoticeable slope or a very obvious
incline. Contour lines forming a ridge tend to be Ushaped or V-shaped. The closed end of the contour
line points away from high ground.
Ridge
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A depression is a low point in the ground or a a
sinkhole. It could be described as an area of low
ground surrounded by higher ground in all
directions, or simply a hole in the ground. Usually
only depression that are equal to or greater than the
contour interval will be shown. On maps,
depressions are represented by closed contour lines
that have tick marks pointing toward low ground.
Depression
A draw is a less developed stream course than a
valley. In a draw, there is essentially no level
ground and, therefore, little or no maneuver room
within its confines. If you are standing in a draw,
the ground slopes upward in three directions and
downward in the other direction. A draw could be
considered as the initial formation of a valley. The
contour lines depicting a draw are U-shaped or Vshaped, pointing toward high ground.
Draw
A spur is a short, continuous sloping line of higher
ground, normally jutting out from the side of a
ridge. A spur is often formed by two roughly
parallel streams, which cut draws down the side of
a ridge. The ground is sloped down in three
directions and up in one direction. Contour lines
on a map depict a spur with the U or V pointing
away from high ground.
Spur
A cliff is a vertical or near vertical feature: it is an
abrupt change of the land. When a slope is so steep
that the contour lines converge into one “carrying”
contour of contours, this last contour line has tick
marks pointing toward low ground. (8) Cliffs are
also shown by contour lines very close together
and, in some instances, touching each other.
Cliff
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A ridgeline is a line of high ground, usually with
changes in elevation along its top and low ground
on all sides. Along the ridgeline there are hilltops
with saddles between them. The term ridgeline is
not interchangeable with the term ridge. A ridge is
on either end of the ridgeline.
Ridgeline
Appendix
Distance Conversions
Area Conversions
1 foot
1 mile
1 mile
1 kilometer
1 kilometer
0.3048 m
1 acre
43,560 ft2
1 acre
4,046.8564 m
1 hectare
107,639.1042 ft2
1 hectare
10,000 m2
5,280 ft
1.6093 km
3,280.8399 ft
0.6213 mile
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Check Your Understanding
Questions 1 - 12 refer to the Plano Quadrangle.
1.
When was the map produced? When were the revisions? How are revisions indicated on the
map?
2.
o
What lines (ddd mm’ ss”) of latitude and longitude form the borders?
N
S
E
W
3.
What is the highest and lowest elevation?
highest
lowest
The total relief would therefore be
ft.
4.
Due to the aspect of this relief, streams on the map are flowing in what direction?
5.
What cities are shown on the map?
6.
What major creek flows through the quadrangle?
7.
What heavy-duty roads are present?
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8.
What medium-duty roads are present?
9.
What railroads are present on the Plano Quadrangle?
10.
What is approximate elevation of the Spring Creek Campus in Plano?
feet
meters
11.
Find a depression on the Plano Quadrangle. What is associated with the depression?
12.
What is the straight line distance between the “P” in Plano and the “A” in Allen?
miles
kilometers
13.
What does it indicate when contour lines are close together?
14.
What does it indicate when contour lines are far apart?
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15.
What is the slope between points A. and B.? (1 inch = 1000 ft.)
A.
B.
For questions 16 – 20 refer to the map below.
B.
A.
F.
C.
E.
D.
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16.
What is the contour interval of the above map?
17.
What contour line has the highest elevation?
18.
What contour line has the lowest elevation?
19.
The total relief of the map is
20.
Identify the topographic features.
ft.
A.
B.
C.
D.
E.
F.
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