Map Reading
Note: Remember you are responsible for graphs, charts and
other items that form part of the overall summary of this topic.
A map is a graphic representation of a
portion of the earth's surface as seen from above, drawn to scale. A map
provides information on the existence; location; and distance between ground
features such as towns and roads. While there are literally thousands of
different types of maps, they basically fall into one of these categories:
- Topographic maps: This
is your typical USGS (United States Geological Survey) map. It portrays
variations in terrain; heights of natural features; and extent of vegetation
cover. Relief is represented by contour lines.
- Terrain models: This
is a scale model of natural and man-made terrain features. It provides a
means to visualize the terrain depicted on a map in three dimensions.
- Photo and Photomosaic maps:
This is a reproduction of an aerial photograph or a series of photographs,
upon which grid lines; marginal data; place names; route numbers; important
elevations; boundaries; and scale and direction have been added.
Topographic maps not only depict
a portion of the earth's surface to scale, but also incorporate other
information to assist the map user. The following list details information
that is currently standard to all USGS topographic maps.
- Map Producer Identification:
The agency which produced the map is identified in the top left-hand corner of
the map sheet.
- Map Sheet Name: The sheet
name is found in two places; the top right-hand corner and the lower
right-hand corner of the map sheet. A map sheet is named after the most
prominent man-made or natural physical feature depicted on that sheet.
Also found here is the area of the earth's surface portrayed on the map.
For example, if a map sheet is a 7.5 minute series or quadrangle, then it
covers 7.5 minutes of latitude and longitude. This can also be
determined by observing the latitude and longitude coordinates of the four
corners of the map.
- Map Specific Legend: This
legend depicts features specific to this map sheet and is located in the lower
right-hand corner.
- The State Locator:
Graphically depicts the location of the map in respect to a state's boundaries
and can be found in the lower right-hand side of the map.
- Scale: The scale of the map
is found in the bottom center of a map sheet. The scale gives the ratio
of map distance to the corresponding distance on the earth's surface.
- Graphic Bar Scales: Found at
the bottom center of the map sheet, these rulers are used to convert map
distances to ground distances and to convert distances between different units
of measure.
- Contour Interval:
Also found at the bottom center
of the map sheet below the graphic bar scales. These show the vertical
distance between contour lines on the map and the unit of measure used.
- Declination Diagram:
Sometimes referred to as the G-M, (grid-magnetic angle) is found in the lower
margin of the map sheet to the left of the graphic bar scales. The
declination diagram is the angular difference between true north and either
magnetic or grid north, for the particular portion of the earth's surface
depicted on the map. Declination diagrams are seldom plotted exactly to scale.
The relative position of the directions is obtained from the diagram, but the
numerical value of the declination should never be measured from the angles.
Use the written value of the declination provided to the left and right of the
diagram.
- Datum Information:
Found below the contour interval
note and contains information that can be used with a GPS unit.
- Map Information Note:
Contains information about the production of the map and can be found in the
lower left-hand corner of the map sheet.
- Index To Adjoining Sheets
Diagram: Found in the lower right-hand portion of the map sheet and
depicts the location of the map sheet you are using in respect to adjoining
sheets.
Locations of features depicted on a
topographic map can be determined by using latitude and longitude (also known as
the geographic coordinate system); township and range land survey system; or the
military grid reference system (MGRS).
- Latitude & Longitude: The
fundamental way of expressing the location of any spot on earth is with a
global reference system that is accepted by all people. This accepted
reference system is a coordinated grid system that uses imaginary east-west
and north-south lines that encircle the earth. The east-west lines are
called lines of latitude. Lines of latitude are parallel to each other
(and hence are also called parallels) and to the equator, which is a special
parallel that circles the earth exactly midway between the north pole and the
south pole. The location of a given parallel is designated as the
angular distance between the parallel and the equator as measured from the
center of the earth. This angular distance ranges from zero degrees at
the equator to ninety degrees at either pole. Therefore, latitude
indicates how far a place is located to the north or south of the equator.
Lines of longitude (or meridians) run north and south perpendicular to the
equator (and all other parallels) and meet at both poles. The location
of a given meridian is designated as the angular distance between the meridian
and the prime meridian, which is a line of longitude running through
Greenwich, England. This angular distance ranges from zero degrees at
the prime meridian to 180 degrees on the opposite side of the globe.
Therefore, longitude indicates how far a place is located to the east or west
of the prime meridian. Together, parallels and meridians comprise a
coordinated grid system, called the graticule, which makes it possible to
locate any spot on the globe. In locating a particular place on earth,
the latitude is always stated first, in degrees, minutes, and seconds
(depending on the accuracy required) and followed by a designation of N (for
north) or S (for south) relative to the equator. Next, the longitude is
given in degrees, minutes, and seconds, followed by E (for east) or W (for
west) relative to the prime meridian.
- Township and Range Survey System:
The federal government land office survey system was established in 1812 to
facilitate the surveying and selling of land in the newly acquired lands west
of Ohio. It is a system unique to the mid-western and western portions
of the United States. As the area became consolidated into territories
and eventually into states, the land there was divided into 6 mile square
units of land. Each 6 square mile unit was numbered corresponding to its
position relative to a parallel of latitude that served as the base line of
the survey, as well as a selected meridian of longitude called a principal
meridian (not to be confused with the Prime Meridian). Thus each 6 mile
square piece of land was designated as either north or south and east or west.
The north or south designation is referred to as township and the east or west
designation is referred to as range. The entire 6 mile square piece of
land however, was referred to as a township. Each 6 mile square piece of
land was further subdivided into 36 1-mile by 1-mile units called sections.
Each of the 36 sections are designated numerically beginning in the northeast
corner of the township. Each section can be subdivided still further
into halves and quarters.
- The Military Grid Reference
System (MGRS): As the lines of latitude and longitude are curved (as is
the earth), they are not depicted well on flat surfaces (such as a maps).
The sections of the earth enclosed by intersecting latitude and longitude
lines are not all of the same size and shape. This is particularly true
in the extreme northern and southern areas of the earth. These problems
complicate the location of points and measurements of directions. To
overcome these problems a rectangular grid is made out of the earth's surface,
between 84 degrees north latitude and 80 degrees south latitude. It divides
the earth's surface into 60 grid zones. Each zone is 6 degrees in
longitude and numbered from 1 to 60, west to east, beginning at 180 degrees
longitude (the international date line). This system then divides the earth's
surface into 20 zones by latitude, each of which is 8 degrees in latitude and
lettered from C to X (the letters I & O are omitted) beginning at 80 degrees
south latitude. Note: Row X is 12 degrees in latitude. Thus, this
portion of the earth's surface has been divided into 1,200 areas each of which
can be identified an alpha-numeric label called a grid zone designation. An
area's grid zone designation is determined by reading right and then up.
MGRS enables greater accuracy in expressing locations by breaking the
flattened (and hence inaccurate) globe into small units of measure.
The modern system of map symbols owes
much to the reign of Napoleon Bonaparte. During the first part of the 19th
century France became the leader in cartography with the introduction of such
cartographic innovations as a comprehensive system of incorporating symbols onto
topographic maps.
On a standard topographic map,
the colors used and the features they represent are:
Black:
Indicates man-made features, such as buildings.
Gray:
Indicates built-up areas; relief features; and elevation. This
color enables the map to be red-light readable.
Red:
Depicts major roads and highways.
Brown: Depicts contour lines.
Green: Identifies vegetation such as woods; orchards; and
vineyards.
Blue:
Identifies water features such as lakes; swamps; rivers; and streams.
Intermittent water features are depicted with a dashed line.
The main advantage of a topographic
map is that it portrays the elevations and relief of the portion of the earth's
surface it depicts.
The representation of the shapes of
natural terrain features (the shape of the ground) is called the relief.
Contour lines are the most common
method used to depict relief; elevation; and depression on a topographic map.
A contour line is an imaginary line on the ground above or below a datum plane
(sea level - which is measured as either "0 Feet" or "O Meters" depending on map
scale). There are three (3) types of contour lines:
- Index: starting at mean sea level
(zero), every fifth contour line is a heavier line with the elevation for that
line printed on it. For most maps the contour interval is 10 feet, thus
the index contour lines are every 50 feet.
- Intermediate: these are lighter
contour lines that fall in between the index contour lines and do not have the
elevation printed on them.
- Supplementary: these are dashed
lines used to show sudden changes in elevation of at least half the value of
the contour interval.
In addition to contour lines, bench
marks are surveyed elevations at specific points and are usually indicated by
the letter "X." Finally, spot elevations are points that may be surveyed
and are used to depict elevations at prominent terrain features, such as hill
tops and road intersections.
Although the use of contour lines
provides a more scientific and accurate portrayal of elevations, the relief that
is depicted by contour lines can be difficult to visualize in 3-D. The
following methods are used in depicting certain terrain features:
- Hills: depicted by closed
rings of contour lines that get successively smaller towards the top of the
hill.
- Valley: 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 level ground of a lower
elevation at its bottom where the water course is found. Contour lines
forming a valley are either 'U' or 'V' shaped with the closed end of the V
points upstream towards higher ground.
- Ridge: a sloping line of
high ground. The contour lines forming a ridge tend to have a 'U' or 'V'
shape with the closed end point away from the high ground.
- Depression: a low point in
the ground or a sinkhole surrounded by higher ground on all sides. It is
depicted by closed contour lines with hachures pointing inward toward the
lower ground.
- Draw: a less developed
stream course than a valley. It has high ground on three sides with no
level ground in the draw. It is depicted by 'U' or 'V' shaped contour
lines, with the closed end of those lines pointing towards the top of the draw
(higher ground).
- Spur/Finger: a sloping line
of higher ground with lower ground on three sides. It is depicted by 'U' or
'V' shaped contour lines with the closed end of those lines point towards the
bottom of the finger (lower ground).
- Cliff: a vertical or near
vertical abrupt change in the land. It is depicted by very closely
spaced contour lines or by converging contour lines into one contour line that
is some times marked with hachures that point toward the lower ground.
- Man-made terrain features: a
man-made terrain feature created by digging through high ground, usually for
transportation routes, is called a cut. It is depicted by a
contour line drawn along the cut line with hachures pointing toward the lower
ground. Another man-made feature is a fill which results from the
filling of a low area, usually to accommodate transportation routes. It is
depicted by a contour line drawn along the fill line with hachures on the
contour line pointing toward lower ground.
As we have stated, relief
is basically the shape of the land. As such, the spacing of the contour
lines also tells much about the relief. For example, you already
understand how contour lines can represent a hill, but how steep is the hill and
what type of slopes can be found on the sides of the hill?
Uniform Gentle Slope:
This type of slope is depicted by contour lines that are evenly spaced and wide
apart.
Uniform Steep Slope:
This type of slope is depicted by contour lines that are evenly spaced and close
together.
Concave Slope:
This type of slope is depicted by contour lines that are close together towards
the top of the terrain feature and are widely spaced towards the bottom of the
slope.
Convex Slope: This
type of slope is depicted by contour lines that are widely spaced towards the
top of the terrain feature and are close together towards the bottom of the
slope.
Mercator Map
The shortest distance between two
points is a straight line, but on the surface of a globe the shortest distance
is an arc known as a great circle. If you were to plot the route of a
great circle on most maps, it would not appear to be a direct route. This
is due to the distortion caused by projecting the curved surface of the globe
onto a flat surface. Perhaps the most widely used map is the Mercator
Cylindrical Projection. The Mercator map is very useful for navigation
because a straight line on the map corresponds to a compass heading. If
you look carefully at the map below, both parallels and meridians are straight
lines and cross at right angles. The meridians are equally spaced, but
parallels are not. This is because the Mercator projection is constructed
by straightening the lines of longitude and by increasing the space between
latitude equal to the space of longitudinal widening. This projection is
most accurate within 15 degrees of the equator. Distortion is so severe
near the pole that the northern and southern limits of the map are fixed at the
84th parallel. The Mercator projection has given many people a distorted
perception of the size of the continents. Greenland, for example, appears
larger than South America when actually Greenland is only one eighth of the size
of South America.
Because lines of
longitude merge at the poles, the distance represented by one degree of
longitude is greatest at the equator and decreases as latitude increases. For
example, at the equator a degree of longitude is about 111 kilometers (or 69
miles), while at 60 degrees latitude, a degree of longitude is only about 56
kilometers (or 35 miles). At the poles, the meridians intersect, so a degree of
longitude is zero kilometers (that's because there are no degrees of longitude
at the poles). Nevertheless, we can easily determine the distance between two
places on the globe even if their latitude is not the same.
Global Positioning
Systems (GPS)
GPS was developed by the U.S.
Department of Defense as a system that would provide global, all-weather, 24
hour positioning capability. GPS is now also used in civil applications
mostly for navigation and mapping.
Presently, 24 GPS satellites orbit the
earth at a distance of about 20,000 km. Four satellites are in each of six
different orbital planes. This arrangement insures that at least four and
as many as eight satellites are visible above the horizon from any spot on
Earth.
Onboard each satellite are atomic
clocks that keep precise time. Each satellite broadcasts its location and
time information as a code on two microwave carrier signals (L1 frequency of
1575.42 MHz and L2 frequency of 1227.60 MHz).
The basis of GPS positioning is that
your location can be determined if you know the distance to four (4) different
satellites. This technique is called triangulating or ranging. The
GPS receiver measures the distance to the satellites using the travel time of a
coded radio signal and the speed of light. Because your handheld GPS unit
does not contain an atomic clock to precisely measure time differences, it
contains a directory (called an almanac) of the projected position of each of
the satellites in the orbital planes. The GPS receiver uses this
information to calculate the time differences and thus the distance to each
satellite.
In order for GPS to work, the antenna
at the end of the GPS unit needs to receive the signals from the orbiting
satellites. Therefore, you need to be outside with view of the sky.
Mountains, trees, buildings and other obstacles can block the satellite signals
or cause the signals to bounce around creating positioning inaccuracies.
When you turn on the GPS unit, it will take some time for enough information
from the satellite to be processed before your position is "fixed" or known.
The military uses an encrypted code on the L2 frequency to obtain ±5m accuracy
on handheld GPS receivers. The signal code carried by the L1 frequency
used by civil GPS units is randomly degraded and scrabbled (called Selective
Availability, or SA). Therefore, a position determined by the handheld
civil GPS units is approximately ±100 m.
Map Projections &
Types
A map projection is the systematic
arrangement of a planet's parallels and meridians onto a plane surface.
These meridians and parallels become the projection graticule. The
graticule takes on different forms depending on the type of projection plane
surface; the point or line of tangency; the aspect; and direction of an
imaginary projection light source. The projection process also involves
the transformation of land features such as coastlines and land boundaries.
All map projections have some type of
distortion or deformation. Depending on the projection properties, the
distortion may be of area; shape; size; distance; direction; or scale. No
projection is free from all distortions, but each contains only some distortion.
The cartographer or mapmaker must select a projection which will result in a
minimum of distortion in relation to the map theme or purpose; the amount
of land area shown; and the portion of the planet's surface being represented on
the map. As previously noted, all map projections contain some types of
distortion. Some projections preserve shape and direction while distorting
area. Others maintain area but distort shape and scale. In many
projections scale may vary from place to place and in all projections distortion
will increase away from the places of tangency. The types of distortions
are a function of the way the projection is constructed. As most
projections have been derived mathematically, the type of distortion is often a
function of certain mathematical relationships specific to a given projection
Map projections are grouped into three
families: Cylindrical, Conic and Azimuthal, with Pseudocylindrical projections
forming a variation on the Cylindrical Family. These families are based on
the configuration of the plane onto which the globe (sphere) is projected.
In addition to map projections, there
are an endless variety of types of maps.
<
Map Reading > |