Transcript Portraying Earth

GEO 200: Physical Geography
Portraying Earth
Portraying Earth
• The Earth’s surface is the focus of the
geographer’s interest.
• The enormity and complexity of the Earth’s
surface would be difficult to comprehend without
tools to systematize, organize, and present the
data.
– Maps are the most important and universal tool of the
geographer.
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The nature of maps, part 1
• A map is a two-dimensional representation of the
spatial distribution of selected phenomena.
• Basic attributes of maps, making them
indispensable:
– Their ability to show distance, direction, size, and shape
in horizontal (two-dimensional) spatial relationships.
– They depict graphically what is where and they are
often helpful in providing clues as to why such a
distribution occurs.
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The nature of maps, part 2
• Basic fault of map:
– No map can be perfectly accurate:
• Maps are trying to portray the impossible—taking a curved
surface and drawing it on a flat piece of paper.
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A matter of scale
• Scale gives the relationship between length
measured on the map and corresponding distance
on the ground. Essential for being able to measure
distance, determine area, and compare sizes.
• Scale can never be perfectly accurate, again
because of the curve of Earth’s surface.
– The smaller the area being mapped, the more accurate
the scale can be.
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Scale types
• Scale indicated in several ways
– Representative fraction, in which numerator indicates X
units on the map, while denominator indicates Y of the
same units on the ground
• For example, 7.5-minute topographic maps are in 1:24,000 or
1/24,000 scale, where one inch on the map would equal 24,000
inches on the ground
– Written scale, such as “one inch equals one mile”
– Graphical scale, such as a line one inch or one
centimeter long, with a legend that indicates how many
units (such as miles) on the ground the line equals
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Large and small scale
• Scale is the relationship of a feature on a map to
its actual size on Earth
– Large-scale maps cover small areas, like neighborhoods
• Smaller area covered
• Representation of area more detailed
– Small-scale maps cover large areas, like continents
• Larger area covered
• Representation of area less detailed
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Role of globes
• Globes have several advantages:
– Can maintain the correct geometric relationships of
meridian to parallel, of equator to pole, of continents to
oceans.
– Can show comparative distances, comparative sizes,
and accurate directions.
– Can represent, essentially without distortion, the spatial
relationships of features on Earth’s surface.
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Map projections, part 1
• A map projection is the system used to transform
the rounded surface of Earth to a flat display.
• The fundamental problem with mapping is how to
minimize distortion while transferring data from a
spherical surface to a flat piece of paper.
• Most maps are derived by mathematical
computation, not by tracing a globe’s depiction
onto a paper.
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Map projections, part 2
• Many ways to manipulate the data to mitigate
distortion:
– Arrange grid system so that the geometric properties of
the globe are retained;
– Have most distorted areas fall in less important parts of
map;
– Interrupt the map with blank spaces in oceanic regions
to decrease distortion of continents.
• Central meridians are meridians that pass through center of
major landmasses and serve as a baseline from which
continents can be mapped.
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Types of map projections
• Conic – Earth’s surface projected onto a cone
• Plane – Earth’s surface projected onto a plane
(also called azimuthal or zenithal projections)
• Cylindrical – Earth’s surface projected onto a
cylinder (example: the Mercator projection)
• Interrupted – Portions of the Earth’s surface
projected more accurately by sacrificing areas not
central to map’s theme
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The Mercator projection, part 1
• The Mercator projection is a special-purpose
projection that was created more than 400 years
ago as a tool for straight-line navigation.
• It has been misused, however, and so creates many
misconceptions about the size of landmasses, as it
makes those landmasses in the high latitudes
appear much larger than they actually are.
– For example, Greenland appears much larger than
Africa, South America, and Australia, although
Greenland is actually smaller than them.
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The Mercator projection, part 2
• Prime advantage: shows loxodromes as straight
lines.
– A loxodrome, also called rhumb line, is a curve on the
surface of a sphere that crosses all meridians at the
same angle. They approximate the arcs of a great circle
but consist of constant compass headings.
• How do navigators use Mercator projection?
– First, navigators must use another type of projection
that shows great circles as straight lines; they draw a
straight line between their starting point and
destination.
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The Mercator projection, part 3
• How do navigators use Mercator projection?
(continued)
– They then transfer that straight-line route to a Mercator
projection by marking spots on the meridians where the
straight-line route crossed them.
– They then draw straight lines between the meridian
points, which are loxodromes or rhumb lines.
– The navigator can use these loxodromes to chart when
periodic changes in compass course are necessary to
approximate the shortest distance between two points.
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The Mercator projection, part 4
• Why does the Mercator projection distort size?
– It is a conformal projection. Although it is accurate in
its portrayal of the equator and relatively undistorted in
the low latitudes, it must distort size in the middle and
high latitudes in order to maintain conformality, that is,
approximate the shapes of landmasses.
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The Mercator projection, part 5
• Why does the Mercator projection distort size?
(continued)
– It shows the meridians as straight, parallel lines instead
of having them converge at the poles as they actually
do. This causes east–west stretching. To compensate for
this stretching and keep shapes intact, the Mercator
projection must also stretch north–south, so it increases
the spacing between parallels of latitude as one goes
further from the equator. Thus landmasses further away
from the equator appear larger than they actually are.
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The major dilemma, part 1
• Resolving the question of equivalence versus
conformality is the central problem in constructing
and choosing a map projection:
– Impossible to perfectly portray both size and shape, so
must strike a compromise between equivalence and
conformality.
• Equivalence is the property of a map projection that maintains
equal areal relationships in all parts of the map.
• Conformality is the property of a map projection that maintains
proper angular relationships of surface features.
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The major dilemma, part 2
• Resolving the question of equivalence versus
conformality (continued)
– Can only closely approximate both equivalence and
conformality in maps of very small areas (e.g., largescale maps).
• Mapmaking must be an art of compromise.
• Robinson projection in Figure 2–11 is one of the most popular
methods for compromising between equivalence and
conformality.
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Equivalent projections
• Equivalent projections portray equal areal
relationships throughout, avoiding misleading
impressions of size.
– Disadvantages:
• Difficult to achieve on small-scale maps, because they must
display disfigured shapes:
– Greenland and Alaska usually appear squattier than they actually
are on equivalent projections.
• Even so, most equivalent world maps are small-scale maps.
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Conformal projections
• Conformal projection maintain proper angular
relationships in maps so the shape stays accurate
(e.g., Mercator projection).
– Disadvantages:
• Impossible to depict true shapes for large areas like continents.
• Biggest problem is that they must distort size (e.g., usually
greatly enlarges sizes in the higher latitudes.
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Automated cartography
• Computer technology has provided several great
benefits to cartography:
– Improved speed and data-handling ability;
– Reduced time involved in map production;
– Ability for cartographer to examine alternative map
layouts.
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Isolines, part 1
• An isoline is commonly used cartographic device
for portraying the spatial distribution of some
phenomenon. Also called isarithm, isogram,
isopleth, and isometric line.
– Refers to any line that joins points of equal value.
• Isolines help to reveal spatial relationships that
otherwise might go undetected.
– They can significantly clarify patterns that are too large,
too abstract, or too detailed for ordinary
comprehension.
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Isolines, part 2
• Most relevant types of isolines to this course:
–
–
–
–
–
Contour lines join points of equal elevation;
Isobars join points of equal atmospheric pressure;
Isogonic lines join points of equal magnetic declination;
Isohyets join points of equal quantities of precipitation;
Isotherms join points of equal temperature.
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Isolines, part 3
• The interval is the numerical difference between
one isoline and the next.
– Size of interval is up to the cartographer’s discretion,
but it is best to maintain a constant interval thorough a
map.
– Their proximity depends on the gradient (that is, the
change in the interval).
• The closer they lie together, the steeper the gradient; the further
apart they lie, the more gentle the gradient.
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Map essentials, part 1
• Maps should include eight essential components;
omitting any of these components will decrease
the clarity of the map and make it more difficult to
read.
• The eight essential components are: Title, Date,
Legend, Scale, Direction, Location, Data Source,
and Projection Type.
– The title should provide a brief summary of the map’s
content or purpose and identify the area it covers.
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Map essentials, part 2
• The eight essential components (continued):
– The date should indicate the time span in which the
map’s data were collected.
– The legend should explain any symbols used in map to
represent features and any quantities.
– The scale should provide a graphic, verbal, or fractional
scale to indicate the relationship between length
measured on the map and corresponding distance on the
ground.
– The direction should show direction either through
geographic grid or a north arrow.
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Map essentials, part 3
• The eight essential components (continued):
– The location should have a grid system, either a
geographic grid using latitude and longitude, or an
alternative system that is expressed like the x and y
coordinates of a graph.
– The data source should indicate the data source for
thematic maps.
– The projection type should indicate the type of
projection, particularly for small-scale maps.
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Global Positioning System, part 1
• The Global Positioning System (GPS) is a
satellite-based system for determining accurate
positions on or near Earth’s surface.
– High-altitude satellites (24) continuously transmit both
identification and position information that can be
picked up by receivers on Earth.
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Global Positioning System, part 2
• The Global Positioning System (continued)
– Clocks stored in both units help in calculating the
distance between the receiver and each member of a
group of four (or more) satellites, so one can then
determine the three-dimensional coordinates of the
receiver’s position.
• Military units allow a position calculation within about 30 feet
(10 meters).
• Also used in earthquake prediction, ocean floor mapping,
volcano monitoring, and mapping projects.
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Remote sensing
• Remote sensing is the study of an object or surface
from a distance by using various instruments.
– Sophisticated technology now provides remarkable set
of tools to study Earth, through precision recording
instruments operating from high-altitude vantage
points.
• Different kinds of remote sensing include aerial photographs,
color and color infrared sensing, thermal infrared sensing,
microwave sensing, radar, sonar, multispectral, and SPOT
imagery.
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Aerial photography, part 1
• First form of remote sensing.
• An aerial photograph is a photograph taken from
an elevated “platform” such as a balloon, airplane,
rocket, or satellite.
• Photos are either oblique or vertical:
– Oblique: camera angle is less than 90 degrees, showing
features from a relatively familiar point of view.
– Vertical: camera angle is approximately perpendicular
to Earth surface (allows for easier measurement than
oblique photographs).
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Aerial photography, part 2
• Photo analysis
– Photogrammetry is the science of obtaining reliable
measurements from photographs and, by extension, the
science of mapping from aerial photographs.
– Two vertical aerial photographs, when properly aligned
and overlapping, can produce three-dimensional
appearance.
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Orthophoto maps
• Orthophoto maps are multi-colored, distortion-free
photographic maps produced from computerized
rectification of aerial imagery.
– Show the landscape in much greater detail than a
conventional map, but are like a map in that they
provide a common scale that allows precise
measurement of distances.
– Particularly useful in flat-lying coastal areas because
they can show subtle topographic detail.
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Color and color infrared sensing
• Color refers to the visible-light region of the
electromagnetic spectrum.
• Color infrared (color IR) refers to the infrared
region of the spectrum.
– Color IR film is more versatile; its uses include
evaluating health of crops and trees; but it cannot detect
much of the usable portion of the near infrared.
– Landsat is a series of satellites that orbit Earth and can
digitally image all parts of the planet except the polar
regions every nine days.
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Thermal infrared sensing
• Thermal Infrared Sensing (thermal IR) uses the
middle or far infrared part of electromagnetic
spectrum; these wavelengths cannot be sensed
with film.
– Thermal scanning is used for showing diurnal
temperature differences between land and water and
between bedrock and alluvium, for studying thermal
water pollution, for detecting forest fires, and, its
greatest use, for weather forecasting.
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Microwave sensing
• Microwave radiometry senses radiation in the 100micrometer to 1-meter range.
– Useful for showing subsurface characteristics such as
moisture.
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Radar and sonar sensing
• Radar (radio detection and ranging) senses
wavelengths longer than 1 millimeter, and now
provides images in photo-like form.
– Radar is unique in its ability to penetrate atmospheric
moisture, so it can analyze wet tropical areas that can’t
be sensed by other systems.
• Radar is particularly useful for terrain analysis.
• Sonar (sound navigation ranging) permits
underwater imaging.
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Multispectral remote sensing
• Multispectral scanning system (MSS) is a system
that images Earth’s surface in several spectrum
regions.
– Landsat Sensory Systems use an MSS; can gather more
than 30 million pieces of data for one image 183-by170 kilometers (115-by-106 miles).
• Thematic mapper uses seven bands to improve
resolution and greater imaging flexibility.
– Images in eight spectral bands with a resolution of 15
meters became available with Landsat 7 in 1977.
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SPOT imagery
• SPOT (Système pour l’Observation de la Terre)
newest sensor system, using a high-resolutionvisible (HRV) sensing system that significantly
improves resolution and performs stereoscoping
imaging.
– SPOT 5 was launched in 2002 and has a resolution of
2.5 to 5 meters in multispectral mode.
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EOS and Terra satellites
• NASA’s Earth Observing System (EOS) satellite
Terra was launched in 1999.
• The satellite contains a moderate resolution
imagery spectroradiometer (MODIS) that gathers
36 spectral bands.
• The latest device is a multiangle image
spectroradiometer (MIS) that is capable of
distinguishing various types of atmospheric
particulates, land surfaces, and cloud forms.
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GIS, part 1
• Geographic information systems (GIS) is an
automated systems for the capture, storage,
retrieval, analysis, and display of spatial data.
– Uses both computer hardware and software to analyze
geographic location and handle spatial data.
– Virtually, libraries of information that use maps instead
of alphabet to organize and store data.
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GIS, part 2
• Geographic information systems:
– Allows data management by linking tabular data and
map.
– Mainly used in overlay analysis, where two or more
layers of data are superimposed or integrated.
– First uses were in surveying, photogrammetry,
computer cartography, spatial statistics, and remote
sensing; now being used in all forms of geographic
analysis, and bringing a new and more complete
perspective to resource management, environmental
monitoring, and environmental site assessment.
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GIS, part 3
• Geographic information systems (continued):
– GIS was also used to compile structural data on the
rubble at Ground Zero at the World Trade Center
disaster.
• The technology allowed the building damage to be mapped
and provided details on the outage of various utilities in the
area.
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Role of the geographer
• In using remote sensing and its images, the
geographer works as an interpreter.
– The new technologies provide new tools for the
geographer, but they do not function as substitutes for
field study, geographic description, and maps.
– No single sensing system works for all problems; each
has its own use for particular purposes and so
geographers must be careful in selecting and obtaining
the best type of imagery for their individual needs.
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