GEO 200: Physical Geography

Portraying Earth

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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., large-scale 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 100-micrometer 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-by-170 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-resolution-visible (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.