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GEOG 268: Cartography

GEOG 268: Cartography. Ohio Northern University Winter 2001-2002 Hill 200. Geodesy. Mapping involves determining locations on earth, transforming positions onto flat map, graphically symbolizing those positions

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GEOG 268: Cartography

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  1. GEOG 268: Cartography Ohio Northern University Winter 2001-2002 Hill 200

  2. Geodesy • Mapping involves • determining locations on earth, • transforming positions onto flat map, • graphically symbolizing those positions • Geographic locations determined by geographic coordinates  Latitude and Longitude • to establish a system of geographic coordinates, we first have to know the Earth’s size and shape

  3. Designing base map & overlay • Generalizing process: • Selection • Classification: process in which objects are placed in groups with similar features • Simplification: smoothing natural lines • Symbolization: • replicative, or • abstract • where do we start?

  4. Size and Shape of the Earth • The development of a base map begins from a small model of the real earth. • Cartographer needs knowledge of earth’s geographic grid as shown on perfect sphere to create projection • Today?: irregular surface approximating an ellipsoid

  5. Geodesy • Earth is very smooth geometrical figure • smoother than a bowling ball ! • Cartography begins with approximating the Earth’s size and shape: • increasingly accurate approximations of Earth’s shape: • sphere • ellipsoid • geoid

  6. Geodesy • Earth’s shape? • Pythagoras (6th century BC), and • Aristotle  round Earth (sailing ships) • Earth’s size? • Eratosthenes (250 BC): • Deep well in Aswan, sun overhead Solstice • Next solstice: angle of sun Alexandria • Circumference? 28,750 mi. (15% more) • Real circumference? ~ 24,000 miles

  7. Spherical Earth • Earth not perfect sphere but ellipsoidal • cartographers use sphere with same surface area as ellipsoid: • authalic sphere: basic figure for mapping • 3,959 mi. standard radius (WGS 84 ellipsoid)

  8. Ellipsoidal Earth • Until 1670s, Earth perfect sphere • Newton: gravity causes flattening at poles • amount ~ 1/300th polar radius vs eq. Radius • satellite measurements = 1/298 • 3D fig. oblate ellipsoid (or oblate spheroid) • at least 11 different values used worldwide based on location. • Example: WGS 84 & GRS80 satellites • Example: Clarke 1866 ground observations

  9. Geoidal Earth • Even more accurate figure of the earth: • Geoid (earth-like) • 3D “equipotential” surface (mean sea level) • gravity everywhere = mean sea level gravity • geoid shape - irregular surface features • geoid deviates from ellipsoid because of • rock density & topography • deviates up to 300 ft. in certain places

  10. Cartographic use of Sphere, Ellipsoid, Geoid • All 3 are different approximations of the Earth’s surface • Authalic sphere used as reference surface for small scale maps • Ellipsoid used as a ref. for large scale mapping • distances, directions and areas would be more correct at individual locations than sphere • GPS compute lat/long and elevation using WGS 84 ellipsoid as reference surface • Geoid • length of degree varies from equator to poles • used for local large scale ground based surveys

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