Geog 268 cartography
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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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Geog 268 cartography

GEOG 268: Cartography

Ohio Northern University

Winter 2001-2002

Hill 200


Geodesy

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


Designing base map overlay

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?


Size and shape of the earth

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


Geodesy1

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


Geodesy2

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


Spherical earth

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)


Ellipsoidal earth

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


Geoidal earth

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


Cartographic use of sphere ellipsoid geoid

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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