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GEOG 268: Cartography

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GEOG 268: Cartography

Ohio Northern University

Winter 2001-2002

Hill 200

- 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

- 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?

- 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

- 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

- 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

- 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)

- 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

- 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

- 3D “equipotential” surface (mean sea level)

- 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