Geodesy Science of earth measurement The earth is not flat but in some cases

Geodesy science of earth measurement the earth is not

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Geodesy: Science of earth measurement The earth is not flat , but in some cases (very large-scale mapping) it might as well be The earth is not a sphere , but in most cases it might as well be a spheroid… The earth is a big lumpy loveable mass of stuff that we can approximate with ellipses (spheroids), geoids, and datums
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Geodesy: Earth as a sphere Circumference Eratosthenes Alexandria à Syene Angle – 7.2 o (1/50 th of globe) Distance – 5000 stadia Circ. = 50 x 5000 stadia = 250,000 stadia 39250 km – 52250 km +/- 15% of true value
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Geodesy: Earth as an ellipsoid GRS 1980 dimensions a = 6,378.13 km b = 6,356.75 km Diff. (a-b) = 21.38 km Circumference Equator = 40,075 km Polar = 39,940.64 km Diff. (Eq. – Pl.) = 134.36 km b b N S a a = Semi-major axis (equatorial radius) = Semi-minor axis parallel to the rotational axis of the earth (polar radius) (After Dent, Thematic Map Design, 1990)
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Geodesy: Earth as a geoid Satellites (1958 à ) Large depressions, bulges Not noticeable, but affect precise survey work Geoid affects cartographer's reference ellipsoid
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Geodesy: Earth as an ellipsoid vs. geoid North Pole 7.62 m (25 ft) 7.62 m (25 ft) 7.62 m (25 ft) 7.62 m (25 ft) 15.24 m (50 ft) 15.24 m (50 ft) Fischer reference ellipsoid 1 / 298.25 South Pole
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Geodesy: Earth as an ellipsoid vs. geoid North Pole 90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 20 10 10 Equator Latitude Latitude +13 +10.2 +7.3 +4.6 +2.4 +2.2 +6.4 +7.1 +5.7 +5.0 –2.7 –2.8 –14.3 –24.5 –30.0 –6.1 –7.6 –3.6 –0.4 South Pole The Geoid- Deviation from the GRS 1980 Ellipsoid Average deviation in meters at each 10 degree latitude
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Datum: Orients ellipsoid to geoid Datums: Describe size & shape of earth (ellipsoid) Anchor ellipsoids to earth’s surface Describe origin & orientation of coordinate systems Ex. NAD 27, NAD 83, WGS84
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