t8 - 1 Geography 231 Introduction to Geospatial Methods...

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Unformatted text preview: 1 Geography 231 Introduction to Geospatial Methods Topic 8: Georeferencing II: Ellipsoids, Datums, and GPS Goals Have a closer look at geoids, ellipsoids, and Have a closer look at geoids, ellipsoids, and datums. Do you know the difference? Look at strategies for measuring latitude, longitude, and elevation on the surface of the earth. Not as easy as you might expect! earth. Not as easy as you might expect! Global positioning systems 2 Geoids, Ellipsoids, and Datums Geoid The equipotential surface with respect to gravity The surface where the force of gravity is perpendicular NOT the actual surface of the earths crust Geoid would be equal to sea level, if undisturbed by tides and weather Would be described on land if the seas were allowed to criss cross the continents in a series of narrow canal criss-cross the continents in a series of narrow canals 3 Geoid, Cont Measured by precise gravitational instruments Too complex a surface for efficient mapping! Ellipsoid A regular geometric surface (oblate spheroid) used to approximate the geoid There are many reference ellipsoids in common use around the world Ellipsoid Ellipticity Note Clarke (1866) 294.9787 North America Used for NAD27 Geodetic Reference System (GRS80) 298.257 North America Used for NAD83 World Geodetic System (WGS84) 298.257 Slight Modification of GRS80; used for GPS 4 Geoid Undulations and Reference Ellipsoids 5 Why Different Ellipsoids? 1. Improvement over time in our ability to measure the dimensions of the earth Clarke 1866: 6,378,206m x 6,356,584m Clarke 1880: 6,378,249m x 6,356,515m WGS-84 (1984): 6,378,137m x 6,356,752m 2. Different ellipsoids fit the local geoid better in various parts of the world OK, So Whats a Datum? Two types: horizontal datums, and vertical datums Horizontal for describing horizontal position (lat/long, etc) Vertical for describing vertical position (elevation) In essence, datums are the errors we agree to live with for a particular mapping application, and involves three choices (for horizontal, at least!): 1. Choice of reference ellipsoid 2. Position of reference ellipsoid 3. Orientation of reference ellipsoid 6 The North American Datum of 1927 (NAD 27) Based on the Clarke reference ellipsoid of 1866, fixed...
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t8 - 1 Geography 231 Introduction to Geospatial Methods...

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