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Georeferencing -Datums, Coordinate Systems & Map Projections
Recap - Datums•Datums = mathematical model of the earth upon which geodetic computations are based•Consists of 2 major components:1.The specifications of the ellipsoid being used2.Set of highly accurate surveyed points & lines•Datums always define the following attributes:1.Size & shape of the earth (e.g., a reference ellipsoid)2.Tie-point that fixes ellipsoid to Earth’s surface or Earth’s center3.Origin & orientation of coordinate system
Recap –Datums•Vertical datums :–National Geodetic Vertical Datum (NGVD29)–North American Vertical Datum of 1988 (NAVD88) –used almost exclusively•Horizontal datums:–North American datum of 1927 (NAD27) –North American datum of 1983 (NAD83) –World Geodetic System of 1984 (WGS84)
Learning OutcomesUnderstand and be able to explain the following:•How do Datums relate to Coordinate Systems & Map Projections•Develop an understanding of the important properties of Map Projections •Understand how the parameters in map projections affect the outcome and be able to adjust these parameters correctly
Georeferencing•Georeferencing - refers to the ability to locate features accurately in a geographic space•Georeferencing includes information about a feature’s:–Datum–Geoid–Ellipsoid–Coordinate System–Projection
Georeferencing•Earth is an irregularly shaped sphere•Need to be able to accurately represent X, Y, & Z position measurements for many reasons–Datum reference or foundation surface against which accurate position measurements are made –Coordinate System to assigned specific x,y,z locations•Two main types of coordinate systems (most common)–Global or spherical (based on Lat/Long)–Projected (based on map projections –State Plane and Transverse Mercator)
Georeferencing•The Earth’s shape is not truly spherical, but can be described in 3 different approximations:SpheroidEllipsoidGeoid
Great Circles•Great Circle = line that connects two points on the surface of a globe and divides the globe into two halves–Pairs of longitudes form great circles–Ex. Prime Meridian International Date line, & Equator?term=great%20circle
Great Circles•A great circle (or a portion of it) is also the shortest distance between two points Source:
Different Norths•Grid North is the Direction of the Eastings on a Map•True north is the direction to the Geographic North Pole•Magentic North is the direction to the Magnetic North Pole
Different Norths•An Extreme example of the difference between grid and True North
Map ProjectionsHow do datums and coordinate systems apply to map projections?