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 used 2. 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 center 3. 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 Outcomes Understand 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: Spheroid Ellipsoid Geoid
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 Projections How do datums and coordinate systems apply to map projections?
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- Fall '08
- Map projection, map projections, equal area