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coordinates - Understanding Coordinates NJDEP ESRI...

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Understanding Coordinates NJDEP & ESRI: Understanding Map Projections & Coordinate Systems
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Department Standards Spheroid GRS80 Datum NAD83 Projection New Jersey State Plane (based on Transverse Mercator) Units Feet
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Parameters for Mapping A mathematical model of the earth must be selected. Spheroid The mathematical model must be related to real-world features. Datum Real-world features must be projected with minimum distortion from a round earth to a flat map; and given a grid system of coordinates. Projection
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Spheroid Simplistic - A round ball having a radius big enough to approximate the size of the earth. A mathematical model of the earth must be selected. Reality - Spinning planets bulge at the equator with reciprocal flattening at the poles. e.g.
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Different Spheroids
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Why use different spheroids? The earth's surface is not perfectly symmetrical, so the semi-major and semi-minor axes that fit one geographical region do not necessarily fit another. Satellite technology has revealed several elliptical deviations. For one thing, the most southerly point on the minor axis (the South Pole) is closer to the major axis (the equator) than is the most northerly point on the minor axis (the North Pole).
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The earth's spheroid deviates slightly for different regions of the earth. Ignoring deviations and using the same spheroid for all locations on the earth could lead to errors of several meters, or in extreme cases hundreds of meters, in measurements on a regional scale. GRS80 (North America) Clark 1866 (North America WGS84 (GPS World-wide) International 1924 (Europe) Bessel 1841 (Europe)
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Datum A smooth mathematical surface that fits closely to the mean sea level surface throughout the area of interest. The surface to which the ground control measurements are referred. Provides a frame of reference for measuring locations on the surface of the earth. A mathematical model must be related to real-world features.
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How do I get a Datum? To determine latitude and longitude, surveyors level their measurements down to a surface called a geoid . The geoid is the shape that the earth would have if all its topography were removed. Or more accurately, the shape the earth would have if every point on the earth's surface had the value of mean sea level.
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Geoid vs Spheroid Coordinate systems are applied to the simpler model of a spheroid. The problem is that actual measurements of location conform to the geoid surface and have to be mathematically recalculated to positions on the spheroid.
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