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Download Coordinate Systems and Projections Part I
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Coordinate Systems Coordinate Systems – key concepts ► Projections and Coordinate Systems ► Data Quality ► Meta Data Projections and Coordinate Systems: Geographic Coordinate System ► Uses 3D spherical surface to define locations ► Often incorrectly called a datum ► Includes angular unit of measure, prime meridian and datum ► Point referenced by longitude/latitude ► Angles measured by degrees Parallels – Lines of Latitude Latitude lines are parallel The equator defines the line of zero latitude Every degree of latitude is theoretically equal Parallels run east/west; measure distances north and south of equator Meridians – Lines of Longitude Meridians converge at the poles Line of zero longitude is called the Prime Meridian Distance of 1° longitude decreases toward the poles Meridians run north/south; measure distance east & west of Prime Meridian Graticular Network Network of Lat/Long called a graticule Origin of graticule (0,0) where Equator and Prime Meridian intersect 4 geographic quadrants based on compass bearings from Origin Degrees, Minutes, Seconds (DMS) ► Point on Earth’s surface referenced by Lat/Long values ► Lat/Long based on 360° ► Each degree has 60 minutes ► Each minute has 60 seconds Decimal Degrees (DD) ► Similar to DMS ► Minutes and seconds expressed as decimal values ► ESRI products require DD in geodatasets Converting from DMS to DD 37° 36' 30" (DMS) ► Divide each value by the number of minutes or seconds in a degree: 36 minutes = .60 degrees (36/60) 30 seconds = .00833 degrees (30/3600) ► Add up the degrees to get the answer: ► 37° + .60° + .00833° = 37.60833 DD Spheres and Spheroids Sphere Spheroid •Shape and size of GCS surface defined by sphere or spheroid. •Mathematical calculations easier on a sphere. •Sphere can be used for small-scale maps (< 1:5,000,000) •Spheroid gives better accuracy for large-scale maps (>1:1,000,000 Major and Minor Axes of Ellipse Minor Major Axis Axis Semiminor Axis Semimajor Axis Shape of ellipse defined by two radii. Longer radius: Semimajor Axis Shorter radius: Semiminor Axis Rotating spheroid around semiminor axis creates a spheroid Spheroids for Accurate Mapping ► Earth has been surveyed many times ► Surveys result in many spheroids ► Spheroid chosen to fit one country ► Best fit for one regions not same for another region ► Earth is neither perfect sphere nor spheroid ► Changing coordinate system’s spheroid changes all previously measured values Datums ► Spheroids approximate earth’s shape ► Datum defines position of spheroid relative to center of the earth ► Datum defines origin and orientation of lat/long lines ► Local datum aligns spheroid to fit surface in a particular area Datum Comparisons Local geographic Coordinate system Earth-centered geographic Coordinate system Earth’s surface Earth-centered datum Local datum North American Datums ► NAD27 – uses Clarke 1866 spheroid Origin – Meade Ranch Kansas Manually calculated control points ► NAD83 Based on earth and satellite observations Uses GRS80 spheroid Origin is earth’s center of mass Previous control points shift as much as 500’ Projected Coordinate Systems ► Defined on flat, 2D surface ► Has constant lengths, angles and area ► Always based on geographic coordinate system ► X,Y coordinates on grid What is a Map Projection? ► Transformation of 3D surface to 2D flat sheet ► Causes distortion in the shape, area, distance or direction of data ► Uses mathematical formulas to relate spherical coordinates to planar coordinates ► Different projections cause different distortions ► Map projections designed for specific purpose – i.e. large-scale data in limited area Relevance to GIS ► maps 1) 2) are a common source of input data for a GIS often input maps will be in different projections, requiring transformation of one or all maps to make coordinates compatible thus, mathematical functions of projections are needed in a GIS ► often GIS are used for projects of global or regional scales so consideration of the effect of the earth's curvature is necessary ► monitor screens are analogous to a flat sheet of paper 1) thus, need to provide transformations from the curved surface to the plane for displaying data Conclusion ► What is a coordinate system? A coordinate system is a grid that may be used to define where a particular location is ► Connection between projection and coordinate system The projection defines the coordinate system by defining the 2-D surface of the earth
