地图投影5.pptVIP

Geodesy, Map Projections and Coordinate Systems Geodesy - the shape of the earth and definition of earth datums Map Projection - the transformation of a curved earth to a flat map Coordinate systems - (x,y) coordinate systems for map data Learning Objectives:By the end of this class you should know: the role of geodesy as a basis for earth datums how to calculate distances on a spherical earth the basic types of map projection the properties of common map projections the terminology of common coordinate systems how to use ArcGIS to convert between coordinate systems Spatial Reference = Datum +  Projection +  Coordinate system For consistent analysis the spatial reference of data sets should be the same. ArcGIS does projection on the fly so can display data with different spatial references properly if they are properly specified. ArcGIS terminology Define projection. Specify the projection for some data without changing the data. Project. Change the data from one projection to another. Types of Coordinate Systems (1) Global Cartesian coordinates (x,y,z) for the whole earth (2) Geographic coordinates (f, l, z) (3) Projected coordinates (x, y, z) on a local area of the earth’s surface The z-coordinate in (1) and (3) is defined geometrically; in (2) the z-coordinate is defined gravitationally Global Cartesian Coordinates (x,y,z) Global Positioning System (GPS) 24 satellites in orbit around the earth Each satellite is continuously radiating a signal at speed of light, c GPS receiver measures time lapse, Dt, since signal left the satellite, Dr = cDt Position obtained by intersection of radial distances, Dr, from each satellite Differential correction improves accuracy Global Positioning using Satellites Geographic Coordinates (f, l, z) Latitude (f) and Longitude (l) defined using an ellipsoid, an ellipse rotated about an axis Elevation (z) defined using geoid, a