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
N E W S (N, W) (S, W) (N, E) (S, E)
Global Cartesian Coordinates (x,y,z) O X Z Y Greenwich Meridian Equator •
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 surface of constant gravitational potential • Earth datums define standard values of the ellipsoid and geoid
Latitude and Longitude Latitude - distance from the equator along the Y axis Longitude - distance from the prime meridian along the X axis. .
Equator Latitude 0 o Latitude: ( 90 o N to 90 o S) Latitude 23½ o North Tropic of Cancer Latitude 23½ o South Tropic of Capricorn Longitude 30 o East Longitude 60 o East Longitude 30 o West Longitude 60 o West Positioning on the Earth ’ s Surface East is the direction of rotation of the Earth North Pole South Pole 23½ o 23½ o 66½ o 90 o 90 0 21 st June 22 nd December 22 nd Sept 20 th March 30 o E 60 o E 90 90 o W 30 o 60 o Longitude 90 o East Longitude 90 o West Prime Meridian 0 o Longitude Longitude: (180 o E to 180 o W) Latitude and Longitude together enable the fixing of position on the Earth ’ s surface. Geographical coordination system
Shape of the Earth We think of the earth as a sphere It is actually a spheroid , slightly larger in radius at the equator than at the poles