# 1. Earth and its representation

Created Monday 10 November 2014 - Map Class

# Earth

• Shape
• Theoretically
• Sphere
• Effect of gravity and rotation
• Spheroid or oblate ellipsoid
• The equatorial diameter is nearly 1/300 longer than polar diameter
• Assuming surface has a constant radius, any point on it is uniquely identified using a polar two-coordinate system
• Selected parallels (in red) and meridians (in blue), here spaced 15° apart, comprise a spherical graticule.
• Longitude is measured from GMT, Latitude from equator

courtesy

# Representaion of Earth

• Map is a Representaion [ Globe(3d) to Map(2D) ]
• Projection - Conversion process and logic
• A geodetic datum is a set of parameters (including axis lengths and offset from true center of the Earth) defining a reference ellipsoid. For each mapped region, a different datum can be carefully chosen so that it best matches average sea level, therefore terrain features.
• Maps are never free from errors
• Distance Measurement
• Location
• Area distortion
• Shapes
• Directions
• Mercator projection - cylindrical map projection
• EPSG:4326 refers to WGS84 earth as an ellipsoid
• Has metric coordinates, uses lat and long
• Used by GPS
• WGS84 Bounds: -180.0000, -90.0000, 180.0000, 90.0000
• For example bangalore is

• EPSG:900913 (EPSG:3857) Web Mercator refers to WGS84 earth is sphere
• Spherical mercator maps use an extent of the world from -180 to 180 longitude, and from -85.0511 to 85.0511 latitude
• Projection bounds -20026376.39 -20048966.10 20026376.39 20048966.10
• Initially by Google Maps for tiled maps
• Named by openlayers project - 'googlE' (substitute '9' for 'g' and squint a bit)
• Originally rejected by standards body, then accepted with 3857
• Used by OpenStreetMap and in general web