Thejesh GN's Notes'

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