Spatial References (projections/coordinate systems)


Useful YouTube introduction to coordinate systems and UTM projection

Four parts of most Earth coordinate systems:

  1. To ‘figure out the size and shape of Earth’, we must define a reference ellipsoid
    –  Done so by averaging the geoid surface (geoid is an imaginary surface of constant gravity)
  2. Next, break the Earth’s surface into an evenly-spaced grid
    –  Done so using lines of longitude and latitude
    –  Must first assign the origin of our grid (0 longitude, 0 latitude; line running north-south through Greenwich England and crossing the Equator)
    –  Theoretically, placing these grid lines is  is easy; however it much more difficult to do on the ground since the exact lines must be referenced to a known point like the line of longitude running through Greenwich England.
  3.  Define network of benchmarks representing points of known latitude/longitude locations (datums)
    –  common global datums are WGS84 and NAD1983
    –  critical to define the datum you use
  4. Flattening the Earth’s round surface on a flat map (projections).
    –  some geometric relationship (location, shape, distance) will be distorted during this projection process
    –  terms & info:  UTM zone, false easting, x and y coordinates always positive, central meridian, measurements are meters

Why do projections matter?

  • Any time you are viewing data on your computer screen, you are viewing projected data
  • If your layers do not have properly defined projections, your data will not align
  • Distances, areas, and shapes may be distorted through the projection process
  • All geographic data have an underlying coordinate system