Four parts of most Earth coordinate systems:
- 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)
- 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.
- 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
- 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