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