Most of the search engine based “geographies” assume that WGS84 is enough – that other coordinate systems are not required. This note is to explore the validity of this approach.
Several issues need to raised at the outset:
- There are large amounts of data today which exist in hundreds of coordinate systems and conversion of all of this data is not going to happen.
- WGS84 is limited in accuracy (today) to a best 1 metre. For many applications this is fine – for many more this is not close. Land surveys are typically being done to 10 cm or better. You would not want to drive your car +/- 1 M on a road and you would not want to lose 1 M of your property.
- WGS 84 may eventually be replaced.
- IF I express the coordinates of a “straight line” segment (meaning linear interpolation) in WGS84 it will NOT be a shortest line on the surface of the earth. Consider taking two points (45,100), (45, 120) (45 = 45 degrees North latitude). Linear interpolation in the (lat,lon) coordinates will generate a small circle arc connecting the two points – but the shortest distance would be a Great Circle (or nearly a Great Circle) since the earth is modelled as an Ellipsoid in WGS84 and not as a sphere. So, I MAY want to use other projective coordinates in some cases.
- In many problems I don’t really care where things are located relative to the entire earth – just where they are relative to one another. Things like highway exits, road signs, rest stops are usually given in terms of driving distance along the highway from some road intersection. These so called “linear” reference systems tend to dominate the discussion in the transportation sector.