We were discussing what exactly a straight line was. There is no such thing as a "straight line in the database", because, as you correctly state, the database only stores the end points of a line. If you draw a line from point lat=10;lon=10 to lat=30;lon=30, then it is unclear whether that line visits point lat=20;lon=20. Some might think yes, some might think no.
I think this is exactly the key question. When there is a line segment in the database, in WGS84 lat/lon, with points (lon1,lat1) and (lon2,lat2), then we need to have a definition of what that representation means. Obvious candidates are: 1) linear in lon,lat space 2) great circle in wgs84 3) linear in google spherical mercator 4) linear in WGS84 UTM 5) linear in your own country's local grid, or US state plane coordinate system 6) we don't define it, and if any of the above are different in any discernible way, you need more points. In the 10,10 30,30 example above, we are clearly in this state. I think we are in state 6, but it would be good to have a formal definition.
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