Latitude and Longitude Are Not the Final Answer
A coordinate expressed in latitude and longitude is genuinely useful, and for navigation, mapping at a broad scale, or exchanging data between systems, it is often exactly what is needed. It is a poor fit, however, for the kind of work most field crews actually do every day: laying out a curb line, checking a distance between two points, describing an easement in feet.
Latitude and longitude describe angles on a curved surface. Most engineering and construction work is measured in flat, straight-line distances. A coordinate has to be translated again before it is convenient for that kind of work.
Angles Do Not Subtract Like Distances
A difference in latitude and a difference in longitude cannot simply be subtracted the way two building corners can be measured with a tape. Longitude, in particular, represents a very different ground distance depending on how far a point sits from the equator, since lines of longitude converge as they approach the poles. Two points a fixed number of degrees apart in Alaska sit much closer together on the ground than two points the same number of degrees apart in Florida.
A degree of longitude is not a fixed ground distance. It shrinks steadily from a maximum near the equator to nothing at all at the poles, which makes raw latitude and longitude an awkward unit for everyday distance and area calculations.
What a Field Crew Actually Wants
Most practical work wants something closer to graph paper: two straight, perpendicular axes, distances that subtract cleanly, and areas that can be calculated the way they were taught in school. Producing that kind of flat, rectangular coordinate system out of a curved, angular one is not a small step. It requires deliberately flattening a curved surface, and that operation cannot be done for free.
If the Earth's surface is curved, can it be represented on a flat plane without changing anything about it? No. Some distortion, in distance, in shape, or in area, is unavoidable. The only choice is which kind of distortion to accept, and where to put it.
Latitude and longitude are accurate but impractical for most everyday distance and area work, because degrees of longitude do not represent a constant ground distance. Producing a flat, rectangular coordinate system requires an additional, deliberate step: a map projection.
That step has a name, and it comes with an unavoidable trade-off. The next chapter explains exactly what a map projection does, and why every single one of them distorts something.