Projected, and Sometimes Adjusted Once More
By the end of the last chapter, this companion's actual question has an answer. A solved position exists in ECEF, tied to a specific reference frame and epoch, and re-expressed as latitude, longitude, and ellipsoid height. That is a complete, physically meaningful description of where the position came from. It is not, however, the number that ends up on most plans, stakeout files, or screens, and it's worth knowing, briefly, what still happens to it on the way there.
Getting a usable coordinate onto a screen or a plan typically involves a few more deliberate translations after the position itself is already solved. None of them make the position more accurate. Each one exists to make an already-complete position more convenient to use.
Flattening the Curve
Latitude and longitude describe angles on a curved surface, and a degree of longitude represents a shorter ground distance the farther a point sits from the equator, shrinking steadily toward the poles. Most practical field work wants straight, subtractable distances instead, so the curved description gets deliberately flattened onto a plane through a map projection. State Plane and UTM are the two most common answers in the United States, built around different priorities, and every projection distorts something in the process: distance, shape, or area, depending on the choice being made.
Grid Distance and Ground Distance Are Not Automatically the Same Number
Once a position is projected, a second, separate gap opens up: a projected grid distance and a distance measured on the actual, elevated ground are not guaranteed to match. Part of the gap comes from the projection itself, part comes from the project simply sitting above the reference ellipsoid rather than on it. Together these are called the combined scale factor, and it is usually invisible over a short distance and increasingly noticeable over a long one.
A grid coordinate and a taped ground distance disagreeing by a few tenths of a foot over a long baseline is not automatically a sign that something went wrong. It's frequently just an unapplied scale factor, doing exactly what a scale factor does.
Sometimes a Project Builds Its Own System Entirely
A project can also fit its GNSS positions directly to a small set of trusted control points, called a localization or site calibration, building a coordinate system tailored to exactly that job site, fit to local control rather than a standard regional grid. This is a genuinely different technique from a standard projection, and it carries its own risk: a bad control point gets quietly built into every coordinate the fit produces afterward.
A projected coordinate and a locally calibrated coordinate for the same point can both be internally correct and still not match each other. Does that mean either one is wrong? Not necessarily. They may simply be two different, deliberately chosen descriptions of the same position.
Why This Companion Only Mentions These
Each one of these topics, map projections, State Plane and UTM specifically, grid-versus-ground scale factors, and localization, is a real, substantial subject with its own failure modes and its own field habits worth building. Going deep into any one of them here would turn this companion into a different book than the one its title promises. Grid-versus-ground and map projections get the dedicated, careful treatment they deserve in Companion 5 of this series.
A solved position gets projected onto a flat plane, and sometimes adjusted again to a local project grid, before it becomes the number most people actually see. Neither step changes the position's underlying accuracy. Both are worth knowing exist, and both deserve more depth than a quick mention, found elsewhere in this series.
So far, this companion has assumed a single, nearby base station supplying the correction that made all of this possible. Many projects instead rely on an entire network of stations, and something that does not physically exist: a virtual base.