4. Relative to What?
A SparkPNT RTK receiver sits over a survey monument and reports a clean FIX. A nearby control sheet lists coordinates for that same monument, and the two don't quite match. The receiver is genuinely fixed. The antenna is centered correctly. The correction stream is healthy. The result repeats.
So which coordinate is wrong? Quite possibly neither one.
A coordinate is not a permanent label stapled onto a point forever. It is a description of that point inside a specific reference frame, and two different, equally correct descriptions can disagree without either one being wrong.
A Grid Needs Something Holding It Still
Draw a grid on a clear sheet of plastic and lay it over a map. Every town and road gets a pair of numbers from that grid. Slide or rotate the sheet, and none of those towns or roads actually move, but every one of their grid coordinates changes anyway.
The three-dimensional coordinate grids GNSS relies on work the same way. Before latitude, longitude, and height can carry any precise meaning at all, that grid has to be defined and anchored to the Earth in the first place.
Naming the Grid Itself
The practical, real-world coordinate framework used to assign positions has a name: a reference frame. It defines how the coordinate axes are positioned, how their scale is set, how the whole frame is oriented, and how it connects to actual, physical stations and observations on the ground.
The word "realized" tends to trip people up here, and it shouldn't. It simply means a coordinate system has moved past being a proposed, theoretical idea and become a concrete, generally adopted system the GIS and surveying industry actually uses. A reference frame is a reference system that has been realized.
A related term, reference system, sounds interchangeable and isn't quite.
| Term | Plain-language meaning | Useful analogy |
|---|---|---|
| Reference system | The rules, definitions, and mathematical ideas behind the coordinates | The blueprint for a grid |
| Reference frame | The real-world realization of those rules through actual stations, coordinates, and observations | The grid, built and anchored in place |
For most practical work, reference frame is the term that actually matters, since it identifies the specific coordinate realization a receiver, a base station, or a correction service is actually using.
Two Honest Ways to Anchor the Same Grid
One way to define coordinates centers the whole frame on the Earth itself and watches the planet from space. In a frame like this, tectonic plates drift through the coordinate grid, and a monument fixed in North America slowly changes coordinates as its continent moves. Global satellite navigation and precise orbit work both lean on this kind of Earth-centered frame naturally, since the entire planet sits inside one consistent system.
For mapping, engineering, and land records, a coordinate that keeps drifting is simply inconvenient. A regional frame can instead be built to move with a tectonic plate, the plate-fixed idea from Chapter 2, so a stable monument in North America keeps reporting nearly the same coordinate year after year, because the grid itself is traveling right along with the continent.
Neither approach is more correct than the other. They're built to answer different questions.
Names Worth Recognizing
Real GNSS work runs into a handful of named reference frames constantly, especially across North America.
| Name | General role | What a new user should remember |
|---|---|---|
| ITRF | A family of precise, global, Earth-centered reference frames | Built for a changing, moving Earth |
| WGS84 | The global reference used by GPS and many navigation products | Often treated loosely as a universal GPS label, though precise realizations still matter |
| NAD83 | A North American datum and family of realizations for mapping and surveying | Built to stay useful for coordinates tied specifically to North America |
The bare name alone isn't enough for the highest-accuracy work. A specific realization, and often a specific epoch, may matter too. None of this needs to be memorized in full. It's enough to recognize that these labels each describe a genuinely different way of anchoring a coordinate to the Earth.
Why Two Correct Receivers Can Disagree
One receiver reports a position in a current, global Earth-centered frame. A published control point nearby is expressed in a North America-fixed frame instead. Both positions can be entirely correct on their own terms, and because the two frames don't place the coordinate grid identically, the numerical latitude, longitude, and height can still come out different.
An RTK FIX does not mean a coordinate must match every map and every control point it gets compared against. FIX describes the quality of the relative carrier-phase solution. It says nothing at all about whether the result shares the same reference frame, epoch, or height system as whatever it's being compared to.
What to Check Before Blaming the Equipment
A SparkPNT rover produces an extremely repeatable, centimeter-level position. Compared against an older survey record, a GIS layer, or coordinates pulled off a web map, the numbers don't line up.
Before suspecting the antenna, the receiver, or the correction source, what's worth confirming first? Whether both coordinate sets actually share the same reference frame, the same specific realization of that frame, the same epoch, and the same height type. Only once all four agree does a direct, centimeter-level comparison actually mean anything.
Moving a Coordinate Between Frames
A coordinate can be converted from one reference frame into another through a coordinate transformation, which can account for shifts, rotations, scale differences, tectonic motion, and the specific epochs involved on either side.
At ordinary, low-accuracy work, software quietly hides all of this. At centimeter-level accuracy, those hidden details are exactly what determine whether a transformed coordinate can actually be trusted. The physical monument never moves during any of this. Only its numerical description does.
Two different coordinate values can both correctly describe the exact same physical point. Before comparing any two precise positions, the question worth asking first is simply: relative to what?
A precise coordinate now needs three questions answered at once: where is it, when is that description true, and relative to what is it being measured? Chapter 5 connects all three directly to RTK, and looks closely at exactly what a FIX solution actually proves, and what it does not.