3. When Is a Coordinate?
Chapter 2 established that a survey monument can sit perfectly still while the continent carrying it drifts. Measure that same monument with a SparkPNT RTK receiver and save three values, latitude, longitude, elevation, and those numbers describe exactly where the point was. They still leave out one detail that turns out to matter just as much: when were they true?
At meter-level accuracy, a date attached to a coordinate is ordinary recordkeeping, easy to ignore. At centimeter-level accuracy, that date stops being incidental. It becomes part of what the coordinate actually means.
A Coordinate Is a Snapshot
A photograph captures where everything was at one specific moment. Years later, the same objects can still exist, arranged slightly differently than the picture shows.
A high-accuracy coordinate works the same way. It describes a point at a particular moment, even for a point that looks completely stable to anyone standing next to it.
Naming the Missing Number
GNSS professionals have a specific word for that missing piece: the epoch. An epoch is simply the date or time a coordinate is actually valid for, sometimes written as a calendar date, sometimes as a year carrying a decimal fraction, where 2026.50 means roughly the midpoint of 2026. The exact convention matters once real precision is on the line. The underlying idea does not: every high-accuracy coordinate carries a timestamp, whether or not anyone bothered to write it down.
Treat a coordinate's epoch the same way a photograph's date stamp gets treated. The image itself doesn't change without it, but knowing exactly when it was taken is what lets someone compare it meaningfully against a different photograph of the same scene.
Why a Few Years of Drift Actually Adds Up
Take a point moving with its tectonic plate at roughly two and a half centimeters a year. After one year, that shift is easy to ignore. After ten years, the accumulated motion reaches about twenty-five centimeters, comfortably larger than the accuracy an ordinary centimeter-level RTK fix claims for itself.
Both the original measurement and the one taken a decade later can be entirely correct. They are simply describing the same physical point at two different moments.
Two Dates That Sound Alike, and Aren't
Working with precise coordinates eventually surfaces two related but genuinely different kinds of epoch.
| Type of epoch | What it tells you | Plain-language question |
|---|---|---|
| Measurement epoch | When the receiver actually observed the point | When did I measure it? |
| Reference epoch | The date coordinates in a reference frame are expressed as of | What date do these published coordinates represent? |
A coordinate measured today can sometimes be mathematically shifted to represent where that same point would have been at an earlier or later reference epoch, provided a model of how the point or region moves through time is available. Reference frames and that kind of transformation get their own full treatment later in this series. For now, the useful lesson is narrower: an epoch is not automatically just the day the data happened to be collected.
An Old Directory Is Not a Wrong One
A store moves across town. Ten years later, an old printed directory and a brand-new one both correctly list a location for that store, each one accurate for the year it was published. A street address alone, without knowing which edition it came from, is incomplete.
Coordinates behave the same way. A coordinate with no epoch attached can be perfectly accurate and still leave a future user with no way to know exactly when that accuracy actually applied.
"The date is just metadata" holds up fine for ordinary, everyday mapping. At centimeter-level GNSS accuracy, that same date can shift the actual numerical position by a meaningful amount. The epoch isn't a label stapled onto a coordinate. It's part of what the coordinate means.
What This Actually Means to Save
A single important RTK position deserves more than latitude, longitude, and height saved on their own.
A saved coordinate has excellent, centimeter-level precision but no recorded date. Years later, is that coordinate still fully usable? Only partially. The number itself may be exactly right, but without an epoch, nobody downstream can know for certain when it was right, which matters as soon as the point in question is on a surface that moves at all.
At minimum, a reusable high-accuracy coordinate deserves the observation date and time, the reference frame or datum it was measured relative to, the correction source used to produce it, and whether the recorded height is ellipsoid height or an elevation already run through a geoid model. Some software saves part of this automatically behind the scenes. Other tools export only the bare coordinate fields and nothing else. Before trusting an old saved point years later, it's worth confirming exactly what that file actually kept.
The Old Coordinate Did Not Become Wrong
When a coordinate shifts with time, it's tempting to say the earlier version simply became inaccurate. That framing usually isn't the right one.
The earlier coordinate can still be a perfectly accurate description of the point at its own original epoch. The newer one describes the same point at a later epoch. The actual problem only shows up when the two get compared as if they were describing the exact same moment, which they never claimed to be.
A precise coordinate is a position with a timestamp attached. Latitude, longitude, and height answer where. The epoch answers when, and at centimeter-level accuracy, leaving that second question unanswered is not a small oversight.
A complete high-accuracy coordinate now has to answer two questions at once: where was the point, and when was that description true? One question still remains unaddressed. Chapter 4 takes on reference frames, the systems that give a coordinate its actual meaning, and explains exactly why two perfectly correct receivers can still report different numbers for the same physical point.