Real-time kinematic (RTK) technology has been used in terrestrial surveying for years to improve the accuracy of Global Navigation Satellite System (GNSS) data and provide high relative accuracy. However, to achieve centimetre precision in absolute accuracy when flying with RTK, greater care is needed.
What is Relative and Absolute Accuracy?
Relative Accuracy — the accuracy when comparing two points within the same dataset to each other, versus the same two points measured in the real world. For example, if you measure the carriageway width of a road from kerb to kerb in the model, and then take the same measurement on-site, the difference between the two should be small, typically within a couple of centimetres.
Absolute Accuracy — the accuracy when the digital model's coordinates are compared to the true real-world coordinates of the same feature, relative to a specific Coordinate Reference System (CRS / EPSG).
It's important to remember that while RTK provides high relative accuracy, it does not automatically guarantee high absolute accuracy.
RTK — Relative Accuracy
Using a DJI RTK drone and base station workflow as an example: the base station is set up before the drone is flown, and provides real-time corrections to the drone throughout the flight. The drone should never lose connection with the base station — if it does, the high-accuracy corrections are lost for that period.
How you set up the base station will determine whether you end up with both high absolute and relative accuracy, or relative accuracy alone.
In RTK, the base station serves as the main reference point for the drone. Coordinates collected by the drone are always accurate relative to the base, but that isn't the same as being accurate relative to your chosen coordinate system (EPSG). If the base station's own position is not accurately established in that reference frame, the whole flight inherits that error, every point stays internally consistent with every other point, but the entire dataset can be shifted from where it truly sits in the real world.
If the base station is set up without manually entering a known coordinate (or without using an NTRIP provider), this is known as Single Point acquisition, and the flight will not obtain high absolute accuracy.
RTK — High Absolute Accuracy
When setting up the base station, you have the option to manually enter the coordinate of the point the base is occupying. Since the base station itself isn't intended to establish a known point to survey-grade accuracy on its own, that point needs to be established independently — either with survey-grade equipment, or by logging the base station's raw data during setup and post-processing it (PPK) against a nearby reference station.
There is also the option to use an NTRIP provider to obtain RTK corrections without needing your own base station at all, using services such as Trimble VRS Now.
Using either a properly established base coordinate or an NTRIP correction service will get you both high relative and high absolute accuracy in the processed data — for the horizontal component. Vertical is worth treating separately, and is discussed below.
A note on vertical accuracy
Everything above applies to both horizontal and vertical position, but vertical is the component that most often falls short in practice, for a few reasons:
GNSS vertical accuracy is inherently weaker than horizontal — as a rule of thumb, expect vertical error to run roughly 1.5–2x the horizontal figure for the same equipment and setup.
The vertical component of a GNSS fix also converges more slowly than the horizontal component. A receiver can show a stable horizontal position well before the vertical solution has fully settled, so a fast commencement of a drone flight, tends to cost you vertical accuracy specifically, even when the horizontal result looks fine.
A correctly configured RTK or PPK setup (accurate base coordinate or NTRIP, good satellite geometry, no multipath) will generally get your vertical absolute accuracy a lot closer than flying without any correction at all — but "RTK/NTRIP is connected and reporting Fixed" is not the same as "vertical accuracy is guaranteed to be within X cm."
The achieved vertical accuracy on any given flight depends on satellite geometry, occupation time, multipath conditions, and the correction source itself, and it can vary meaningfully from one flight to the next even with identical equipment. Without RTK/PPK at all — relying on the drone's onboard GNSS alone — absolute accuracy (horizontal or vertical) is typically far worse, at the metre level rather than the centimetre level.
Because of that variability, don't treat RTK/NTRIP as giving you a known, fixed absolute accuracy figure. Ground Control Points remain the most reliable way to pin the model to true ground coordinates, since they're measured directly with dedicated survey-grade equipment rather than depending on the aircraft's GNSS solution and correction link quality throughout the flight. And regardless of which method you use, the only way to know what accuracy you actually achieved on a given site is to check it: see How To Use Ground Control for how GCPs are placed and measured, and its Checkpoints section for how to independently verify the finished model's real-world accuracy rather than relying on assumed or spec-sheet figures.
Summary
If you're planning to use a base station without establishing a known point, and you're not using an NTRIP provider, you will not achieve high absolute accuracy, this setup is not suitable for operations that require it.
If your project needs high absolute accuracy (for example, engineering deliverables, volumetric calculations against a CAD plan, or comparisons between flights over time), don't rely on RTK/PPK setup alone: add Ground Control Points, and use checkpoints to confirm the result.
See How To Use Ground Control for guidance.