Why RTK Alone Isn't Always Enough
RTK Fix accuracy describes how precisely the aircraft knows its own antenna position, not how precisely that translates to the ground point a pixel represents. The lever-arm distance between antenna and camera introduces its own uncertainty, on the order of 10 to 15cm if uncorrected, and the bundle adjustment that stitches a mapping project together can curve an otherwise flat site upward at its edges by several centimetres if nothing constrains it. GCPs anchor the model at known points, correcting that curvature rather than just reporting it.
Whether this matters depends on the job. Repeat missions over a fixed, monumented site, stockpile volume comparisons between flights, or internal progress monitoring with no third-party sign-off can run on RTK alone, since the systematic bias mostly cancels out when the same points are compared to themselves. A deliverable that needs absolute accuracy, especially vertical, or that has to stand up to independent verification, needs ground control.
Control Points vs Checkpoints
A control point constrains the model during processing, DJI Terra adjusts the bundle to match it. A checkpoint is a separately surveyed point held back from that adjustment entirely, used only afterward to check how far the finished model is out at a point it was never told about. Using the same point as both defeats the purpose of the check, since the model was built to match it in the first place.
A typical layout uses more control points than checkpoints, but keeping at least a handful of genuinely independent checkpoints is what turns "the model looks right" into a number that can be reported with confidence.
How Many Points and Where to Place Them
There's no single fixed number, it scales with site size, terrain complexity and how much of the accuracy budget RTK has already covered. As a general starting point, spreading points evenly around the site perimeter with at least one further into the interior gives the bundle adjustment something to hold onto across the whole area rather than just at the edges. Concentrating extra points in areas with less image overlap, dense vegetation or where the terrain changes sharply strengthens the model exactly where it's weakest, rather than adding redundant points where the data was already solid.
An RTK or PPK-equipped aircraft like the Matrice 4E needs meaningfully fewer points than a standard GPS drone would for the same site, since the aircraft's own positioning is already doing most of the work; a smaller number of well-placed checkpoints to verify that positioning held is often enough rather than the dense GCP grid a non-RTK survey would need.
Marking and Surveying the Point
A rigid, high-contrast target, a Goecke 300 x 300mm aerial target plate at £10.00 is a common choice, gives DJI Terra a sharp, unambiguous feature to match against the aircraft's captured images. The plate's physical position then needs its own independent, centimetre-accurate coordinate, which is where a separate GNSS receiver comes in rather than relying on the aircraft's own RTK for the point that's meant to check it.
An Emlid Reach RS3 or RS4 surveys the plate directly, occupying the point in FIX and logging its coordinate the same way it would for any other survey shot. A D-RTK 3 in Rover Station mode does the same job over Bluetooth to the DJI Enterprise app, useful for a team that already owns one for base station work and doesn't want a second GNSS system on site.
Importing and Matching GCPs in DJI Terra
DJI Terra imports ground control points from a spreadsheet listing each point's name, latitude, longitude and elevation, with the coordinate system specified so it matches the project. Rather than marking every point on every image by hand, marking one point on a single photo lets Terra's automatic detection find and mark the same point across the rest of the dataset on its own, a substantial time saving on any project with more than a handful of images.
Automatic marking works on the plate-style targets covered above and on distinct existing features (kerb corners, manhole covers) without a physical target at all, though a genuine target plate gives it a cleaner, less ambiguous feature to lock onto. Manual marking is still sometimes needed where a point is obstructed in some of the images, a building or piece of plant blocking the view from certain angles.
What This Means in the Field
The decision to use ground control comes down to what the deliverable is for, not a blanket rule. Internal monitoring and repeat comparisons on the same site can run on RTK alone; anything needing absolute vertical accuracy, or handed to a third party who'll check it, needs real ground control with genuine checkpoints held back from the adjustment.
Getting the RTK link solid during capture still matters even when GCPs are in use, since a weak or dropped correction during the flight adds noise the control points then have to correct for, rather than replacing the need for a clean flight in the first place.
Free Gifts Included
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Goecke Ground Control Point - 13-LP300-2
Improves Drone Survey Accuracy · Large, Easy-to-Spot Format
Common Questions
Depends on the deliverable. RTK alone is fine for repeat comparisons and internal monitoring on the same site; anything needing absolute vertical accuracy or independent verification needs real ground control.
No, using it for both defeats the purpose. A control point is used to adjust the model; a checkpoint is held back specifically to check the model afterward, and a point that's already been used to build it can't also independently verify it.
No fixed number, it scales with site size, terrain and how much of the accuracy already comes from the aircraft's own RTK. A general approach spreads points around the perimeter with at least one further inland, then adds more in weaker areas like dense vegetation or steep terrain.
No, marking one point on a single image lets Terra's automatic detection find and mark it across the rest of the dataset, though obstructed points sometimes still need manual marking.
The point needs an independent survey so it can genuinely check the aircraft's data. Surveying it with the same RTK system the aircraft uses would mean any systematic error in that system carries into the check as well, rather than being caught by it.
