A ground control point, or GCP, is a marked target on the ground whose position has been surveyed independently of the drone, and which is identified in the imagery during processing so the survey can be tied to a real coordinate system.
It is the oldest and most reliable reference in aerial surveying, and the one that works regardless of what the aircraft can receive.
What a GCP does
Two separate jobs, and they are worth keeping apart because different jobs need different amounts of each.
- Georeferencing. The target's surveyed coordinate tells the processing software where this pixel sits in the project's horizontal coordinate system and vertical datum. Without an external reference of some kind, a photogrammetric model has shape but no absolute position.
- Constraining the shape. Photogrammetric reconstruction is prone to systematic deformation, most commonly doming or a bowl across the interior and drift at the edges. Control distributed through the site pulls that out. This is why placement matters more than count.
What a GCP looks like in the field
A high-contrast target with a well-defined centre, large enough to be unambiguous at the flight's ground sample distance. A printed or painted chequerboard, a plastic target, a cross marked on pavement, or an existing feature with a sharp identifiable centre such as a survey monument or a painted line intersection.
Its position is established by survey: a GNSS rover occupation, a total station traverse, or an existing monument with published coordinates. The quality of that survey sets the ceiling on the whole job, because the drone survey cannot be more accurate than the control it is fitted to.
GCP versus check point
Both are surveyed targets. The difference is entirely in how they are used, and it is the difference between a claim and evidence.
- A control point is given to the processing software, which fits the model to it.
- A check point is withheld. The software never sees it. Afterwards, the finished model's value at that location is compared against the surveyed value.
Only check points provide independent evidence of accuracy. The residuals reported on control points describe how well the model fits data it was already told to match, which is a measure of the fit rather than of the truth. On any job that will be audited, disputed or used for payment, some surveyed points should be held out and the comparison reported.
How many, and where
There is no universal number. The minimum useful arrangement encloses the mapped area and includes at least one interior point; beyond that the count rises with site size, vertical relief, how broken up the site is, and how tight the tolerance is. Placement drives accuracy more than count does: perimeter first, at least one in the middle, spread through the site's vertical range, and inside the photo block rather than on its edge. The full reasoning on GCP count and placement goes through each of those.
When RTK or PPK replaces ground control
RTK and PPK position the camera to centimetre grade at the moment of each exposure, which georeferences the photo block without a dense target network. On many jobs that genuinely removes the bulk of a control campaign.
What they do not do is confirm where the model landed on the ground, and they do not catch a vertical offset introduced by the ellipsoid-to-orthometric height conversion, which moves the whole surface uniformly and is invisible in the imagery. The practical middle ground is to drop the dense network and keep a small number of surveyed points as check points and as vertical ground truth.
Ground control is also the fallback that needs no signal at all, which is why it becomes the primary method rather than a supplement on sites where the sky is obstructed or there is no correction stream.
Why buyers should care
Ground control is the line item that separates a survey you can rely on from a picture with coordinates attached. If a quote carries an accuracy figure, the control plan behind it is what makes the figure real, and what that accuracy number actually means depends on whether anything independent was measured on the ground at all.