Modern drones with onboard RTK and PPK have transformed field efficiency, and it is tempting to conclude that ground control points (GCPs) are obsolete. In practice, GCPs remain the most reliable way to guarantee, and prove, absolute accuracy.
GCPs serve two roles. As control, they anchor the photogrammetric block to a known datum. As checkpoints, independent markers not used in processing let us measure true accuracy rather than estimate it.
Distribution matters more than quantity. A handful of well-distributed points, including the corners and centre of the site, with elevation control on terrain extremes, outperforms a dense cluster in one area.
For corridor projects, we place control in alternating pairs along the route to constrain the long-axis drift that single-line captures are prone to.
Every deliverable we issue includes a checkpoint residual report, so the accuracy figure on the cover page is a measurement, not a marketing number.