You get to the site, unload the drone, and the question comes up before the first battery goes in: do we lay out ground control on this one? The jobs where the answer isn't obvious are where pilots lose time, either by skipping control they needed or by walking targets across a field that never required them. This guide covers how drone mapping crews actually use ground control points: when a job calls for GCPs, how many the processing vendors suggest, where to put them, and how they work with RTK and PPK.
What a ground control point is
A ground control point (GCP) is a spot on the ground with coordinates measured independently of the drone, usually with survey GNSS equipment, that you can also pick out clearly in the photos. During processing, the software ties those image locations to the measured coordinates, which pins the whole map to the ground. Pix4D's article on tie points in photogrammetry projects (checked Oct 30, 2026) puts it simply: "Ground control points (GCPs) are used to georeference the model" and "GCPs improve the relative and absolute accuracy of the model."
Those two words, relative and absolute, matter for deciding whether you need control at all. If the difference is fuzzy, our post on absolute vs relative accuracy explains it. This post sticks to planning and placement.
When a job needs GCPs
Start with what the client will do with the map, not with the drone. A few practical questions sort most jobs:
- Will the map be overlaid on other data? If it has to line up with design files, a previous flight, a property survey or a GIS layer, it needs to sit in the right place on the earth. That's an absolute positioning question, and control (or a correction workflow you can check) is how you answer it.
- Will anyone measure from it and act on the number? Volumes, elevations and distances that drive payment or design decisions deserve independent control or at least check points.
- Is it a repeat site? Progress flights compared week to week need to line up with each other. Permanent or re-surveyed control makes that comparison far more trustworthy.
- Is it just a visual record? Marketing imagery, a quick condition overview or a roof photo set usually doesn't justify a control layout.
One firm line: if the work touches property lines, boundaries or anything with legal weight, that's a licensed surveyor's job. A drone map with GCPs can support a surveyor's work, but it doesn't replace their stamp, and your quote should say so.
How many GCPs vendors suggest
There isn't one magic number, but the major processing vendors publish similar starting points. Here's what their own documentation says.
Pix4D. The PIX4Dmapper article on getting GCPs on the field (checked Oct 30, 2026) says: "A minimum number of 3 GCPs is required for them to be taken into account in the reconstruction. Each one should be marked in at least 2 images." It then recommends more: "A minimum number of 5 GCPs is recommended. 5 to 10 GCPs are usually enough, even for large projects. More GCPs do not contribute significantly to increasing accuracy." The same page adds an exception: "In cases that the topography of the area is complex, then more GCPs will, indeed, lead to better (more accurate) reconstruction."
OpenDroneMap. The OpenDroneMap documentation on ground control points (checked Oct 30, 2026) lands in the same range: "A minimum of 5 GCP works for most of the jobs, and for larger projects 8 – 10 are sufficient."
So the honest summary is: three is a technical floor in Pix4D, five is the common starting recommendation, and both vendors treat roughly ten as plenty for most sites, with more only when the terrain is complicated.
Where to place them
Count matters less than layout. Pix4D uses a good mental picture: "Imagine the area as a large table and the GCPs as the legs that will support it." Bunch the legs in one corner and the table tilts. Spread them out and it stands level. The same Pix4D article recommends you "place one GCP in the center of the area" and warns: "Do not place the GCPs exactly at the edges of the area, as they will only be visible in a few images."
OpenDroneMap's guidance points the same way, with a few extra specifics:
- "Locate some points near the corners and others in the center"; don't cluster them.
- Spread them out "covering high and low elevations"; that matters on sloped sites, pits and stockpiles.
- "GCP spacing should be larger than the image footprint so that you can't see more than one GCP in a single image."
- To get each point into enough photos, "separate the points 10 to 30 meters from the perimeter of the project." ODM notes that distance depends on your overlap.
That last point ties back to flight planning. Points near the edge of the flight area show up in fewer overlapping images, which is why both vendors keep them in from the boundary. If you're adjusting front and side overlap for a control job, our post on drone mapping overlap settings covers that side.
Targets: size, contrast and what makes a good point
A GCP is only useful if you can mark its exact center in the photos. Pix4D says a photogrammetric target "should have about five to ten times the dimensions of the GSD." So target size follows from the ground sampling distance you plan to fly; if you're working that number out, see our guide to ground sampling distance.
A few field habits help:
- Use high-contrast targets with a clear center: checkerboard or X patterns on flat ground.
- Keep them off steep slopes, out from under trees and away from shadows cast by vehicles or buildings.
- Weigh them down or pin them. A target that shifts after it's measured is worse than no target.
- Natural features can work if you have to. OpenDroneMap notes: "Sharp corners are good picks for GCPs." Pix4D cautions that natural points can be harder to identify and mark.
Check points: hold some back
Pix4D recommends "measuring more GCPs, so some can be used as checkpoints to assess the accuracy of the project." A check point is measured exactly like a GCP, but you leave it out of the georeferencing and compare where the map puts it against where you measured it. That gives you an independent check instead of a self-graded one. Results from points the processing never saw are far easier to stand behind with a client.
GCPs with RTK and PPK
RTK and PPK improve the position recorded for each photo. Our explainers on what RTK is in drone mapping and what PPK is in drone mapping cover how each correction method works. What they don't do is check themselves. A base station set up on the wrong coordinates, a datum mix-up or a bad correction file can shift a whole map without anything looking wrong in the imagery.
That's why many crews flying RTK or PPK still lay out a smaller set of points, often used as check points rather than full control, so they can confirm the corrected positions landed where they should. How many depends on the job and the client's spec; record the decision either way.
A simple field checklist
- Decide from the deliverable whether the job needs control, check points or neither.
- Plan five or more well-spread points, including one near the center, kept in from the edges.
- Cover high and low ground on sloped sites.
- Size targets from your planned GSD.
- Measure every point in the coordinate system and datum the client expects.
- Hold a few points back as check points.
- Send legal or boundary work to a licensed surveyor.
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