Discussion Topic: Do I need Ground Control Points (GCP) if I have RTK GPS on my drone?
While having an RTK GPS on your drone can significantly improve the accuracy of the geolocation data collected during aerial surveys, there are still several reasons why using ground control points (GCPs) with Pix4D or similar photogrammetry software is recommended:
Why use GCPs when you already have RTK?
- Accuracy verification: GCPs are surveyed reference points with known coordinates. They let you independently verify the accuracy of your model and catch errors from atmosphere, multipath, or satellite geometry.
- Georeferencing: RTK gives accurate drone positions, but GCPs align the imagery to the project coordinate system so the entire dataset lands where it should.
- Correcting systematic errors: Camera distortion, timing offsets, and other biases can creep into photogrammetry. GCPs help the solver correct these so features match reality.
- Vertical accuracy: Horizontal precision from RTK is usually stronger than vertical. GCPs (and/or independent checkpoints) tighten up elevations for DEMs, volumes, and flood studies.
- Consistency on large/varied sites: On big areas or complex terrain, accuracy can drift. GCPs distribute control so accuracy holds across the site.
- Quality assurance: Compare computed vs. surveyed GCPs to quantify residuals and document accuracy in deliverables.
- Regulatory & client requirements: Some specs require GCPs or checkpoints for acceptance.
Practical approach
- Fly RTK/PPK to minimize absolute error and reduce the number of GCPs needed.
- Place a small set of well‑distributed GCPs and independent checkpoints (for QA).
- Report horizontal/vertical residuals, CRS, and geoid model in your methods note.
Bottom line: RTK greatly improves positioning, but GCPs (plus checkpoints) validate, correct, and georeference your dataset so it integrates cleanly with engineering workflows.