How far apart should ground control points be for your flight height?
A simple starting point is twice your flight height. Propeller Aero, which makes AeroPoint ground control targets, advises spacing GCPs about that far apart, so a 75 m flight gives 150 m and a 100 m flight gives 200 m. The planner uses 150 m by default, or twice your flight height if you enter one.
This is a practical rule and not an ASPRS requirement. ASPRS Edition 2 sets no rule for how many GCPs to use or how far apart, and says maker advice should be checked against research (ASPRS Addendum V, A.7.1).
How does the planner set the spacing?
If you enter a flight height, the planner doubles it. If you set the spacing yourself, the default is 150 m, which matches a 75 m flight. Either way it places at least 5 GCPs, the minimum both Pix4D and Propeller Aero recommend. Pix4D says 5 to 10 GCPs are usually enough. Research on a large site found accuracy kept improving with more, so the planner lets the number grow with the size of the site.
How are the GCPs arranged across the site?
On a 17 ha test site, Martínez-Carricondo et al. (2018) found that GCPs round the edge gave the best horizontal accuracy and GCPs spread through the interior gave the best height accuracy. On a much larger 1,225 ha site, Sanz-Ablanedo et al. (2018) found that GCPs spread evenly over the whole area worked best, and that bunching them at the edge or the centre gave poor results.
The planner puts GCPs round the edge, pulled in by a 10 m buffer (less on narrow sites) because Pix4D notes that targets right on the edge appear in few images. It then fills the inside until nothing is more than about 70% of the spacing from a GCP. It also adds one near the centre if there is none, as Pix4D recommends. Once the GCPs are in place, the planner adds checkpoints for accuracy testing. See the difference between GCPs and checkpoints and why checkpoints are kept away from GCPs.
What about long, narrow sites such as roads?
On a 2.1 km road, Ferrer-González et al. (2020) found that GCPs alternating between the two sides, with a pair at each end, gave the best balance of accuracy and fieldwork. GCPs on one side only gave the worst results. The planner only merges edge points that are closer than 15% of the spacing, so both sides and both ends of a narrow site keep their GCPs.
References
American Society for Photogrammetry and Remote Sensing (2024) ASPRS Positional Accuracy Standards for Digital Geospatial Data, Edition 2, Version 2. Available at: https://publicdocuments.asprs.org/PositionalAccuracyStd-Ed2-V2 (Accessed: 7 October 2026). doi: 10.14358/ASPRS.PAS.2024
Ferrer-González, E. et al. (2020) “UAV Photogrammetry Accuracy Assessment for Corridor Mapping Based on the Number and Distribution of Ground Control Points,” Remote sensing (Basel, Switzerland), 12(15), p. 2447. Available at: https://doi.org/10.3390/rs12152447.
Martínez-Carricondo, P. et al. (2018) “Assessment of UAV-photogrammetric mapping accuracy based on variation of ground control points,” International journal of applied earth observation and geoinformation, 72, pp. 1–10. Available at: https://doi.org/10.1016/j.jag.2018.05.015.
Pix4D (no date) Step 1. Before Starting a Project > 4. Getting GCPs on the field or through other sources (optional but recommended). Available at: https://support.pix4d.com/hc/en-us/articles/202557489 (Accessed: 6 October 2026).
Propeller Aero (no date b) How Many AeroPoints and Ground Control Points Do I Need for My Survey? Available at: https://help.propelleraero.com/hc/en-us/articles/19384158185495 (Accessed: 6 October 2026).
Sanz-Ablanedo, E. et al. (2018) “Accuracy of Unmanned Aerial Vehicle (UAV) and SfM Photogrammetry Survey as a Function of the Number and Location of Ground Control Points Used,” Remote sensing (Basel, Switzerland), 10(10), p. 1606. Available at: https://doi.org/10.3390/rs10101606.
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I am a Geospatial Engineer with five years of real estate consulting experience. I studied for an MSc in Geographical Information Science at the University of Exeter, United Kingdom.
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