GCPs and checkpoints: what is the difference?
GCPs tie the survey to the ground. Checkpoints test it. GCPs are fed into the processing to place, scale and straighten the model. Checkpoints are surveyed the same way but kept out of the processing, so they can show how accurate the finished map really is (ASPRS C.1, 7.13).
Checking a model against the same GCPs that made it flatters it. In one large study the real error at independent checkpoints was about three to six times the error at the GCPs.
What do GCPs do?
A drone survey first builds a 3D model with no real-world position, size or direction. GCPs have surveyed coordinates and are marked in the photos, so the processing software can fit the model to them (Martínez-Carricondo et al., 2018). Fitting to GCPs also reduces the slight dome shape that can creep into drone models (Sanz-Ablanedo et al., 2018).
Why can't I test accuracy with the GCPs?
Because the model bends to fit the GCPs, so they will always look accurate. Sanz-Ablanedo et al. (2018) tested thousands of combinations on a 1,225 ha site. With only 3 to 5 GCPs the error at the GCPs was a few millimetres, while the real error at independent checkpoints was over 1 m and as high as 8 m. More GCPs narrow the gap but do not close it. The study put the real error at about 3 times the GCP error with a very high number of GCPs, 4 to 5 times with a medium number and 6 times with a medium to low number.
What does ASPRS ask of checkpoints?
Checkpoints should come from an independent set of points that was not used to process or calibrate the product (ASPRS C.1). They must be at least twice as accurate as the product being tested (ASPRS 7.13). A checkpoint layout designed to ASPRS Edition 2 plans where those points go. Whether the project meets the standard depends on the field survey and the accuracy report. See how the files are used on site with an RTK controller.
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
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.
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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