Why are checkpoints kept away from ground control points?
A checkpoint next to a GCP tests the one place the model is forced to be right. Drone models are most accurate near the GCPs and drift further away, so a checkpoint beside a GCP makes the map look better than it is. ASPRS says to place checkpoints away from the GCPs used in processing (ASPRS 7.13) and well spread over the project (ASPRS 7.14).
How does error change with distance from a GCP?
Martínez-Carricondo et al. (2018) compared a drone point cloud with a reference built from 72 survey targets. The smallest differences were where the GCPs were, and height error grew the further away from them. Sanz-Ablanedo et al. (2018) saw the same on a larger site. With few GCPs, the worst of the dome-shaped error sat as far as possible from every GCP. A checkpoint beside a GCP would miss it.
How does the planner keep them apart?
After the GCPs are placed, the planner adds checkpoints one at a time, each at the spot farthest from every GCP and every checkpoint already placed. By default checkpoints are also at least a tenth of the site diagonal apart, a guideline from the US National Standard for Spatial Data Accuracy, but never more than a fifth of the GCP spacing.
Every target also stays 3 m clear of buildings, water and your no-go areas. That is the planner's own default, based on ASPRS 7.13, which says height checkpoints should preferably be 3 m or more from vertical features and abrupt changes in height.
How can I check the spread on my plan?
The Summary Report lists the closest distance from any checkpoint to a GCP and between any two checkpoints. It also checks the four quarters of the site. Each quarter that holds a fair share of the site should hold at least 20% of the checkpoints, following the same national standard.
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
Federal Geographic Data Committee (1998) Geospatial Positioning Accuracy Standards, Part 3: National Standard for Spatial Data Accuracy. FGDC-STD-007.3-1998. Available at: https://www.fgdc.gov/standards/projects/accuracy/part3/chapter3 (Accessed: 6 October 2026).
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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