Ground Control Points are visual markers on the ground with known coordinates that increase orthomosaic accuracy and maintain alignment of multiple surveys. Before you fly, place the GCPs in the desired locations (Figure 1).
When you are ready to upload your survey, you will need to add and tag the GCPs using the Precisionviewer's Infield View.
For step-by-step instructions to do so, refer Adding GCPs Using Precisionviewer.
Also please see this video for reference.
Figure 1: GCP placed in a farm (Clear and distinguishable)