Placing the Ground Control Points (GCPs) and measuring their exact location is an important task to ensure data quality.
Special attention has to be paid to:
- the use of proper materials for the GCPs.
- the GCP placement on the ground
- the GCP distribution throughout the field
- the actual measuring of the GCP location
GCP placement on-ground
When positioning the GCPs on the ground, ensure that:
- they are completely flat
- they are secured to prevent any movement
- no overhanging vegetation or dirt is covering the GCP
- a region of 1m*1m around the GCP is kept free from vegetation
- an additional colored marker is used to identify GCPs that are close to each other (within 30m)
At the beginning of the season, make sure to position the GCPs in such a way that they remain clear of vegetation and machinery as much as possible and are anchored in the same location for every flight.
Before every flight, remove all overhanging vegetation to ensure a clear line-of-sight from the drone at every angle.
GCP distribution throughout the field
The number and location of GCPs depends on the type of drone (RTK or not), the shape of the field and the size of the field.
GCPs should be spread across the whole area as uniformly as possible and should not be placed too close to the border of the area of interest.
The 'regular' GCPs should be placed at each corner of the field, with one in the centre. Position the 'corner' GCPs approximately between 50 to 10 meters from the field boundaries.
The 'control' GCP should also be placed in the centre, but at a distance from all other GCPs.
In case your area of interest is larger than 2ha: 1 additional GCP should be placed for every additional hectare.
If you are using a non-RTK drone, you must place at least 5 'regular' GCPs and 1 'control' GCP in the field (totalling 6 GCPs).
If you are using an RTK drone, you must place at least 1 'regular' GCP and 1 'control' GCP in the field.
In case your area of interest Is not rectangular, additional GCP(s) should be placed at all corners.
GCP location measurement
All GCP locations should be measured:
- at the center of the GCP
- using a measurement pole (ensure it is not inclined when measuring)
- using an RTK-enabled GPS device (ensure the RTK correction service is functional when measuring, providing cm-level accuracy)
- or using a (robotic) total station (providing mm-level accuracy)
- according to the format requested for MapEO