This article deals with the best practices to follow while installing and using GCP (Ground Control Points) hardware on field, prior to fly a drone, in order to get the best possible experience and results while processing collected data with delair.ai.
1. How to spread targets on ground ?
An appropriate distribution of the GCP targets on ground is essential for the operation to be successful.
First principle : The quantity of required GCP to be set on ground depends on two factors :
- The site size to be surveyed
- The altitude you want to fly
Tip 1 : Use a starred distribution - with one GCP every 300m
Targets will act as a traverse ; one single target would not be as accurate as a network spread over the site. It is recommended to start from the site boundaries prior to add GCPs inward.
Tip 2 : Take topography into account
Targets shall be spread in high and low points. It is ideal to place GCP in both extremes so as to capture data in downs and maintain data accuracy all the way up.
Tip 3 : In a quarry, pay special attention to stockpiles area
Stockpiles area is usually a zone of potential higher uncertainty as a result of GCPs bad practices. The accuracy of volume calculations depends heavily on the steadiness of the targets during operation. It is highly recommended to increase the density of targets in such areas.
Tip 3 Bis : In a Agriculture field
Make sure that there is one GCP at each parcel border or corner and one in the center of the field
2. Ensuring a qualitative positioning
Second principle : the visual quality of the GCPs remains imperative to keep accuracy at best throughout the project.
Tip 4 : Keep your GCPs in place anytime
GCPs shall not be moved during the operation. Once targets positions have been measured, each displacement of a target would result in a risk of increased inaccuracy during data processing. It is recommended to brief all sites stakeholders that could potentially interfere with the targets.
In case of accidental movement, we strongly advise to inform us at firstname.lastname@example.org so that provision shall be made to ignore moved GCP, or to update position in processing for the new one if it could be measured. We advise to remove obsolete positions and reload a file of latest positions.
Tip 5 : Keep your GCPs clean
Cleaning targets especially in dusty areas is key to ensure best accuracy possible.
Tip 6 : Use easily detectable GCPs - with a single central point
- We recommend to use checkerboard type GCPs to have an easily identifiable center
- Painted crosses or strips are not recommended since not providing a good level of precision
3. Measuring GCP Position and Transferring data to delair.ai
To reach a reliable, centimetric, accuracy level.
Tip 7 : GCPs in same projection system as the project
This practice avoids to convert geographical coordinates with the risk to introduce a potential bias.
Tip 8 : Track Vertical Reference System
Ensure to note and report in file in which vertical system the GCPs positions have been measured.
Tip 9 : File name
Adopt a standard file naming such as :
- Example : GCP-20191030-Paris-EPSG_3944-RAF09.csv
Tip 10 : Upload GCPs for each new survey in delair.ai
It is required to upload GCPs for each new photogrammetry run so that they are taken into account.
For processing steps with GCP in delair.ai, please refer to this article