*2.2. Data Acquisition*

The data collection was performed by a custom quadcopter drone equipped with a 20-megapixel (4096 × 2160 resolution) CMOS mechanical shutter RGB camera, which was used to generate the low-altitude aerial imagery dataset. The images were captured and saved in the 16-bit JPG format, accompanied by the georeferencing metadata of each image. A total of 3 flight missions were executed during the time window between 11:00 and 13:00 (when the solar elevation angle was greater than 45◦), in order to avoid drastic deviations in solar illumination between the flights. The flights were executed with similar flight parameters across all of the flights, operating at a fixed altitude of 10 m AGL ±0.5 m (GPS vertical hovering error), which is considered to be consistent towards the ground because the field was levelled by a flattening cultivation roller approximately 4 months prior to the measurements. The sensor was set to capture images at a fixed interval of 2 sec/capture, and therefore the flight plans were designed around this parameter. The frontal and lateral overlaps were both selected to be 80%, resulting in a cruising speed of approximately 1.1 m/s. Finally, the orientation of the flight lines was selected to be parallel to the orientation of the planting rows. The generated flight plan is also presented below (Figure 4), and was executed three times consecutively by the UAV.

**Figure 4.** The flight mission executed by the UAV.

Before the start of the data collection flights, a total of 45 ground truth targets were deployed and stabilised across the field, in order to support the annotation stage (described in the following section). The targets indicated the maturity level of the selected broccoli crops, as they were categorised by an expert agronomist who participated in the process. The human expert indicated a total of 15 broccoli heads of 3 different broccoli maturity classes. These classes ranged from 1 to 3, with class 1 representing immature crops that would not be harvested for at least the following 15 days, class 2 representing heads that are estimated to reach harvesting level within a week, and finally, class 3, which contained exclusively "ready to harvest" heads. Due to the high humidity of the air near the surface and the constantly wet soil, the targets were enveloped inside transparent plastic cases in order to protect them from decomposing, as they would remain on the field for approximately two hours. The cases had been tested in the university campus to verify that the labels (numbering) remained visible in the UAV imagery. In case a ground truth label was covered heavily by the surrounding leaves, a bright-yellow point-like object was also placed nearby as a pointer. Examples of the deployed targets are presented below (Figure 5).

**Figure 5.** The ground truth targets for each plant's maturity class and their respective pointers (when deployed, in high plant coverage areas). (**a**) Maturity class 1; (**b**) maturity class 2; (**<sup>c</sup>**,**d**) maturity class 3.
