1.2. Field Setup
❏ Early in the crop season, distribute at least 10-15 ground control points (GCP) uniformly throughout the field. Four GCPs should be fixed at the field corners and the rest distributed inside the field, preferably, in the alley ways for better visibility. Survey the GCPs with either RTK or a DGPS GNSS systems (whichever available) multiple times during the season to reduce the day/time bias. Make sure the surveyed GCPs are not disturbed/moved from their fixed positions throughout the crop season.
Generally, the white color GCPs are easy to spot, hence, should be preferred. Some photogrammetry software support automatic GCP detection. When available, these software customized GCPs can also be used. Here are a few examples of GCPs that have worked well for us:
i. Metal sheets screwed on top of the iron rods (example)File:Example.jpg. ii. White color tile (12" x 12") placed on top of the brick/paver base (example)File:Example.jpg: White tiles and bricks can be purchased from any hardware store. A base with 2-2.5" thickness should be preferred so as to avoid any dust deposition on the white surface.
❏ Place the reference calibration panel on a flat surface close to the first mission waypoint at a
fixed position (File:Figure 1.1). It is important to put the reference panels in the exact same orientation and position for each flight. This is to facilitate automatic processing of the panels based on coordinate positions. Small-size stakes/flags can be used to mark the corners of panels.
❏ The panel can also be utilized as a launchpad especially when it is wide.
Also, in order to minimize the variations due to varying sun illumination and overcast, maximum aerial shots of calibration panel at regular intervals are recommended. This can be achieved either by using multiple calibration targets or by capturing images of a target at every landing and takeoff event during the mission. A 20% gradient grayscale calibration panel can be used as a reference panel. The details to build a calibration panel are described in the following sections.
Figure 1.1: Example of a grayscale calibration panel attached on a metal frame.