Field-setup Guideline

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1.2. Field Setup Guide

1.2.1 Ground Control Points: Early in the crop growing season, distribute at least 12-15 ground control points (GCP) uniformly throughout the field.
Six to eight GCPs should be installed in the outside periphery and the rest can be distributed uniformly inside the field, preferably, in the alleyways, at the row-column intersections for better visibility (Figure 1.2a)
Figure 1.2a: Example layout of the uniformly distributed Ground Control Points in a phenotyping field.
. In case the field layout permits leaving a few blank plots, the GCPs can also be installed in the middle of those plots.
Survey the GCPs with either RTK or a DGPS GNSS systems (whichever available) multiple times during the season to reduce the day/time bias. For best results, we strongly recommend recording multiple readings per GCP to get the precise positioning values.
Some GPS units allow recording of raw NMEA GNSS strings at high frequency (1-10Hz). Therefore, these units should be preferred, depending upon the availability. The averaged Latitude, Longitude, and Altitude readings can then be used in the photogrammetry softwares to generate highly accurate orthomosaics and Digital Elevation Models or DEMs.
It is highly important not to move the surveyed GCPs from their fixed positions throughout the crop season.
Generally, the white color GCPs are easy to spot, hence, should be preferred. Some photogrammetry softwares support automatic GCP detection. When available, these software customized GCPs can be used. Here are a few examples of GCPs that have worked well for us:
Metal sheets screwed on top of the iron rods (Figure 1.2b)
Figure 1.2b: Ground Control Point made by screwing metal sheet on top of a thin metal rod.
White color tile (12" x 12") placed on top of the brick/paver base: 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. (Figure 1.2c)
Figure 1.2c: White tile sitting on top of a concrete paver used as a Ground Control Point.
1.2.2 Reference Panels: Place the reference calibration panel on a flat surface close to the first mission waypoint, at least 2-meter away from the plot (Figure 1.2d)
Figure 1.2d: A foldable color reference panel.
It is very 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.
These panels can also be utilized as a launchpad.
In order to minimize the variations due to changing 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 (for bigger fields where multiple flights are necessary).
A 15-20% gradient grayscale reference panel can be used as a reference panel. The details to build a reference panel are described below.
Materials required:
1. 7mm thick lightweight plywood. 20 x 20inch (6 pieces).
2. Flat black and white water based color (1 liter each).
3. Wood primer (1/2 liter)
4. Paint mixing trays (6 units).
5. Brush rollers (6 units)
6. 100 ml liquid syringes with 10ml markings (2 units)
7. 5-7mm metal hinges with 2-3 holes (10 units)
8. Painters tape

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