For aerial imagery providers like TerrAvion, certain elements are essential for successful image capture. One necessary component is good weather/flight conditions, a topic that was discussed in our recent blog post ‘Weather and atmospheric effects - challenges for aerial imagery capture‘. A more fundamental part of capturing imagery is sunlight. TerrAvion records the same amount of light into its sensors at all times (meaning each image is directly comparable to any other image), but a certain amount of light is necessary to produce quality imagery. Not all sunlight is adequate for capturing quality imagery. There may be 12 hours of daylight in a day but only 8 hours of usable light for aerial image capture. Optimal capture conditions for agricultural aerial imagery are not merely based on time of day but on solar elevation or sun angle, which varies by region and time of year.
The angle of the sun above the horizon and at which the rays of the sun hit the earth’s surface determines sun intensity and sun angle. Sun rays that hit the earth’s surface from directly overhead, at a 90-degree angle measured from the horizon, are the most intense and provide the most light. The ideal sun angle for agricultural aerial imagery is anything higher than 40 degrees. Image quality can degrades with sun angles lower than 40 degrees but it depends on the systems used. Near-infrared light, which is essential for capturing agricultural image data, is affected by low sun angle light more than visible light (RGB). Therefore Vigor or NDVI data quality degrades exponentially as sun angle decreases.
Most locations on the planet receive less than 90-degree of sun angle in a day. Typically, the sun angle is highest along the equator. Sun angle decreases as you move away from the equator and closer to the poles. This generally means that southern locations in the northern hemisphere have better aerial image capture conditions than northern locations, and vice versa for the southern hemisphere.
Seasonality and time of day
Sun angle and total light are also affected by the time of year. On average, lighting conditions in June and July are better than in April and September, although the sun angle still depends on the time of day. Regardless of latitude or time of year, the sun’s angle reaches closest to 90 degrees at the midpoint of the day. The sun angle is typically greater than 40 degrees between 9 AM to 4 PM, but this depends on location and changes throughout the year.
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