Correction of Low-altitude Thermal Images applied to estimating Soil Water Status

A system based on a thermal camera was developed for monitoring soil water status. This study used a thermal camera to take images of a field from a low-altitude helicopter. Thermal infrared energy captured by cameras tends to include errors because of atmospheric effects. This means that the image must be corrected for atmospheric transmissivity. The transmissivity can be determined from the ambient temperature, the humidity and the distance between the target object and the sensor. A method for correcting the image was developed using a principle of thermal imaging. The imaging accuracy was improved by the correction method. Remote-sensing experiments using an unmanned helicopter were conducted at an agricultural field. The experimental field was a paddy field with uniform soil quality, and the images of bare soil in the field were obtained at 10 a.m. and 3 p.m. on the same day. The correlation between the thermal image temperature and soil water content was evaluated. The determination coefficient between water content and temperature at 10 a.m. was 0·69 and that at 3 p.m. was 0·64. The difference in temperature between the two images was examined. The determination coefficient between water content and temperature difference was 0·42. Finally, a map of ground surface water content was generated using the captured image obtained at 10 a.m. The thermal imagery was found to be useful in determining within-field variability in ground surface soil moisture status.

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