USING UNMANNED AERIAL VEHICLE PHOTOGRAMMETRY TO CHARACTERIZE THERMAL ANOMALIES WITHIN AGRICULTURAL KARST IN BRIDGEWATER, VIRGINIA

Bridgewater, like other localities in Rockingham County, Virginia, is known to have karst development that produces features such as springs and sinkholes. Some karst features can produce thermal anomalies that could be visible on the surface. In this study we used a UAV to characterize a suspected thermal anomaly by taking wide angle and infrared (IR) photos of the anomaly and of the entire 23.6 ha farm.

A total of 3,274 wide angle and IR photos were captured of the farm on 6/13/2023 from 8:17 AM to 10:17 AM. Aerial photos were georeferenced using real-time kinematic (RTK) processing, meaning that each photo taken had a highly accurate ( 2 cm) latitude, longitude, and elevation associated with it. Using Agisoft Metashape, wide angle images were used to construct a digital elevation model (DEM) and orthomosaic of the study area. To make a thermal orthomosaic, IR images first had to be converted from R-JPEG format to TIFF format to be usable as thermal data. Once the processed TIFF images were made into an orthomosaic, temperature data was extracted from the orthomosaic by applying an equation to the raster dataset. GPS points were taken around the perimeter of the thermal anomaly to provide a reference for its location in historical aerial imagery.

The thermal orthomosaic confirmed that there was a thermal anomaly. The anomalous area was up to 2° C cooler than the surrounding land. IR data was collected when the weather was partly cloudy, and the air temperature was 15° C. The wide angle orthomosaic indicates that the thermal anomaly is discolored from surrounding vegetation. The discoloration is evident in historical aerial imagery, with evidence of discoloration being visible in 2007. Future work may involve conducting IR scans at different times of the year to measure if the temperature of the anomalous spot changes depending on the season.