Spatiotemporal monitoring of surface temperature in an urban area using UAV imaging and tower-mounted radiometer measurements

An extended UAV campaign that covered the historic city center of Heraklion, Greece has been performed during two consecutive days in July 2018. Heraklion city center is equipped with a permanent micrometeorological tower measuring net radiation and turbulent heat and CO ₂ fluxes. UAV cameras with RGB/NIR bands, as well as an integrated thermal (IR) band were used during the campaign. A calibrated and orthorectified RGB/NIR image mosaic (5 cm) was produced and a Support Vector Machine (SVM) algorithm was applied for the classification of the urban surface materials. The emissivity of the different materials was obtained by spectral libraries and used to calibrate the thermal maps (10 cm) by the IR camera. The first results show the pronounced effects of urban canyon orientation, building density and materials to the surface temperature. Some roofing materials present significantly lower temperature than other materials (cool materials). The surface temperature maps by the UAV were evaluated using tower-mounted net radiometer measurements. Increased differences between the two methods were found, attributed mainly to the different field of view of the two instruments, the increased thermal anisotropy of the urban environment and the uncertainty regarding the emissivity of the different materials.