Thermal infrared remote sensing of urban heat: Hotspots, vegetation, and an assessment of techniques for use in urban planning

- Thermal infrared (TIR) remote sensing can identify urban hotspots at appropriate scales.
- Increasing urban vegetation cover can reduce land surface temperatures (LST).
- LST and air temperature broadly follow similar patterns at coarse spatial resolution.
- TIR remote sensing is a useful tool for planning and informing urban greening.
- Defining the scale of application should inform TIR remote sensing product selection.

In order to mitigate areas of high urban air temperature (or 'hotspots'), advice and tools are currently being sought to help inform urban planning and decision making around urban greening. One potential tool receiving growing interest is the use of thermal imagery for identifying hotspots, however this relies on the assumption that patterns in land surface temperature (LST) coincide with patterns in air temperature. This study explores the capacity of very high resolution (VHR), airborne thermal infrared (TIR) remotely sensed data to identify hotspots at a neighbourhood and street scale resolution. As such it assesses whether VHR TIR remote sensing is an appropriate tool for urban planning and urban greening decision making. In partnership with a local municipality in Melbourne, Australia, VHR (0.5 m) daytime and night time TIR images were captured during warm summertime conditions in February 2012. We found that VHR TIR data certainly identified locations of high LST that could be prioritized for urban greening, however at very high resolutions, VHR TIR data could not identify hotspots because patterns in LST did not strongly correlate with patterns of high air temperature. When VHR TIR data was aggregated to a coarser resolution, it could be used at the neighbourhood-scale to identify hotspots due to greater confidence that areas of high LST do represent air temperature hotspots. Increased vegetation proportion was associated with a reduction in LST for both the day and night meaning urban greening can be used to mitigate hotspots and should be prioritized in wide, open streets where building shade is less. While VHR TIR remote sensing may be an attractive option, this study shows it is not a suitable tool to help inform urban planning and urban greening decision making as the capture, post-processing and interpretation requirements and costs of delivering a high quality product are prohibitive for many end users. We suggest that those seeking to use thermal imagery to identify hotspots in the urban landscape, strongly consider more accessible and cheaper satellite remote sensing products (such as Landsat). VHR TIR derived LST may be useful in designing urban spaces for improved human thermal comfort and focusing on a small region of interest, but the numerous complexities and limitations of the data must be recognized.