Two-step fusion of operational land imager and thermal infrared spectrometer data based on nested hypersharpening and pansharpening stages

We propose a two-step spatial enhancement procedure for the two 100-m thermal infrared (TIR) bands of Landsat 8/9, captured by its TIR spectrometer (TIRS), approached as a problem of fusion of heterogeneous data or multimodal fusion. The fusion algorithm is guided by the statistical similarity between the TIR and visible/near-infrared (VNIR) and short-wave infrared (SWIR) bands provided at 30 m by the operational land imager (OLI). In the first step, hypersharpening is applied from the 100- to 30-m scale (3:10 scale ratio): the two TIRS bands are spatially enhanced by means of two linear combinations of the 30 m VNIR + SWIR bands, devised to maximize the correlation with each thermal band at its native 100-m scale. In the second step, the thermal bands, previously hypersharpened at 30 m, are pansharpened through the 15-m panchromatic (Pan) band of OLI. The proposed approach is compared with plain 100- to 15-m pansharpening carried out uniquely by means of the Pan image of OLI. Both visual evaluations and statistical indexes measuring radiometric and spatial consistency at the three scales are provided and discussed. The superiority of the two-step approach is highlighted.