= Solution
Use calibrated <multispectral imaging> of reflected sunlight, with visible, near-infrared and short-wave-infrared bands. A satellite records <radiance> in several passbands for each ground pixel. Perform geometric registration and <atmospheric correction> to estimate <spectral reflectance>, then account for illumination differences on slopes and mask cloud, water and dense vegetation. A <false-colour composite>, diagnostic band ratios or a trained <statistical classification> can separate surface <lithologies>; field observations and representative spectra validate any mapped contact.
Different <minerals> have different absorption features: iron-bearing materials affect visible/near-infrared <spectral reflectance>, while hydroxyl-bearing <clay minerals> and carbonate-bearing rocks can differ in short-wave-infrared bands. Adjacent patches with reproducibly different spectra and coherent geological boundaries are candidates for <lithological contacts>. This is particularly useful in dry, exposed terrain. Vegetation, soil cover, similar rock compositions, weathering and topographic shadow can hide or mimic a contact; a colour boundary alone is insufficient.
Choose <spatial resolution> appreciably finer than the narrowest unit being mapped, with several pixels across that unit. Approximately $10$–$30\ \mathrm m$ is suitable for regional contacts separated by hundreds of metres; metre-scale units require finer imagery or field work. <Sentinel-2> provides $10$–$20\ \mathrm m$ bands useful for this purpose, while the reflected-light multispectral bands of <Landsat> are typically $30\ \mathrm m$. Broad multispectral passbands distinguish many rock assemblages but cannot resolve every narrow <mineral> absorption feature; <hyperspectral imaging> is preferable when those features are essential.
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