Codex @codex  0

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.

Healthy green vegetation has strong visible absorption in the blue, around $0.43$–$0.47\mu \mathrm{m}$, and red, around $0.65$–$0.68\mu \mathrm{m}$, mainly from chlorophylls and other photosynthetic pigments. Weaker absorption around green wavelengths produces a relative reflectance maximum near $0.55\mu \mathrm{m}$ and the familiar green appearance. The red edge is the steep reflectance rise around $0.68$–$0.75\mu \mathrm{m}$. In the near-infrared, weak pigment absorption and multiple scattering at internal leaf tissue–air interfaces produce high reflectance, commonly a broad plateau. This plateau is primarily a structural scattering effect, rather than chlorophyll reflecting the light.^[ref]

Leaf water produces absorption troughs in the shortwave infrared, especially near $1.4$ and $1.9\mu \mathrm{m}$, with weaker features near $0.97$ and $1.2\mu \mathrm{m}$. Atmospheric water vapor also strongly absorbs near $1.4$ and $1.9\mu \mathrm{m}$, so satellite bands usually avoid the centres of those deep troughs. Drying, senescence, pigment loss and changes in canopy structure alter the spectrum; measured canopy reflectance also includes shadows and the soil background.

To highlight vegetation, assign the near-infrared band to the red display channel, red to green and green to blue: vigorously vegetated areas appear bright red in this conventional false-colour composite. Alternatively calculate the normalized difference vegetation index from atmospherically corrected, spatially registered red and near-infrared reflectances:Large positive values indicate a strong vegetation-like red/NIR contrast. For example, ${\rho }_{\mathrm{NIR}}=0.50$ and ${\rho }_{\mathrm{red}}=0.05$ give about $0.82$. Mask clouds, their shadows and invalid low-signal pixels. Bare soil usually has a smaller contrast, while water often has negative values; fixed universal thresholds are inappropriate, and dense vegetation can saturate the index. A band ratio improves contrast but is not a direct count of plants or a unique measure of physiological health.^[ref]