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What do the different band combinations mean for the corrected reflectance images?
A digital color image displayed on a monitor is composed of three different color channels: red, green, and blue. The Corrected Reflectance satellite images from MODIS and VIIRS are made by combining the reflected light detected by the sensor at various wavelengths (spectral bands) and making them into a single image. Worldview Snapshots makes use of the MODIS and VIIRS broad range of spectral observations by creating both true-color and false-color images, each tailored to highlight different land surface, atmospheric, and oceanic features.
Advantages: Natural-looking images of land surface, oceanic and atmospheric features.
Long Answer
True-color imagery uses MODIS Bands 1, 4, and 3 or VIIRS Bands I1, M4, M3 respectively corresponding to the red, green, and blue range of the light spectrum, are assigned to the red, green, and blue channels of a digital image. These images are called true-color or natural color because this combination of wavelengths is similar to what the human eye would see. The images are natural-looking images of land surface, oceanic and atmospheric features.
MODIS Bands 3, 6, 7 or VIIRS Bands M3, I3, M11 combination
Advantages: Distinguishing liquid water from frozen water, for example, clouds over snow, ice cloud versus water cloud; or floods from dense vegetation.
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Long Answer
The MODIS Band 3, 6, 7 or VIIRS Bands M3, I3, M11 combination are assigned to the red, green, and blue components of a digital image. This combination is used to map snow and ice. Snow and ice are very reflective in the visible part of the spectrum (MODIS Band 3/VIIRS Band M3), and very absorbent in MODIS Bands 6 and 7/VIIRS Bands I3 and M11 (short-wave infrared, or SWIR).
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Snow and Ice
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Since the only visible light used in these images (MODIS Band 3/VIIRS Band M3) is assigned to red, snow and ice appear bright red. The more ice, the stronger the absorption in the SWIR bands, and the more red the color. Thick ice and snow appear vivid red (or dark pink), while small ice crystals in high-level clouds will appear pinkish in the MODIS imagery and reddish-orange or peach in the VIIRS imagery.
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Vegetation
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Vegetation is absorbent in MODIS Bands 3 and 7/VIIRS Bands M3 and M11, but reflective in MODIS Band 6/VIIRS Band I3, and so will appear greenish in this band combination. Bare soil and desert will appear bright cyan in the image since it much more reflective in MODIS Band 6/VIIRS Band I3 and MODIS Band 7/VIIRS Band M11 than MODIS Band 3/VIIRS Band M3.
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Water
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Liquid water on the ground will appear very dark since it absorbs in the red and the SWIR, but small liquid water drops in clouds scatter light equally in both the visible and the SWIR, and will therefore appear white. Sediments in water appear dark red
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MODIS Bands 7, 2, 1 or VIIRS Bands M11, I2, I1 combination
Burn scar: Red to reddish-brown, depending on the nature of the pre-fire vegetation and the severity of the burn.
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Advantages: Distinguishing burn scars from naturally low vegetation or bare soil. Enhancing floods.
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Long Answer
MODIS Bands 7, 2, and 1/VIIRS Bands M11, I2, I1, are assigned to the red, green, and blue portions of the digital image. This combination is most useful for identifying burn scars from naturally low vegetation or bare soil and enhancing floods. This combination can also be used to distinguish snow and ice from clouds. Snow and ice are very reflective in the visible part of the spectrum (MODIS Band 1/ VIIRS Band I1), and absorbent in MODIS Band 2/VIIRS Band I2 (near infrared) and MODIS Band 7/VIIRS Band M11 (short-wave infrared, or SWIR). Thick ice and snow appear vivid sky blue, while small ice crystals in high-level clouds will also appear blueish, and water clouds will appear white.
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Vegetation and bare ground
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Vegetation is very reflective in the near infrared (MODIS Band 2/VIIRS Band I2), and absorbent in MODIS Band 1/VIIRS Band I1 and MODIS Band 7/VIIRS Band M11. Assigning that band to green means even the smallest hint of vegetation will appear bright green in the image. Naturally bare soil, like a desert, is reflective in all bands used in this image, but more so in the SWIR (MODIS Band 7/VIIRS Band M11, red) and so soils will often have a pinkish tinge.
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Burned areas
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Burned areas or fire-affected areas are characterized by deposits of charcoal and ash, removal of vegetation and/or the alteration of vegetation structure. When bare soil becomes exposed, the brightness in MODIS Band 1/VIIRS Band I1 may increase, but that may be offset by the presence of black carbon residue; the near infrared (MODIS Band 2/VIIRS Band I2) will become darker, and MODIS Band 7/VIIRS Band M11 becomes more reflective. When assigned to red in the image, MODIS Band 7/VIIRS Band M11 will show burn scars as deep or bright red, depending on the type of vegetation burned, the amount of residue, or the completeness of the burn.
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Water
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Liquid water on the ground appears very dark since it absorbs in the red and the SWIR. Sediments in water appear dark blue. Ice and snow appear as bright turquoise. Clouds comprised of small water droplets scatter light equally in both the visible and the SWIR and will appear white. These clouds are usually lower to the ground and warmer. High and cold clouds are comprised of ice crystals and will appear turquoise.
