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Vegetation Analysis Via SkySat-1 Near Infrared Imagery
Wednesday, January 15, 2014
SkySat-1 collects imagery using 5 channels: blue, green, red, near infrared (NIR), and panchromatic. Everyone is familiar with the first three channels because the human eye is sensitive to this range of the electromagnetic spectrum and our brains have evolved to interpret this information intuitively. For this reason, cameras and satellite data are typical viewed in this “true-color” scheme that we see in platforms like Google Maps.
The NIR channel (740 - 900 nm) is designed to capture light in a range just beyond the visible spectrum (390 - 700 nm). To visualize a “color infrared” image we typically map the NIR channel to red, while assigning the red to the green and the green to the blue channels. This allows us to visualize the near infrared channel as red, so materials that reflect well in this wavelength range appear very red.
One of the more interesting characteristics of the NIR channel is that lush vegetation reflects very strongly relative to other materials or woody vegetation. Healthy vegetation generates more chlorophyll in the leaves, which reflects well in the NIR, while less healthy leaves are much less reflective. Many interesting metrics depict vegetation health based on ratios of the red to NIR channel.
One of the most common vegetation health metrics is the Normalized Difference Vegetation Index (NDVI) which works well in consistently vegetated areas. The Modified Soil Adjusted Vegetation Index (MSAVI) takes this metric one step further by correcting for the amount of exposed soil in each pixel in agricultural areas where vegetation is surrounded by exposed soil.
The example in Figure 1 illustrates a color infrared rendering of some center pivot irrigated fields in Saudi Arabia. Figure 2 illustrates the MSAVI results from this same dataset. I have rendered the colors to depict healthy vegetation with hot colors and less healthy vegetation in cool colors. While we could qualitatively determine that one of the center pivot irrigated fields was well watered and the others appear to be fallow in the color infrared image, we now can quantify this with metrics that can be compared throughout the seasons or years and correlated with the factors measured on the ground like water usage, fertilization, seed varieties, crop yield, etc.
Mineral deposits rich in Iron Oxide are another common source of reflective NIR values in satellite images. This can be very interesting to geologists searching for assemblages of minerals indicative of higher concentrations of valuable metals.
Figure 1: Color Infrared Image of a Center Pivot Irrigated Field in Saudi Arabia. Captured by SkySat-1 on 12/26/13.
Figure 2: MSAVI Results of a Center Pivot Irrigated Field in Saudi Arabia. Captured by SkySat-1 on 12/26/13.
Posted by Ty Kennedy-Bowdoin, Product Manager
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