What is NDVI?
SWAN uses the NDVI (see illustration below), calculated from satellite imagery data, as the basis for monitoring how healthy (or green) a crop or field of turf is. NDVI is calculated from the red and near-infrared light bands, producing an index of between 0 and 1, which correlates with the greenness of the crop and its biomass.
SWAN allows you to choose parts of a field to monitor, by configuring Site Health Points or Site Health Areas. Data is obtained from two satellites.
Sentinel-2 – 10 x 10m pixels, data collected every ~5 days
Landsat-8 – 30 x 30m pixels, data collected every ~16 days.
The orbit of both satellites is sun-synchronous – this ensures the angle of sunlight hitting the Earth’s surface is maintained throughout the data set, enabling comparison between data sets obtained at different dates. More information on the Sentinel-2 satellite can be found on the ESA Sentinel website, while information on Landsat-8 is available on the USGS Landsat Missions website.
Sentinel-2 provides higher spatial resolution and is a good choice for smaller fields, or if you are interested in a particular area of your field. Landsat-8 data is intended for users who wish to have a broader overview of their site, and is more suited to larger operations. Additionally, there is a larger historical data set available.
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