What is NDVI, and how does SWAN use it?
NDVI - Normalised Difference Vegetation Index - is calculated from satellite imagery data and provides an indication of overall plant health.
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 three satellites.
- Landsat-8 – 30 x 30m pixels, data collected every ~16 days
- Sentinel-2 – 10 x 10m pixels, data collected every ~5 days
- Planet - 3.7 x 3.7m pixels, data collected daily*
*Frequency is dependent on data quality (e.g., cloud cover, haze, smoke).
Landsat-8 and Sentinel-2 data are available to all SWAN Systems subscribers. Planet data are available via a separate subscription.
The orbit of all 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, information on Landsat-8 is available on the USGS Landsat Missions website, and information on Planet satellites is available on their website.
Planet data is the highest resolution data set currently offered by SWAN Systems. This data set offers a cost-effective remote system to observe and compare block, variety, and farm performance, and to detect variability across fields. Sentinel-2 data provides good spatial resolution suitable for uniform crops and is a good choice for users who do not need daily data on their crop health. Landsat-8 data is intended for users who wish to have a broader overview of their site and is ideal for broad-acre farming.
