It’s like we learned from The X-Files: the truth is out there.
But it’s not that far out there. For those of us searching for terrestrial truth, we need not look any further than our planetary orbit. That’s where NASA has two satellites constantly analyzing and photographing Earth’s atmosphere, weather systems, land, oceans and even radiation.
The two satellites—named Terra (as in earth) and Aqua (as in water)—have been orbiting Earth since 1999 and 2002 respectively, all the while delivering key information to government and research agencies around the world.
For the purposes of this project, we are particularly interested in the ability of these satellites to provide a constant and nearly real-time analysis of wide areas of vegetation. Thanks to their onboard Moderate Resolution Imaging Spectroradiometers (MODIS), Terra and Aqua can tell us how healthy and/or abundant the vegetation is within a defined area.
Measuring Red to Find Green
MODIS measures light that is reflected from the surface of the earth. In simpler terms, it measures the “greenness” of the earth to determine the health pasture. Because photosynthetically active (green) vegetation absorbs most of the red light that hits it—and reflects near-infrared light that is invisible to the human eye—MODIS can measure infrared and near-infrared light reflected off the surface of the earth to calculate an average score of light reflectance. This score is then compared against the Normalized Difference Vegetation Index (NDVI) to determine the amount of active or dead photosynthetic (plant) life in the sampled area.
The National Oceanic and Atmospheric Administration (NOAA) makes NDVI information publicly available at a reasonable cost. Because of this, and the quality and reliability of the data, NDVI is a favourite measurement for geological and geographic researchers and government agencies around the world. One large government organization using NDVI data is the U.S. Geological Survey, which explains its use of NDVI in the following way:
NDVI values range from +1.0 to -1.0. Areas of barren rock, sand, or snow usually show very low NDVI values (for example, 0.1 or less). Sparse vegetation such as shrubs and grasslands or senescing crops may result in moderate NDVI values (approximately 0.2 to 0.5). High NDVI values (approximately 0.6 to 0.9) correspond to dense vegetation such as that found in temperate and tropical forests or crops at their peak growth stage.
By transforming raw satellite data into NDVI values, researchers can create images and other products that give a rough measure of vegetation type, amount, and condition on land surfaces around the world. NDVI is especially useful for continental- to global-scale vegetation monitoring because it can compensate for changing illumination conditions, surface slope, and viewing angle.
– From the U.S. Geological Survey.
MODIS is an incredibly advanced high-tech piece of equipment, but it has its limitations. Due to the distance of the satellites from earth, the spectroradiometer can only measure reflected light in 250 square meter sections, or “pixels”.
For example, if we were looking at a 250 x 250 meter field, and half of the vegetation was healthy and the other half dead (due to flood, let’s say), MODIS would give us an NDVI reading somewhere between healthy and dead for the whole field. Depending on the circumstance and size of total pasture (which would likely include multiple 250 square meter pixels) this average may be adequate. But it’s possible that an important part of the story could be lost when looking at such a large picture.
The NDVI readings provided by MODIS provide us with an excellent opportunity to quickly and remotely analyze large areas of pasture at a reasonable cost. It is now our job to determine how accurate these readings are and align our results with the perspectives of our producers.
In our next post [link], we’ll explain how we’re manually collecting samples on the ground and comparing them with satellite NDVI readings to determine the viability of satellite imaging for pasture analysis.
For a complete explanation of the science behind MODIS, visit http://modis.gsfc.nasa.gov/about/design.php.