The SpectroSense 2 GPS meter by Skye Instruments is one of two critical tools being used by our researchers to determine whether we can accurately measure pasture growth electronically (the other tool being the MODIS satellite). Here’s a closer look at the device, and an explanation of its features and the way we use it.
The SpectroSense and MODIS spectrometers both achieve a measurement of the greenness of the ground. They do this by way of an NDVI score that is calculated by measuring light reflecting off of the surface of the Earth. But while the MODIS satellite spectrometer can measure the Earth in 250 square-metre pixels, the SpectroSense allows us to measure NDVI scores 0.5 metres at a time. By measuring our half-metre rings across our 250 square-metre sample sites, we can determine whether readings from MODIS are sufficiently accurate to rely upon when assessing individual pastures.
So how does the handheld SpectroSense spectrometer work? As shown in the photo gallery above, there are two sensors on the mount—one that measures the light directly above the target on the ground (the sky), and another that measures the light being reflected off of the ground below. The difference between the two readings represents light being absorbed by vegetation on the ground. The NDVI equation compares light absorbed by the surface and reflected back from the surface.
While on, the spectrometer is constantly measuring light. To record a specific one-metre sample ring, the researcher positions the spectrometer over the area on a level plane and presses “store”. In order to ensure we have accurate readings for each sample ring, the research team records NDVI readings in a specific pattern and double checks the results soon after recording them.
Grass is taken from each sample ring and carefully labeled in accordance with a predetermined sample pattern so that each bag of grass can be matched with the spectrometer readings when taken back to the sorting facility. We can then compare the amount of green grass in a single sample and compare to the NDVI reading.
The handheld spectrometer provides a reading over the area marked by our sample rings. After sorting the grass we will hopefully see a strong the correlation between the NDVI readings taken in the field and the green of the grass. If correlation is evident between the on-the-ground spectrometer NDVI readings and the green grass samples, that same correlation can be applied to the MODIS spectrometer.
Ultimately our research on the ground is working towards a decision on the viability of using the MODIS spectrometer platform to measure pasture growth. We hope to find MODIS a cost effective tool to measure pasture growth (through NDVI readings that could be converted into something like pounds per acre of grass) and one that could be used to build an effective pasture insurance program for ranchers—and maybe used in other applications as well.