-Cheatgrass Study: Idaho Falls, ID
The Problem: Cheatgrass (Bromus Tectorum) is an invasive grass that rapidly out-competes native grasses and vegetation. Management is difficult because it is phenotypically similar to other native grasses. Also, the awns of the seed heads are very sharp and can injure grazing wildlife and livestock. Little research on cheatgrass has been done in the past.
-Our study site. All of the cheatgrass is brown, while almost all of the other vegetation is green. This will make spectral analysis possible.
Outline: The purpose of the cheatgrass project is to try and quantify the percent cover of cheatgrass in a given area. Previous spectral analysis has been difficult because all of the imagery (via air photos and satellites) is taken from a nadir perspective (straight down). Therefore, the brown, bare ground blends in with the brown cheatgrass— making it hard to distinguish between the two. This project was completed after the cheatgrass had senesced, because most of the other vegetation in the area was still green, making spectral analysis possible.
NASA's MODIS satellite sensor collects 16 images in varying oblique angles for a given area to create a composite image as close to nadir as possible. These off-nadir images are used to view the cheatgrass from an angle that shows the cheatgrass but not the bare ground. The sensor also collects data in 36 spectral bands, making it possible to identify a unique spectral signature for cheatgrass. The distinctive phenology of the cheatgrass should distinguish its presence from other vegetation. Groundtruthing photos were needed to have some baseline data to compare the satellite images to. This was accomplished by taking oblique angle images (similar to the angle of the MODIS images) from a 2.4 meter vertical boom. The photos are analyzed with a program to break down the RGB spectrum and determine the percent cover of cheatgrass in each photo.
Procedure: We established the study site as a 350x350 meter area in some BLM land west of Idaho Falls, ID. It was chosen because it was relatively flat and had a high density of cheatgrass. A systematic grid was devised to cover the greatest area without overlap and therefore provide a good representative sample of the area. At each “pivot point” on the grid, four photos were taken in each of the four cardinal directions from a 2.4 meter vertical boom. The camera was set at an angle similar to that of the MODIS satellite sensor. We placed red flags at 25 and 50 meters from the camera in each photo to provide reference points during analysis. At each photo pivot point the area photographed was walked and the percent cover of cheatgrass was determined and recorded.