Why do I need Earth observation data processing?
While our eyes only detect a fraction of all light available, satellite sensors can actually capture – and send back – much more information. Furthermore, this information is relayed back to us in a format quite different from the photographs we are used to. For each band, satellites capture the spectral reflectance of the area within a specific narrow band of the light spectrum.
True-colour composite images use the red, green, and blue bands gathered by satellites to mimic the range of vision for the human eye, showing us images closer to what we would expect to see in a normal photograph.
Satellites also capture information in the non-visible part of the light spectrum. Different features: rock, bare soil, vegetation, burned ground, snow, sediment-rich water, etc. all have different reflectance properties in each band. This a called a 'spectral signature'.
To highlight specific features, one or more of the RGB bands can be substituted for another, such as infrared, or near infrared, which are not visible to the human eye. These images are referred to as false-colour images.
Additionally, to better discriminate between features and highlight changes in time, mathematical models can be applied to the data to produce a new kind of processed image. These are referred to as indexes.
When should I use GEMI?
GEMI stands for Global Environmental Monitoring Index. The GEMI algorithm was developed by Pinty and Verstraete (1991). They attempt to eliminate the need for a detailed atmospheric correction by constructing a stock atmospheric correction for the vegetation index. Pinty and Verstraete (1991) provide no detailed reasoning for this index, other than that it meets their requirements of insensitivity to the atmosphere empirically. A paper by Leprieur et al. (1994) claims to find that GEMI is superior to other indices for satellite measurements. Qi et al. (1994) shows a violent breakdown of GEMI with respect to soil noise at low vegetation covers.
How to obtain GEMI data using EarthCache
Note: If you do not have an account, you can sign up for one here.
To obtain a true-colour image using EarthCache, simply select GEMI processing as an output when creating or editing a pipeline through the dashboard.
To obtain a true-colour image using code, you can create or edit a pipeline using the following parameters: