Fractional cover refers to estimating the proportion of an area that is covered by each member of a pre-defined set vegetation or land cover types. For mapping fractional cover, the proportions of the different classes should sum to one. In terms of remote sensing, the area being considered is generally a pixel (although fractional cover estimation can be applied to object-based image analysis), and the estimation of fractional cover is considered a type of "spectral unmixing" or sub-pixel classification.
There are many different techniques for estimating vegetation fractional cover from remotely-sensed data. Fernandes et al. (2004) and Scanlon et al. (2002) described the following subpixel mapping techniques applied to estimating fractional cover:
Fractional cover estimation requires that the classes to be mapped be defined ahead of time. Accordingly, the output of fractional cover estimation is an estimate of the proportion (i.e., percentage of the total) of each class for each pixel (or object).
Fractional cover mapping has been applied in a number of rangeland systems, but mostly as test cases for research, and not as management applications. In many cases, these were experimental techniques that may or may not be suitable for deriving data layers for rangeland management.
One limitation of fractional cover estimates is that all of the classes for which cover proportions are to be estimated must be defined prior to the analysis. This generally is only a problem in cases where you encounter a different cover type that you weren't expecting. Also, the number of classes that you define and predict fractional cover of can affect the accuracy of the result - especially if the spectral reflectance of those cover types is very similar or highly variable.
The data inputs for estimating fractional cover will vary depending on the method used.
Like the data inputs, the software and hardware requirements for estimating fractional cover will also vary depending on the methods used. Any attempt to produce fractional cover layers will require remote-sensing/image processing software (e.g., ERDAS Imaging, ENVI). Calculating fractional cover is normally a complex exercise, and all but the most simple implementations of it (e.g., Marsett et al.'s (2006) conversion of SATVI to fractional vegetation cover) will also require at least statistical software and maybe additional specialized software.
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