ENVI’s vegetation analysis tools classify a scene for agricultural stress, fire fuel distribution, and overall forest health. Thus, you can perform vegetation analysis using tools that guide VI selection for a specific outcome. You can also use the Vegetation Analysis Workflow to guide you through this process.

The scientific basis for the vegetation analysis tools was developed by Dr. Gregory P. Asner of the Carnegie Institution of Washington, Department of Global Ecology.

You should first use the Vegetation Index Calculator to calculate the VIs in an image before processing it with a vegetation analysis tool. This allows you to reuse the data in multiple ways without having to recalculate the VI dataset for each new analysis.

You can also skip creating the VI dataset and, instead, let a vegetation analysis tool calculate the VIs. However, this means the VIs are calculated each time a tool is used, so the VIs cannot be saved or reused for multiple passes through a tool.

Selecting the appropriate VIs from each input category is a critical step in determining the output from the tools. If poor selections are made for the input VIs, the output may contain unnecessary masked areas, the classes may have uneven distributions of output results, or the classification results may be inaccurate. One approach to selecting the best input VIs is to display and examine all of the VIs in a VI category before running the tool. Specifically check for the following attributes when displaying a VI before selecting the VI as input to the analysis tools:

  • Does the VI display in ENVI with good contrast? Good contrast increases the likelihood the analysis tool works effectively on that VI.
  • Is the display from the VI similar to other VIs from the same category? This indicates the results obtained from the VI are similar to the results obtained from other VIs. Occasionally, the result of one VI may provide drastically different results from similar VIs, which may indicate the data in the particular VI is suspect.
  • Does the VI have large masked areas? Masked areas in an input VI can pass through to the classification result, resulting in masked areas for the classification result where other input VIs may have data.

Tip: Because urban materials such as buildings and roads have different spectral signatures than vegetation, they can cause errant results for scenes with mixed pixels. It is best to mask out or ignore urban areas or bodies of water when performing a vegetation condition analysis.

Each vegetation analysis tool uses a different set of three VI categories that are combined to produce a map with classifications showing some vegetative property or state. At least one VI from each of the categories must exist for a tool to be available. Though the VI categories are pre-defined by each tool, you can select any of the available VIs within a category. This provides flexibility within the tool, as you can run it with different combinations of VIs to provide different classification results.

See Also

Spectral Indices, Vegetation Indices, Agricultural Stress Tool, Fire Fuel Tool, Forest Health Tool, Vegetation and Its Reflectance Properties, EO-1 Hyperion Vegetation Indices Tutorial