Spectral Enhancements
Spectral enhancements change of the values of each pixels in the original image by transforming the values of each pixel in a multi-band image. Spectral enhancement is the process of creating new spectral data from available bands. Spectral enhancement techniques are typically used to extract data that is more more interpretable to the eye and more appropriate for analysis. It can also be used to minimize redundancy in multi-spectral data by compressing bands of data that are similar. The most common spectral enhancement techniques are:
- Spectral Ratios and Indices - Band ratios and mathematical transformation that are commonly used in vegetation and mineral studies
- Principal Components Analysis (PCA) - Compresses redundant data values into fewer bands which are often more interpretable than the source data
- Tasseled Cap – Transforms and rotates the data structure axes to optimize data viewing for vegetation studies.
Band Ratios
Band Ratios can be used to enhance the spectral differences between bands and to reduce the effects of topography. Dividing one spectral band by another produces an image that provides relative band intensities. The Band Ratio image enhances the spectral differences between bands and can be useful when trying to discriminate between land cover types.
There are a number of possible ratio combinations for a multispectral sensor, for example with Landsat 8 OLI’s nine reflective bands there are 36 unique ratio combinations possible. These ratio can provide unique information and highlight subtle differences in reflectance between surface materials that are often difficult to detect in a standard image.
Spectral Indices and Band Ratios
Spectral Indices are mathematical combination or transformations of bands that accentuates the spectral properties of features. The Indices accentuate the differences in features so that they appear distinct from other image features. Vegetation Indices are the most common type of index, but there are many other indices available. This includes Indices to indicate man-made (impervious) features, water, minerals and identify burned areas. Most indices require at least two different bands (or wavelengths of data).
Band Ratios
Some indices are relatively simple. For example, band ratios are simply the divide one band by another band to create a spectral index. Band ratios can be used to enhance the spectral differences between bands and to reduce the effects of topography. Dividing one spectral band by another produces an image that provides relative band intensities. The Band Ratio image enhances the spectral differences between bands and can be useful when trying to discriminate between land cover types.
There are a number of possible ratio combinations for a multispectral sensor, for example with Landsat 8 OLI’s nine reflective bands there are 36 unique ratio combinations possible. These ratio can provide unique information and highlight subtle differences in reflectance between surface materials that are often difficult to detect in a standard image.
Others indices are more complicated and are based on mathematical models. A variety of spectral indices can be easily applied in ENVI and ArcGIS. After calculating a spectral index a new data set will be generated. The image produced is always a single band image with the pixel values representing the calculated index. By default spectral indices are displayed in greyscale, although to improve interpretability a color scale can be applied.
