Gram-Schmidt Pan Sharpening to sharpen multispectral data using high spatial resolution data.
The source images must be georeferenced to a standard map projection. If the images have different projections, ENVI reprojects the low-resolution image before performing the sharpening. For RPC-based images (for example, Pleiades and WorldView-2), please use the NNDiffuse or SPEAR pan sharpening tools.
You can also write a script to perform pan sharpening using the ENVIGramSchmidtPanSharpeningTask routine.
Pan-sharpening algorithms are used to sharpen multispectral data using high spatial resolution panchromatic data. An underlying assumption of these algorithms is that you can accurately estimate what the panchromatic data would look like using lower spatial resolution multispectral data.
The Gram-Schmidt and PC spectral sharpening tools both create pan-sharpened images, but using different techniques. Generally speaking, the Gram-Schmidt method is more accurate than the PC method and is recommended for most applications. Gram-Schmidt is typically more accurate because it uses the spectral response function of a given sensor to estimate what the panchromatic data look like.
If you display a Gram-Schmidt pan-sharpened image and a PC pan-sharpened image, the visual differences are very subtle. The differences are in the spectral information; compare a Z Profile of the original image with that of the pan-sharpened image to see the differences in spectral information, or calculate a covariance matrix for both images. The effect of pan sharpening is best revealed in images with homogenous surface features (flat deserts or water, for example).
The low spatial resolution spectral bands to use to simulate the panochromatic band must fall in the range of the high spatial resolution panchromatic band or they will not be included in the resampling process.
ENVI performs Gram-Schmidt spectral sharpening by: