User Manual - pancroma

25. ENHG Algorithm – Manual Method________________________________________________________________The Enhanced Green (ENHG) Channel method is designed to correct a spectraldistortion problem that is characteristic of the pan sharpening process whenusing panchromatic images that are shifted from the blue wavelengths toward theIR spectrum (for example the Landsat panchromatic band.) This can cause ashift of green hues toward blue in the pan sharpened image, especially forvegetated areas. The ENHG method can correct this somewhat. It uses theNDVI index to identify probable vegetation areas, and then increases the value ofthe green channel pixels in these areas. The NDVI index uses a fourth lowresolution near infrared (NIR)) band to help identify vegetation. The NDVI indexis given by:NDVI = (NIR – RED) / (NIR + RED)Where NIR is the near infrared band (0.76 – 0.9 micrometers) and RED is thevisible red band. For Landsat NIR is band 4 and RED is band 3. The idea is tocompute an enhanced green band (ENHG) and then substitute the ENHG forband 2 (green band) in the pan sharpening process.ENHG can be computed manually as a pre-processing step saving the ENHG filefor subsequent processing. In this case the pan sharpening process isconducted as a separate step, substituting ENHG for the green band. The onlyreal advantage of the manual method is additional control as the ENHG file canbe image processed separately if desired. Automatic ENHG processing isdescribed in the next section. This example of the manual process will assumethat a Landsat image is being processed.The first step is to download five band files: bands 1, 2, 3 and panchromatic asusual for pan sharpening plus an extra band: band 5 (NIR). The easiest way toproceed is to subset the band files first. This allows you to experiment with theENHG scale factor in order to iteratively produce the optimal ENHG file withouttaking up a lot of computing time. PANCROMA TM provides a special five-filesubset utility for this purpose.After loading your five files in the order: band1; band2; band3; panchromatic andNIR band select ‘Band Combination’ | ‘Subset Images’ | ‘Subset Five Bands’.The subsetting procedure is exactly as described for subsetting four bands asdescribed previously except that an additional subsetted file will be producedcorresponding to the NIR band. The usual subsetting techniques can be usedand of course the subset files will be the same size and co-registered.Next, compute the ENHG file. Open three files: NIR, band 2 (green) and band 3(red), in this order. (For the first pass, use the default scale factor setting.) Do65