One of the main advantages of the SPOT1 through SPOT4 over Landsat data is its slightly higher resolution. Landsat band files are approximately 30m resolution with a 15m panchromatic band. SPOT^® on the other hand has 20m band files and a 10m panchromatic band. The disadvantage of SPOT^® is that it does not have a visible blue band. As a result, if you want to create pan sharpened natural color RGB images from SPOT^® , you have to first synthesize the natural color bands from the three false color bands using a utility such as the one offered by PANCROMA. This can work reasonably well but even after a fair amount of effort the results will never be perfect. So one is left with either a true color pan sharpened RGB image at the lower 15m resolution or else a synthetic RGB image at the higher 10m resolution.

A third possibility is to use the SPOT^® panchromatic band to pan sharpen the Landsat band files. PANCROMA version 4.02 has the necessary utilities to do this, and the results can be quite satisfactory, as shown by the example below. (This is good news for Canadian users as they have access to both SPOT^® and Landsat data for all of Canada.)

The first step is to locate your Landsat and SPOT^® files. You will need three Landsat band files and a corresponding SPOT^® panchromatic band. This procedure works best if the following conditions are met:

• Use ortho images if available.

• Both images should share the same projection and if possible the same datum.

• Both images should be relatively cloud free.

• It is OK if there is some mountain snow but it is better if there is none. If snow coverage exists, it should be similar in both images.

Select your SPOT^® scene first. Select the Landsat scene so that it completely surrounds the SPOT scene. That is, the SPOT scene should be located completely within the bounds of the Landsat scene. This is not too hard to do because Landsat scenes cover an area approximately 185 km by 185 km while SPOT scenes cover an area of 60 km by 60 km.

Now select and download the three Landsat band files, i.e. band1, band2 and band3 (blue, green and red) according to the rules above. Subset the Landsat bands by opening the files as usual. In my case I subsetted the Landsat image to the SPOT^® set of corner coordinates. Select 'Band Combination'|'Subset Images'|'Subset Three Bands'. When the Subset Input Box appears select the 'Select by Coordinates' radio button. Now enter the upper left and lower right corner coordinates that you recorded previously. These are the corner coordinates of the SPOT^® scene. If they do not occur within the Landsat image, you have made an error. Select the 'Enter' button. The data entry box will disappear and the subset images will be displayed. Now save the three subset images in GeoTiff format by selecting 'File'|'Save'|'Save Subset Images'|'GeoTiff'.

The next step is to compare your subset Landsat and SPOT^® panchromatic images. They should cover exactly the same area. The SPOT^® rows and columns should be exactly three times those of the Landsat Subset images. Often the rows or columns will be off by one. This must be corrected by resizing whichever is larger down to the smaller dimension. If the panchromatic band is larger, resize it by selecting 'Pre Process' | 'Resize Images'. Select 'Resize One Image' if the panchromatic band is larger or else 'Resize Three Images' if the Landsat bands are larger.

Once you have checked your images and resized if necessary, the Landsat band files can be pan sharpened using the SPOT^® panchromatic band as usual. In my case I entered the three subsetted Landsat band files (blue green and red) and the resized (by one row) SPOT^® panchromatic band. The images to the right show the result of my efforts to pan sharpen Landsat bands p051r020_7dt20010814_z10_10.tif, p051r020_7dt20010814_z10_20.tif and p051r020_7dt20010814_z10_30.tif downloaded from the GLCF website with SPOT^® panchromatic band s5_12508_5721_20061008_p10_1_utm10.tif downloaded from the Canadian GeoBase archive of SPOT^® imagery. The first grayscale image is the SPOT^® panchromatic band. The second is the Landsat band3 grayscale image that has been subsetted so that its corner coordniates match that of the SPOT^® panchromatic image exactly. The third image is the RGB pan sharpened image.

If desired you can also use the five file XIONG algorithm. PANCROMA will subset a four file bundle of the three visible spectrum bands plus the NIR band. After subsetting, the pan sharpening proceeds as normal except of course that the SPOT panchromatic band is again used.