User Manual - pancroma

49. Gap Filling Cloud Masks for Landsat ETM+ Images________________________________________________________________Although it us often possible to find a cloud free Landsat scene for a given area,a completely cloud-free image may not be available for your Landsat location ofinterest and acquisition time. In this case masking the clouds and replacing themasked areas with pixels from a corresponding cloud-free image may be ofinterest.PANCROMA TM has two utilities for detecting clouds and for masking theobscured areas. They are included under the ‘Gap Fill’ menu selection as thefilling of the masked areas is done using the Transfer gap fill method described inthe previous section and in the following Tutorial.The first method for creating cloud masks is called the ‘Three Band Method’.This technique, as its name implies, requires that three band files be input, anduses information in two of those Landsat bands, the blue band 1 and the thermalband 6 to create a mask directly onto the input band. Clouds are both highlyreflective in the short wavelength band 1 and also cold, meaning they will appeardarker in the thermal band.The PANCROMA TM three-band algorithm is loosely based on the method ofMartinuzzi, Gould and Gonzales. It compares the band 1 (blue band) and band 6(thermal infrared or TIR band) digital numbers (DNs) against user-definedthresholds and classifies each pixel as either cloud or non-cloud. The cloudpixels are set equal to zero and the non-cloud pixels are left alone. The result isa band file with the cloud areas blacked out.To use the utility, you must input the band file to be masked, then the band 1 file(blue band), and finally the band 6 (TIR) file in that order, exactly.IMPORTANT NOTE: There are typically two band 6 files in an ETM+ data set. Ifso they will be designated 61 and 62. They differ in the gain setting of thesensor, 61 being the high gain image and 62 the low gain. Higher gain yieldsgreater dynamic range. Lower gain means greater sensitivity.(resolution). In thiscase the resolution of the thermal infrared (TIR) bands 61 and 62 are nominally60m per pixel, half that of the other multispectral band. In order to perform themask, the TIR band must be expanded and interpolated to match the size ofband 1. PANCROMA will expand and interpolate the lower resolution TIR bandto match the multispectral bands automatically.IMPORTANT NOTE: If PANCROMA TM automatic utility fails to handle theautomatic resizing due to an off-by-one condition that it did not anticipate, youmay have to manually resize the images so that the band 1 rows and columnsizes are exactly twice as large as the TIR row and columns. This can be done129