User Manual - pancroma

72. Tutorial – Gap Filling using TERAS Method________________________________________________________________The third PANCROMA TM method for filling gaps in Landsat ETM+ images is theTERAS method. TERAS is a modified sliding window, local optimization method.This means that a square frame is established so that the edges of the frame aretwice the user-specified searchRadius in length and width. The frame ispositioned over the first target Digital Number (DN). Computations areconducted to map the gap DNs to the reference image DNs, using only DNswithin the frame. After filling the gaps, the frame is displaced and the process isrepeated.Because the mapping is very localized (for example within a frame of size 20pixels) with respect to the size of the image, the match is only influenced by theDNs in the immediate vicinity of the center target DN. This can result in a highfidelity match between the filled DNs and the DNs adjacent to the gap.Using the TERAS algorithm is similar to the Hayes described above. You startthe filling procedure by opening first the Reference (grayscale) image (the onewith the gaps), followed by the Adjust image. When the images are loaded,select ‘Gap Fill’ | ‘Gap Fill TERAS Method’. A Gap Fill Data Form will appear.You can select the algorithm parameters: Search Extents, the Gap Thresholdand the Cloud Saturation Cutoff. The default values are 20, 1 and 100,respectively. The Search Extents defines the size of the sliding window withinwhich PANCROMA TM will look for similar pixels. PANCROMA TM will search thetarget +/- the Search Extents, so the size of the window is twice the SearchExtents value.The Gap Threshold determines which pixels in the Reference image will becomputed. Any pixel with a brightness value less than the Gap Threshold will beconsidered a missing pixel and PANCROMA TM will attempt to fill it.The Cloud Saturation Cutoff value is designed to minimize cloud pixel“blooming”. This problem is caused when the Adjust (no gap) image has cloudsthat the Reference (gap) image does not. These highly saturated pixels candistort the matching computation and will propagate across the entire slidingwindow frame. In order to avoid this, the algorithm will ignore any pixel with a DNabove the Cloud Saturation Cutoff value in its computations. Note that this willmitigate the phenomenon but may not completely eliminate it.337