# Antarctic Surface Ice Velocity Retrieval from MODIS-Based Mosaic of Antarctica (MOA)
## 5 Discussion
In this research, the uniformity of time span in a mosaic from area to area complicated this problem. We treated it with a constant of five years for every pixel between two MOA images without special adjustment during processing. However, for most extreme cases, such as a pixel in MOA2004 observed at the very end of February 2004, and a corresponding pixel in MOA2009 observed at the very beginning of December 2008, the time span is nearly three months less than exactly five years. A relative error of up to 5% could be attributed to the above time difference, giving rise to both directional odds in slow moving areas and a large magnitude disparity in fast moving areas. In addition, while seasonal changes in glacier velocity do occur across the Antarctic, their effect is canceled out during the inherent averaging process as well. According to the \"On Thin Ice\" project website, the MOA2014 is in preparation now. After its release, another velocity field tracking from MOA2009 and MOA2014 would make monitoring Antarctic's spatial and temporal possible. To improve the performance of raw tracking displacement, some constraints based on either signal processing or physical mechanisms are indispensable. Filtering of mismatches from a single band image pair is still difficult and remains a challenging topic in post-processing after raw tracking. Usually, the SNR and correlation coefficient are used to detect spurious matches [71]. However, as the SNR and correlation coefficient can vary depending on the visual contrast and feature distribution, the user-specified thresholds might be arbitrary and even remove correct ones [58]. With respect to this deficiency, it seems that the SNR might not be a perfect indicator to label tracking quality on account of insensitivity. Several studies have designed complicated techniques, including manual extraction of glacial areas [72], prior knowledge of flow direction or maximal displacement [62], and statistics of multi-bands within its neighborhood [64], to erase mismatches and preserve as many good matches as possible, leaving thus only those who meet the criteria. Combining the interferometry and offset tracking, [PERSON] et al. achieved a seamless velocity mosaic of the Amery Ice Shelf [73], also offering valuable insight into the ice sheet study. Recently, some researchers have tried to merge displacement measurements from multiple sources by the fusion technique [74]. Here in our research, after excluding outliers by SNR thresholding, low-pass Butterworth filtering in the frequency domain was employed to smooth the remaining field, although physical assimilation based on Navier-Stokes equations is worth further investigation [52]. To leverage the triple-sigma rule of thumb, our approach requires an a priori velocity field with the same spatial coverage to derive standard deviation statistics. Similarly, an algorithm consisting of three processing steps was developed as the general filter framework for the ice surface velocity field deriving from remote sensing methods [75]. The MEaSUREs dataset acts as an excellent reference, although the a priori reference field might be absent in some cases. Apart from outliers, some valid measurements with high strain rates in the shear margin can also be removed inevitably during accuracy control strategy. Although we did not deal with direction directly, the \(X\) and \(Y\) components in the filtering process inherently include the direction constraint indirectly. In addition, it would be better if no significant rotation or torsion [42] existed in two images, since otherwise some other techniques, such as affine transform and the SIFT operator, might be introduced [76]. Such deficiency, such as the aberrant phenomenon in Figure 8, cannot be tackled by any statistical or physical methods independently unless another data source is provided. In addition, featured surfaces with crevasses are usually only present due to the stream confluence to ice shelves, dropping out of the vast upstream inland region.