# Antarctic Surface Ice Velocity Retrieval from MODIS-Based Mosaic of Antarctica (MOA)
## 5 Discussion  
Retrieving the ice velocity via the tracking algorithm is usually conducted over the high-resolution remote sensing image. Since little research involves coarse imagery such as MODIS, we adopted several tailored procedures to achieve a better final result. In this section, the data quality, processing performance, and potential promotions are discussed. It is worth pointing out a couple of co-registration bias is simply ignored in the processing steps and analysis section. This can exert a little bias to pixel alignment, such as ocean tidal motion can entail up to a few meters height anomaly to the floating ice shelf [66]. However, it has to be taken into account in the interferometry method [67]. Negligible effects like this also include atmospheric artifacts, errors in sensor orientation and the ortho-rectification model, and DEM discrepancy through time and image distortion from the altitude effect [68], especially in steep margins of ice sheets. Given that the  
Figure 8: The velocity field in the middle Amery Ice Shelf, with the transparent speed of the MEaSUREs dataset overlapping the enhanced MOA2009 image as background. Black and blue arrows represent the primitive tracking result and the final refined result of our accuracy constraint strategy, respectively. Enhanced original 2004 and 2009 MOA images of the same region are shown on top of the figure. The location is shown in the red box of the inset. magnitude of pixel offsets generated by the above sources are usually two orders less than the spatial resolution of original MODIS image, their effects are not taken into account either. Nevertheless, image coregistration can never be absolutely accurate, and the displacements generated from the misalignments among co-registered images need to be removed from the total composite displacement field. A common strategy to limit such error is to test the displacement field where no movement should be observed, i.e., in rock outcrops. When motions are observed in these areas, the errors can be spatially uniform, or present heterogeneous variations. In the former case, the residual can be used for the removal of co-registration errors. However, for our case, this operation is infeasible since the residual is non-uniform within each drainage due to the large scale across thousands of kilometers, just like between various drainages. Dividing them into a smaller drainage system might help, but that would require either time-consuming work in parameter tuning or locally adaptive matching operators. At the same time, if doing so, the difference distribution in Figure 6 should reveal only one peak in the histogram in a homogeneous flowing regime with a single dominant direction. In terms of different final performance in each drainage division, the detail varies considerably from one drainage system to another, even within the same basin. The worst case refers to the Amundsen drainage, where warm ocean water beneath the continent's floating margins is eating away at the ice attached to the seabed [69]. For example, the Pine Island Glacier, the grounding line of it has been continuously retreating thanks to heavy forcing by the warm ocean from below. It experienced as large as \(\pm\)150 m\(\cdot\)a\({}^{-1}\) variation in velocity at a different location along one specific profile from 2009 to 2015 [70]. Such drastic variation would no doubt sharply alter its surface feature. As shown in Figure 8, surface melting can be blamed for another reason as well. At least for large ice shelves such as Filchner and Ross, the tracking velocity is believed to reveal the real motion, and the result is comparable to [PERSON]'s experiment in the Ross Ice Shelf [49]. In the Larsen-C ice shelf, our result seems even to achieve a better spatial coverage than the original MODIS images [38]. The longer the time delay between images, the lower the error related to the measured displacement per year, but the larger the error in finding an exact match, thanks to the surface pattern changes through time.