# Antarctic Surface Ice Velocity Retrieval from MODIS-Based Mosaic of Antarctica (MOA)
## 4 Result and Analysis
### Comparisons with MEaSUREEs  
Most MEaSUREEs source data were collected between 2006 and 2009 [19], coincidentally falling within the time span of two MOA datasets, which are from the end of 2003 to the beginning of 2009. Assuming that velocity evolves gradually rather than drastically with respect to time (except for surge-type glaciers, which is not typical in Antarctic), a discrepancy resulting from the time difference should be limited except for a few high variable glaciers. To better evaluate the accuracy in the seven drainages, we analyzed the difference between MOA-based velocity and the MEaSUREEs dataset, which is resampled to 750 m by the bi-linear interpolation method, pixel to pixel in the remaining valid area. Figure 5: The SNR distribution histogram, the residual speed with the SNR in the rock outcrop, and the corresponding SNR threshold labeled as a vertical dash-line in seven drainages: (**a**) Amundsen, (**b**) Dronning Maud, (**c**) Filchner-Ronne, (**d**) Lambert, (**e**) Peninsula, (**f**) Ross, (**g**) Wilkes Land. Note that the scale of the left Y axis in each panel differs. Figure 6 represents the histograms of the velocity difference in the \(X\) and \(Y\) components in seven drainages, respectively. Considering the granularity of source images imported into the tracking program, Figure 6c,e shows that the final velocity retrieval from MOA is excellent in Ross drainage, where the mean and median are less than 5 m\(\cdot\)a\({}^{-1}\), and the Filchner-Ronne drainage, where the mean and median range from 4 to 13 m\(\cdot\)a\({}^{-1}\). Variance is less than 30 m\(\cdot\)a\({}^{-1}\) in these two drainages as well, roughly half of the RMSE in comparison between the tie-point pairs and our result. This value of 30 m\(\cdot\)a\({}^{-1}\) is the same as the velocity retrieval difference between MODIS and Landsat, whose outstanding spectral and georeferencing characteristics have already been recognized ([PERSON], 2018). In drainages where moderate scale ice shelves dominate, such as Dronning Maud, Lambert, and Peninsula, the result is yet acceptable since the mean and median of the difference is still relatively low, and the variance of the difference is the same order of that RMSE. Noting that, along the coastline of Dronning Maud drainage, Fimbul and other small ice shelves flow primarily in the positive Y direction but the Brunt Ice Shelf flows toward the negative X direction, the histogram in the Y direction shows a bimodal pattern. This kind of behavior appears most prominent in the Wilkes Land Drainage since it stretches most widely in longitude. In the worst cases--the Amundsen and Wilkes Land drainages--the discrepancy is considerable, resulting from both surface decorrelation and different acquisition times. Featuring intensive acceleration and a retreating grounding line, the Pine Island Glacier (Sanchez-Ronne, 2018) and its neighbor, the Thwaites Glacier (Sanchez-Ronne, 2018) are both located in the Amundsen drainage. There are hardly any valid data available in the main trunk of these two dominant glaciers. The corresponding deformation can not only ruin the matching features but also affect the accuracy of nearby areas in filtering processing. Even worse, if taking the observation time of MEASUREs into account, a great deal of source data from ERS-1 and ERS-2, recorded as early as 1996 in this region, which is more than one decade before MOA's compilation, is integrated. Therefore, we can infer that only a fraction of inconsistent data is caused by tracking outliers. In term of the Wilkes Land drainage, where the vast majority of glaciers span across only dozens of kilometers as a result of steep topography along the coastline, only a few ice tongue or ice shelf motion signals are represented correctly. Six of them are indicated by a digital number from one to six in Figure 2, among which the Mertz Ice Tongue was already broken into two pieces by a floating iceberg B-09B in 2010 ([PERSON], 2018).