## 4 Results and Discussions
### Surface Velocity Validation  
After tuning of the function _TemplateMatch_ of ImGRAFT as reported above, we eventually derived the surface velocity for the whole glacier (see Figure 3, giving Vs during the austral fall/winter of 2016). The estimated values are in general agreement with topography. The Southernmost part of the glacier shows faster velocity (about 3-5 m day\({}^{-1}\)), likely due to narrowing of the flow section, leading to an acceleration of the glacier flow. Along the ablation tongue, the movement and direction are more regular because there are no obstacles or sudden changes in section area or in slope. Close to terminus flow is split in two parts. In front of the Peninsula Magallanes, the lake bottom is higher, and from there, the glacier follows two different directions (i.e., towards _Lago Argentino_ and _Brazo Sur_), where the speed is about 2 m day\({}^{-1}\) higher than in the central area. The accumulation area is poorly studied because of harsh conditions and perennial snow cover. Snow cover makes it difficult to reckon reference points when using feature tracking methodologies (such as those used by _TemplateMatch_). In addition, this area is often covered by clouds, and accordingly, pairs of images with clear-sky conditions at a short distance over time are rare to find. For these reasons, we validated our results only along the ablation tongue. To better assess the velocity fields so obtained, we defined some cross-sections of the glaciers, where velocity profiles were studied, and benchmarked against studies in the literature. We used 10 sections (transects), of which 5 (TA Transects, Figure 3b within the broad ablation area and 5 along the PMG front (last 4 km, TF transects, Figure 3c. In Figure 4, we report for TF transects the estimated surface flow velocity (average of all images) and a comparison between the two satellites LANDSAT and SENTINEL2 and their image resolution, from 30 to 10 m of pixel size. We performed a comparison using all images since there is not a common period between LANDSAT4-5 and SENTINEL2A. Therein, it is shown that images with 30 m resolution (LANDSAT4-5; see Table 1) and at 20 m resolution (SENTINEL2) provide systematic underestimation of flow velocity. Accordingly, we decided to discard the images from LANDSAT4-5 here. Figure 3: _Cont._  
Figure 3: Estimated flow velocity for the Perito Moreno Glacier. (**a**) Whole mapped area, ID6 (LANDSAT8, 8–24 May 2016). (**b**) Ablation area, ID3 (SENTINEL2, 19 August–28 September 2016), and transects TA1-5. (**c**) Glacier front (last 4 km), ID3, and transect TF1-5. Black dots are arrows indicating local flow direction. In Figure 5, we report a more accurate comparison between SENTINEL2 and LANDSAT8, by providing mean flow velocity (here, in transect TA3 in the ablation zones, with similar results in all transects, also in front zone; see [33]) during three broadly overlapping periods in 2016 (winter 2016, ID3, ID19; end of summer 2016, ID15, ID24; and start to mid summer 2016, ID16, ID5; see Table 1). Therein, the two summer periods differ slightly in the shear margins, possibly given the temporal distance of about 1 month. Validation was pursued against estimates of flow velocity of the PMG as reported from [27]. Namely, we compared flow velocity at transect B therein, in Figure 1, by creating a new section TB62, slightly downstream of TA2 here, and subsequently assessing flow velocity thereby. Figure 6 shows the results so obtained. We report therein the different flow velocities estimated at transect B in [27], namely, from (i) tracking of ablation stakes (1995-2003), (ii) assessment of velocity field from SIR-C/SAR-X (October 7, 9, and 10, 1994), (iii) SENTINEL2 (RGB, 10 m pixel size), and (iv) LANDSAT7-8 (panchromatic, 15 m pixel size), in this study. Despite the different periods of assessment, the velocity patterns along the glacier seem to be quite similar. Figure 4: Transects in the front zone TF. Estimated flow velocity (average of all images) along cross-wise ablation transects.