# Mapping Surface Flow Velocity of Glaciers at Regional Scale Using a Multiple Sensors Approach
## 3 Results
### Uncertainties Analysis
#### 3.3.4 Comparison between Sentinel-2, Landsat, Planet Labs and Pleiades in the French Alps  
On the Mont Blanc massif, we can compare the Landsat 8 and Sentinel-2 data with the 50 cm resolution Pleiades sensor obtained as part of the CNES/Kalideos-Alpes project and the measurements made from Planet images.  
We use a surface flow velocity map made from a pair of Pleiades images that were acquired 30 days apart during the summer of 2018 and compare it to time-averaged maps derived from Sentinel-2 and Landsat 8 to assess the level of detail obtained by the latter (Figure 7). We find that the overall spatial distribution of glacier surface speed is preserved on the Landsat 8 and Sentinel-2 maps with respect to Pleiades. Obviously, Pleiades offers a sharpness in the displacement map that cannot be obtained with sensors that acquire images at a spatial resolution 20 to 30 times lower. This is particularly true along glacial margins and for narrow glaciers such as the upper reaches of the Mer de Glace, and for tributaries of the Miage Glacier (inset Figure 7B). There are some differences in the upper part of the Argentiere glacier where few elements can be followed and/or frequent shading is present, and where Pleiades offers better performance (Figure 7). We estimate that the precision on displacement over ice-free areas is 4.3 m/year for Pleiades, while the Sentinel-2 and Landsat 8 maps reach this level of precision for cycles longer than 60 and 160 days, respectively (based on Figure 4C). Note that, to obtain these details, a considerable number of image pairs must be used for Sentinel and Landsat, whereas with Pleiades we used only one pair. This highlights the advantages and need for very high resolution sensors such as Pleiades to map with high accuracy the smallest mountain glaciers that are not currently captured by conventional sensors.  
We can also use the data from Planet Labs which provides orthorectified intermediate resolution images of 3 m and compare the ice speed maps produced by this sensor with those of Sentinel-2 (10 m) and Pleiades (50 cm). We obtained 15 pairs of images between April 2018 and September 2018 with different observation geometries (for example, not in the same orbit) from Planet Labs. Due to the limited number of images that are freely accessible, we are not able to perform the same statistical analysis on sensor precision as for Venus, Sentinel-2 and Landsat by calculating the standard deviation for each pixel between the different measurements. However, we have estimated the standard deviation of displacement on ice-free areas for each pair of images (Figure S3) and find a value of 50 m/year for a 7-day repetition cycle, decreasing exponentially to 15-20 m/year for 40 to 50-day cycles (Figure S6). These values are higher than those expected for 3 m spatial resolution imagery and are attributed to ortho-rectification issues that are exacerbated by the fact that we were  
Figure 7: Comparison of glacier surface velocity fields obtained with (**A**) Pleiades; (**B**) Sentinel-2; and (**C**) Landsat 8. The Pleiades mosaic was obtained from a single 30-day pair of late summer 2018 whereas Sentinel-2 and Landsat 8 are 2017–2018 average mosaics of all 30-day and 32-day image pairs, respectively. Inset shows a zoom in the glacier surface velocity of two tributaries of the Miage Glacier called Dome and Mont-Blanc glaciers.  
unable to use exactly the same orbital paths between image pairs. We believe that additional geometric corrections are therefore necessary to fully utilize the potential of Planet Labs images.