# The Influence of Glacier Mass Balance on River Runoff in the Typical Alpine Basin
## 2 Data and Methods
### Methods
#### 2.3.1 Production of DEM through InSAR Method  
The DEM generated through Sentinel-1A includes the selection of image pairs, DEM generation, and bias correction. The selection of SAR image pairs significantly affects the accuracy of InSAR DEM generation. The optimal interferometric pairs should have a large perpendicular baseline and a small temporal baseline. Interferograms with very small perpendicular baselines (<30 m) are almost useless due to their high sensitivity to phase noise and atmospheric effects. Furthermore, the height ambiguity is inversely proportional \begin{table}
\begin{tabular}{c c c} \hline \hline
**Data** & **Date** & **Data Sources** \\ \hline Sentinel-1A & February 2022 & [[https://search.asf.alsaka.edu/](https://search.asf.alsaka.edu/)]([https://search.asf.alsaka.edu/](https://search.asf.alsaka.edu/)) (accessed on 1 August 2022) \\ C-band NASA DEM & February 2000 & [[https://search.earthdata.nasa.gov/](https://search.earthdata.nasa.gov/)]([https://search.earthdata.nasa.gov/](https://search.earthdata.nasa.gov/)) (accessed on 1 August 2022) \\ X-band SRTM DEM & February 2000 & [[https://goeservice.dlr.de/egp/](https://goeservice.dlr.de/egp/)]([https://goeservice.dlr.de/egp/](https://goeservice.dlr.de/egp/)) (accessed on 1 August 2022) \\ ALOS DEM & February 2009 & [[https://search.asf.alsaka.edu/](https://search.asf.alsaka.edu/)]([https://search.asf.alsaka.edu/](https://search.asf.alsaka.edu/)) (accessed on 1 August 2022) \\ ERA5 & 2000 to 2022 & [[https://cds.climate.copercinus.eu/](https://cds.climate.copercinus.eu/)]([https://cds.climate.copercinus.eu/](https://cds.climate.copercinus.eu/)) (accessed on 15 August 2022) \\ GLDAS & 2000 to 2022 & [[https://ldas.gsfc.nasa.gov/data/](https://ldas.gsfc.nasa.gov/data/)]([https://ldas.gsfc.nasa.gov/data/](https://ldas.gsfc.nasa.gov/data/)) (accessed on 16 August 2022) \\ RGI 6.0 & July 2017 & [[http://www.glims.org/RGI/](http://www.glims.org/RGI/)]([http://www.glims.org/RGI/](http://www.glims.org/RGI/)) (accessed on 1 August 2022) \\ ICESat-2 & September 2021 to February 2022 & [[https://nsidc.org/home](https://nsidc.org/home)]([https://nsidc.org/home](https://nsidc.org/home)) (accessed on 10 August 2022) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The sources of data used in this study. Figure 2: Technology route. to the spatial vertical baseline. In other words, the larger the vertical baseline, the smaller the height ambiguity, resulting in more detailed representation of surface elevation variations and dense fringes in the interferograms. Conversely, shorter spatial vertical baselines lead to larger height ambiguities and coarser representation of elevation variations. Therefore, for mapping using InSAR technology, image pairs with long spatial vertical baselines are preferred within a certain range. However, the baseline of interferometric pairs from Sentinel-1A is not very high, concentrated within the range of \(\pm\)150 m, which falls short of the optimal vertical baseline achieved by ERS satellites (150-300 m) [23]. To ensure temporal consistency and proximity in data acquisition to NASA DEM and ALOS DEM, we used the ASF Baseline Tool to examine the vertical baselines of interferometric pairs during January to March 2022. Based on these criteria, we identified 7 suitable datasets, with most of the interferometric pairs having small vertical baselines (<40 m). Consequently, we selected SAR images acquired on 12 February and 24 February 2022, to generate the DEM. The vertical baseline for this interferometric pair is 60 m, well below the critical baseline of 6402 m, and associated height ambiguity is 265.690 m. The information of the SAR data is presented in the Table 2 below. First, the co-registration of SAR SLC (Single-Look Complex) data aims to ensure that corresponding points in the two images used for interferometric phase calculation correspond to the same point on the ground. The principle behind this process is similar to image matching in photogrammetry.