# Inland Water Level Monitoring from Satellite Observations:
## 2 Methodological Framework
### Charting the Data: Transformation, Analysis, and Interpretation  
The next stage of the scoping review involved \"charting\" key items of information. \"Charting\" refers to a technique for synthesizing and interpreting qualitative data by sorting and charting data according to key themes and issues [38; 47]. The charted information should include a mixture of general information about the study and specific data related to the study question [37]. Thus, each team member worked independently to chart the included articles, and then all reviewers compared their results to agree upon the categories to chart for the study.  
Initially, included articles were charted using specific data relating to the literature databases, the year of publication, and the aim of the study. Apart from the publications in which the performances of different satellite missions are evaluated over inland water bodies using existing and known retracking algorithms, some articles aim for deployment and the evaluation of novel and improved retrackers that can derive stable and accurate inland water levels. By developing new retracking algorithms, researchers aim to overcome the limitations of existing retrackers to accurately measure inland water levels [48]. Consequently, the publications were categorized into two groups based on the utilization of only existing retracking algorithms or also of novel and improved ones. Moreover, the publications associated with specific satellite missions (e.g., Sentinel-3, Jason-2/3) were analyzed in order to examine the literature based on the use of different satellite missions. Although satellite altimetry missions are mainly used to monitor the inland water levels, there are also different techniques, such as Global Navigation Satellite System-Reflectometry (GNSS-R), and gravimetry which could also provide accurate water level measurements. Thus, the yearly number of publications, between 2018 and 2022, associated with specific missions were analyzed since each satellite mission has its own characteristics and applications. The trend of specific missions may mirror the trend of the current research field, inland water level monitoring [36]. In the last years, the publications on radar altimetry missions (e.g., Sentinel-3, CryoSat) and laser altimetry missions (e.g., ICESat-1/2) have exhibited a high growth trend. Therefore, the publications associated with radar and laser altimetry missions were charted in order to illustrate any possible trend in the monitoring of inland water levels.  
Also, the return waveform, the fundamental measurement of radar altimetry, is contaminated over inland water bodies due to the large size of the radar footprint. Thus, the extracted ranges are corrupted which leads to inaccurate water level measurements. As can be understood, to overcome this problem, proper algorithms need to be used to retrack the waveforms. Therefore, publications were charted based on the retracking algorithms (i.e., retrackers): either they are novel and improved or existing and known ones, applied to the waveforms deemed to be from inland water bodies [48]. There are different types of retracking algorithms, such as Ocean, Ice, and Sea-ice, according to the type of waveforms re-tracked. The different retrackers can be more suited to a specific surface. Therefore, due to the fact that each retracking algorithm has its own characteristics, the trend of one retracker may mirror the trend of inland water level monitoring.  
Moreover, in the current study data were organized with respect to the continent and the specific study region, river, lake, or reservoir, where the different satellite missions and retracking algorithms are evaluated so as to examine the different study regions around the world [36].