Co-Orbital Sentinel 1 and 2 for LULC Mapping with Emphasis on Wetlands in a Mediterranean Setting Based on Machine Learning

This study aimed at evaluating the synergistic use of Sentinel-1 and Sentinel-2 data combined with the Support Vector Machines (SVMs) machine learning classifier for mapping land use and land cover (LULC) with emphasis on wetlands. In this context, the added value of spectral information derived from the Principal Component Analysis (PCA), Minimum Noise Fraction (MNF) and Grey Level Co-occurrence Matrix (GLCM) to the classification accuracy was also evaluated. As a case study, the National Park of Koronia and Volvi Lakes (NPKV) located in Greece was selected. LULC accuracy assessment was based on the computation of the classification error statistics and kappa coefficient. Findings of our study exemplified the appropriateness of the spatial and spectral resolution of Sentinel data in obtaining a rapid and cost-effective LULC cartography, and for wetlands in particular. The most accurate classification results were obtained when the additional spectral information was included to assist the classification implementation, increasing overall accuracy from 90.83% to 93.85% and kappa from 0.894 to 0.928. A post-classification correction (PCC) using knowledge-based logic rules further improved the overall accuracy to 94.82% and kappa to 0.936. This study provides further supporting evidence on the suitability of the Sentinels 1 and 2 data for improving our ability to map a complex area containing wetland and non-wetland LULC classes

Measuring Annual Sedimentation through High Accuracy UAV-Photogrammetry Data and Comparison with RUSLE and PESERA Erosion Models

Model-based soil erosion studies have increased in number, given the availability of geodata and the recent technological advances. However, their accuracy remains rather questionable since the scarcity of field records hinders the validation of simulated values. In this context, this study aims to present a method for measuring sediment deposition at a typical Mediterranean catchment (870 ha) in Greece through high spatial resolution field measurements acquired by an Unmanned Aerial Vehicle (UAV) survey. Three-dimensional modeling is considered to be an emerging technique for surface change detection. The UAV-derived point cloud comparison, applying the Structure-from-Motion (SfM) technique at the Platana sediment retention dam test site, quantified annual topsoil change in cm-scale accuracy (0.02–0.03 m), delivering mean sediment yield of 1620 m3 ± 180 m3 or 6.05 t ha−1yr−1 and 3500 m3 ± 194 m3 or 13 t ha−1yr−1 for the 2020–2021 and 2021–2022 estimation. Moreover, the widely applied PESERA and RUSLE models estimated the 2020–2021 mean sediment yield at 1.12 t ha−1yr−1 and 3.51 t ha−1yr−1, respectively, while an increase was evident during the 2021–2022 simulation (2.49 t ha−1yr−1 and 3.56 t ha−1yr−1, respectively). Both applications appear to underestimate the net soil loss rate, with RUSLE being closer to the measured results. The difference is mostly attributed to the model’s limitation to simulate gully erosion or to a C-factor misinterpretation. To the authors’ better knowledge, this study is among the few UAV applications employed to acquire high-accuracy soil loss measurements. The results proved extremely useful in our attempt to measure sediment yield at the cm scale through UAV-SfM and decipher the regional soil erosion and sediment transport pattern, also offering a direct assessment of the retention dams’ life expectancy.Greece and the European UnionPeer Reviewe

Combining SAR and Optical Earth Observation with Hydraulic Simulation for Flood Mapping and Impact Assessment

Timely mapping, measuring and impact assessment of flood events are crucial for the coordination of flood relief efforts and the elaboration of flood management and risk mitigation plans. However, this task is often challenging and time consuming with traditional land-based techniques. In this study, Sentinel-1 radar and Landsat images were utilized in collaboration with hydraulic modelling to obtain flood characteristics and land use/cover (LULC), and to assess flood impact in agricultural areas. Furthermore, indirect estimation of the recurrence interval of a flood event in a poorly gauged catchment was attempted by combining remote sensing (RS) and hydraulic modelling. To this end, a major flood event that occurred in Sperchios river catchment, in Central Greece, which is characterized by extensive farming activity was used as a case study. The synergistic usage of multitemporal RS products and hydraulic modelling has allowed the estimation of flood characteristics, such as extent, inundation depth, peak discharge, recurrence interval and inundation duration, providing valuable information for flood impact estimation and the future examination of flood hazard in poorly gauged basins. The capabilities of the ESA Sentinel-1 mission, which provides improved spatial and temporal analysis, allowing thus the mapping of the extent and temporal dynamics of flood events more accurately and independently from the weather conditions, were also highlighted. Both radar and optical data processing methods, i.e., thresholding, image differencing and water index calculation, provided similar and satisfactory results. Conclusively, multitemporal RS data and hydraulic modelling, with the selected techniques, can provide timely and useful flood observations during and right after flood disasters, applicable in a large part of the world where instrumental hydrological data are scarce and when an apace survey of the condition and information about temporal dynamics in the influenced region is crucial. However, future missions that will reduce further revisiting times will be valuable in this endeavor

Long and Short-Term Coastal Changes Assessment Using Earth Observation Data and GIS Analysis: The Case of Sperchios River Delta

The present study provides information about the evolution of the Sperchios River deltaic area over the last 6500 years. Coastal changes, due to natural phenomena and anthropogenic activities, were analyzed utilizing a variety of geospatial data such as historic records, topographic maps, aerial photos, and satellite images, covering a period from 4500 BC to 2020. A qualitative approach for the period, from 4500 BC to 1852, and a quantitative analysis, from 1852 to the present day, were employed. Considering their scale and overall quality, the data were processed and georeferenced in detail based on the very high-resolution orthophoto datasets of the area. Then, the multitemporal shorelines were delineated in a geographical information system platform. Two different methods were utilized for the estimation of the shoreline changes and trends, namely the coastal change area method and the cross-section analysis, by implementing the digital shoreline analysis system with two statistical approaches, the end point rate and the linear regression rate. Significant river flow and coastline changes were observed with the overall increase in the delta area throughout the study period reaching 135 km2 (mean annual growth of 0.02 km2/yr) and the higher accretion rates to be detected during the periods 1805–1852, 1908–1945 and 1960–1986, especially at the central and north part of the gulf. During the last three decades, the coastline has remained relatively stable with a decreasing tendency, which, along with the expected sea-level rise due to climate change, can infer significant threats for the coastal zone in the near future

Research of geomorphological and environmental changes in the Sperchios’ river basin utilizing new technologies

Remote sensing is today the modernist manner of observation of environment, as it is set off in an effective tool for the monitoring of geomorphologic characteristics. The use of satellite images in combination with Geographical Information Systems (GIS), can help in the reception of information that concerns the land cover/use, the undergoing changes that have taken place in a specific time interval and the creation or updating of correspondent data bases, given that most times the existing thematic maps do not cover the current needs of users because of the age and the serious inaccuracies that they include. Coastal zones are one of the most complicated and dynamic earth-ecosystems with a large number of living and non-living recourses; therefore, coastal zones are areas of major socio-economical importance, worldwide. Among the various morphological types of coastal zone, river deltas represent a highly dynamic land-water interface that is also subjected also to a strong pressure of several natural (e.g. climatic change) and anthropogenic (e.g. urbanization, transportation, tourism, cultivations) factors. Remote sensing satellite data and GIS provide a unique tool for research and monitoring land use/cover, coastal areas and deltaic environments. The aim of the thesis was, mainly, the investigation of satellite and aerial photo data capabilities to detect the undergoing environmental and geomorphologic conditions, to study the diachronic changes of land cover/use at time interval 1984-2007 and the coastline changes at time interval 4.500 BC - 2007. Also, the determination of reasons that caused these changes, the monitoring and analyses of human pressure, especially to the natural environment are examined. The area under investigation is the basin and the mouth area of the River Sperchios that debouches to the eastern coast of the Maliakos Gulf; the latter is a semi-enclosed embayment located on the east coast of the Greek mainland. The basin covers an area of some 1.828 km2, is approximately 60-80 km long, 20-30 km wide with its southern and western flanks characterized by high elevations and steep slopes, whilst its northern flank presents lower elevations and more gently slopes. The whole research generally consisted of 3 main parts, on the basis of the differentiation of the geomorphologic and environmental characteristics.Η τηλεπισκόπηση είναι σήμερα ο πλέον σύγχρονος τρόπος παρατήρησης του περιβάλλοντος, καθώς αναδεικνύεται σε ένα αποτελεσματικό εργαλείο για την παρακολούθηση των επιφανειακών χαρακτηριστικών της Γης. Η χρήση των δορυφορικών εικόνων σε συνδυασμό με τα Γεωγραφικά Πληροφοριακά Συστήματα (ΓΠΣ), μπορούν να βοηθήσουν στη λήψη πληροφοριών που αφορούν την κάλυψη/χρήση γης, τις υφιστάμενες αλλαγές που έχουν λάβει χώρα σε ένα συγκεκριμένο χρονικό διάστημα και τη δημιουργία ή ενημέρωση των αντίστοιχων βάσεων δεδομένων, δεδομένου ότι τις περισσότερες φορές οι υπάρχοντες θεματικοί χάρτες δεν καλύπτουν τις σημερινές ανάγκες των χρηστών λόγω της παλαιότητας και των σοβαρών ανακριβειών που εμπεριέχουν. Οι παράκτιες ζώνες αποτελούν ένα από τα πιο πολύπλοκα και δυναμικά οικοσυστήματα της γης με έναν μεγάλο αριθμό έμβιων και μη πόρων. Για το λόγο αυτό οι παράκτιες ζώνες είναι περιοχές με αυξημένη κοινωνικοοικονομική σημασία, παγκόσμια. Μεταξύ των διαφόρων μορφολογικών τύπων των παράκτιων περιοχών, τα δέλτα των ποταμών αποτελούν ένα δυναμικό σύστημα στεριάς και θάλασσας, το οποίο επιδέχεται ισχυρές πιέσεις, τόσο από φυσικούς (κλιματικές αλλαγές) όσο και από ανθρωπογενείς (αστικοποίηση, μεταφορές, τουρισμός, καλλιέργειες) παράγοντες. H τηλεπισκόπηση και τα ΓΠΣ αποτελούν ένα εύχρηστο εργαλείο για την έρευνα της χρήσης/κάλυψης γης των παράκτιων περιοχών και των δέλτα. Στόχος της διατριβής ήταν, πρωτίστως, η διερεύνηση των δυνατοτήτων που έχουν τα δορυφορικά δεδομένα και οι αεροφωτογραφίες στην αναγνώριση τις υφιστάμενης γεωμορφολογικής και περιβαλλοντικής κατάστασης, στην μελέτη των διαχρονικών αλλαγών χρήσης/κάλυψης γης για την περίοδο 1984-2007, καθώς και τις αλλαγές της ακτογραμμής για το χρονικό διάστημα 4.500 π.Χ. έως 2.007. Ακόμα, διερευνήθηκε ο καθορισμός των αιτιών που οδήγησαν στις αλλαγές αυτές και εξετάστηκε η επίδραση του ανθρώπινου παράγοντα, ιδιαίτερα στο φυσικό περιβάλλον. Η περιοχή έρευνας είναι η λεκάνη απορροής και η παράκτια περιοχή του Σπερχειού ποταμού, ο οποίος χύνεται στον Μαλιακό κόλπο. Ο Μαλιακός είναι ένα ημίκλειστος κόλπος που βρίσκεται στις ανατολικές ακτές της ηπειρωτικής χώρας. Η λεκάνη καλύπτει έκταση, περίπου 1.828 km2, έχει κατά προσέγγιση 60-80 km μήκος, 20-30 km πλάτος με το νότιο και δυτικό τμήμα της να παρουσιάζουν μεγαλύτερα υψόμετρα και απότομες κλίσεις, ενώ το βόρειο τμήμα παρουσιάζει μικρότερα υψόμετρα και ήπιες κλίσεις. Η έρευνα γενικά αποτελείται από 3 κύρια μέρη, στη βάση της διαφοροποίησης των γεωμορφολογικών και περιβαλλοντικών χαρακτηριστικών. [...

The Complementary Use of Optical and SAR Data in Monitoring Flood Events and Their Effects

This paper describes the synergetic use of earth observation satellites optical and radar data to detect flooded areas and explore the impacts of the flood event. A flash flood episode took place in May 2016, in the central-eastern part of West Thessaly (Central Greece). A Landsat-7 ETM+ and a Sentinel-1 SAR image were acquired. For Landsat-7 several water indices were applied and for the Sentinel-1 a threshold method was implemented. Furthermore, Sentinel-2 images were utilized so as to record the land use/cover of the flooded area. The inundated areas and the affected cultivations were delineated with high precision, and the financial effects were evaluated

Synergistic Approach of Remote Sensing and GIS Techniques for Flash-Flood Monitoring and Damage Assessment in Thessaly Plain Area, Greece

This paper describes the synergetic use of earth observation satellites optical and radar data with a high-resolution digital elevation model (DEM) to detect flooded areas and explore the impacts of a flood event. A flash flood episode took place in May 2016, in the central-eastern part of West Thessaly (Central Greece). Landsat-7 ETM+ and a Sentinel-1 SAR images were acquired. For Landsat-7, several water indices were applied and for the Sentinel-1 a threshold method was implemented. Elevation data were also used to improve the delineation of the inundated areas, and to estimate flood water depth. Furthermore, Sentinel-2 images were utilized so as to record the land use/cover of the flooded area. The inundated areas and the affected cultivations were delineated with high precision, and the financial effects were evaluated

Investigating the Correlation of Tectonic and Morphometric Characteristics with the Hydrological Response in a Greek River Catchment Using Earth Observation and Geospatial Analysis Techniques

Morphometric analysis can be used to investigate catchment dynamics and tectonic processes responsible for the development of drainage catchments and to support flood risk assessment. In this study, a comparative GIS-based morphometric analysis between the main southern and northern sub-catchments of the Sperchios River basin, Central Greece, was performed, using geospatial and remote sensing data. The goal was to investigate their correlation with the peculiar geotectonic activity and the frequent flash-flood events that occur in the river floodplain. All sub-catchments characteristics are linked with the geological formation types of the area, in combination with ongoing tectonic activity. The results indicate that drainage network development is significantly controlled by the region’s overall tectonic activity. The morphometric characteristics—i.e., bifurcation ratio, drainage density, circularity ratio, elongation ratio and water concentration–time values, reflect the flood-prone character of the southern part of Sperchios River catchment in comparison to the northern part, especially during intense rainfall events. The study can provide valuable insight into identifying how morphometric characteristics are associated with increased flood hazard