In this work, we aim to evaluate the feasibility and operational limitations of using Sentinel-1 synthetic aperture radar (SAR) data to monitor water levels in the Poço da Cruz reservoir from September 2016–September 2020, in the semi-arid region of northeast Brazil. To segment water/non-water features, SAR backscattering thresholding was carried out via the graphical interpretation of backscatter coefficient histograms. In addition, surrounding environmental effects on SAR polarization thresholds were investigated by applying wavelet analysis, and the Landsat-8 and Sentinel-2 normalized difference water index (NDWI) and modified normalized difference water index (MNDWI) were used to compare and discuss the SAR results. The assessment of the observed and estimated water levels showed that (i) SAR accuracy was equivalent to that of NDWI/Landsat-8; (ii) optical image accuracy outperformed SAR image accuracy in inlet branches, where the complexity of water features is higher; and (iii) VV polarization outperformed VH polarization. The results confirm that SAR images can be suitable for operational reservoir monitoring, offering a similar accuracy to that of multispectral indices. SAR threshold variations were strongly correlated to the normalized difference vegetation index (NDVI), the soil moisture variations in the reservoir depletion zone, and the prior precipitation quantities, which can be used as a proxy to predict cross-polarization (VH) and co-polarization (VV) thresholds. Our findings may improve the accuracy of the algorithms designed to automate the extraction of water levels using SAR data, either in isolation or combined with multispectral images.
A Realidade Aumentada (RA) é uma tecnologia com novo paradigma de interface com os usuários que vem crescendo em importância na apresentação de produtos cartográficos. A visualização de elementos naturais é incorporada à representação estática terrestre em um ambiente digital interativo. Essa transformação provoca mudanças na forma como os usuários percebem os produtos cartográficos que antes eram apresentados em meios analógicos. O uso da Realidade Aumentada potencializa a apresentação de produtos cartográficos tridimensionais dinâmicos, e permite ampliar o aspecto cognitivo de usuários não especialistas em contato com documentos cartográficos. A tecnologia remonta a década de 1960, e nos dias atuais tem se expandido em diversas funcionalidades em vários setores. O artigo discute a incorporação da Realidade Aumentada na construção do modelo 3D do campus da Universidade Federal de Pernambuco. Tal modelo foi utilizado em experimento aplicado a usuários não especialistas, tendo-se como eixos de avaliação: aspecto territorial, interpretação de elementos, e aspecto comunicativo. Os resultados do experimento mostram que o processo de ensino-aprendizagem na área da cartografia é um campo fértil para atuação da RA, tornando-se um ponto de partida para a elaboração de uma base cartográfica aumentada.Palavras-chave: Visualização Cartográfica; Interatividade, Modelagem 3D. Abstract:Augmented reality (AR) is a technology with new user interface paradigm that is growing in importance in the presentation of cartographic products. The display of natural elements is incorporated into the static terrestrial representation in an interactive digital environment. This 791 A realidade aumentada... Bol. Ciênc. Geod., sec. Artigos, Curitiba, v. 22, n o 4, p.790 -806, out -dez, 2016.
Although the single threshold is still considered a suitable and easy-to-do technique to extract water features in spatiotemporal analysis, it leads to unavoidable errors. This paper uses an enumerative search to optimize thresholds over satellite-derived modified normalized difference water index (MNDWI). We employed a cross-validation approach and treated accuracy as a random variable in order to: (a) investigate uncertainty related to its application; (b) estimate non-optimistic errors involving single thresholding; (c) investigate the main factors that affect the accuracy’s model, and (d) compare satellite sensors performance. We also used a high-resolution digital elevation model to extract water elevations values, making it possible to remove topographic effects and estimate non-optimistic errors exclusively from orbital imagery. Our findings evidenced that there is a region where thresholds values can vary without causing accuracy loss. Moreover, by constraining thresholds variation between these limits, accuracy is dramatically improved and outperformed the Otsu method. Finally, the number of scenes employed to optimize a single threshold drastically affects the accuracy, being not appropriate using a single scene once it leads to overfitted threshold values. More than three scenes are recommended.
One of the main causes of land subsidence in the world is the exploitation of groundwater above the regeneration capacity of the aquifer systems. However, the rapid urban growth in estuarine areas can also contribute to the development of this phenomenon. An example of this occurs in the city of Recife, northeastern Brazil. The municipality is built on an estuarine plain composed of several rivers (Capibaribe, Beberibe, Tejipió), which formation results from the occupation of humid areas and mangroves. In recent decades, the excessive removal of water resources from the subsoil has caused the reduction of more than 100 meters of the deep aquifer piezometric level in some places. The presence of these factors in Recife may contribute to land subsidence. To detect this phenomenon, the Persistent Scatterer Interferometric Synthetic Aperture Radar (PSInSAR) technique was used. The dataset consisted of 135 Sentinel-1A Interferometric Wide (IW) Single Look Complex (SLC) images from September 2016 to April 2021. The images were acquired in descending orbits and VV polarization. The results of the PSInSAR analysis reveal that in Recife there are several areas of land subsidence with a rate close to -15 mm/year. The main occurrence of soil settlement is observed in large recently built areas in the west zone, and small areas in the north zone. Minor cases occur in the southern zone due to the exploitation of groundwater. The identification of these land subsidence areas can help in the study of urban drainage to avoid flooding sites and in the adoption of mitigating measures for the suitable use of underground water resources.
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