Abstract
Veredas (palm swamps) are wetland complexes associated with the Brazilian savanna (cerrado) that often represent the only available source of water for the ecosystem during the dry months. Their extent and condition are mainly unknown and their cartography is an essential issue for their protection. This research article evaluates some of the fine resolution satellite data both in the radar (Radarsat-1) and optical domain (ASTER) for the delineation and characterization of veredas. Two separate approaches are evaluated. First, given the known potential of Radarsat-1 images for wetland inventories, the automatic delineation of veredas is tested using only Radarsat-1 data and a Markov random fields region-based segmentation. In this case, to increase performance, processing is limited to a buffer zone around the river network. Then, characterization of their type is attempted using traditional classification methods of ASTER optical data combined with Radarsat-1 data. The automatic classification of Radarsat data yielded results with an overall accuracy between 62 and 69%, that proved reliable enough for delineating wide and very humid veredas. Scenes from the wet season and with a smaller angle of incidence systematically yielded better results. For the classification of the main vegetation types, better results (overall success of 78.8%) were obtained by using only the visible and near infrared (VNIR) bands of the ASTER image. Radarsat data did not bring any improvement to these classification results. In fact, when using solely the Radarsat data from two different angle of incidence and two different dates, the classification results were low (50.8%) but remained powerful for delineating the permanently moist riparian forest portion of the veredas with an accuracy better than 75% in most cases. These results are considered good given the width of some types often less than 50 m wide compared with the resolution of the images (12.5 - 15 m). Comparing the classification results with the Radarsat-generated delineation allows an understanding of the relation between synthetic aperture radar (SAR) backscattering and vegetation types of the veredas.
Highlights
As wetlands, veredas bear an essential role in the environment by enhancing water quality, reducing flood damage, sequestrating carbon and supporting a disproportional part of the biodiversity [1,2,3]
An experimental evaluation of synthetic aperture radar (Radarsat-1) and optical (ASTER) data for delineating and characterizing veredas has been concluded in this paper and the results suggest that both data types show good potential providing that appropriate classification tools are used
An original unsupervised algorithm based on Markov random fields (MAMSEG) proved better than conventional pixel-based classification for delineating veredas using synthetic aperture radar (SAR) data if the processing is restricted to a buffer zone following the hydrographic network
Summary
Veredas (palm swamps) bear an essential role in the environment by enhancing water quality, reducing flood damage, sequestrating carbon and supporting a disproportional part of the biodiversity [1,2,3]. Veredas present varying types, ranging from wet meadows to riparian forest and are associated with the presence of Buriti palms (Mauritia flexuosa L.f., see Figure 1). Their aspect can vary significantly, typical veredas are relatively narrow landscape features that follow a mostly intermittent stream that can either have a diffuse or well-defined channel. Their width can vary from tens of meters to a few hundred. From the contact with cerrado vegetation towards the lowest point of the valley, typical veredas show the following sequence of types: grassland (wet meadows), shrubs and trees often dominated by Mauritia flexuosa
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