Dense wetland sample production at large scale by combining multi-source thematic datasets and visual interpretation
Sample collection is one of key research foundations for wetland mapping.It plays an important role in classifier training and accuracy validation.Generally,wetland samples are produced by visual interpretation based on high-spatial-resolution images or automatic generation based on multi-source existing dataset.The visual interpretation is time and labor consuming and cannot meet the demand for large-scale wetland classification.The automatic sample-generation method is unsuitable to detailed-type wetland mapping due to the diversity of wetlands and classification-scheme inconsistency of existing wetland datasets.Thus,an efficient and accurate sampling method is in demand for large-scale and detailed-type wetland mapping.In our study,we collected a series of auxiliary datasets and developed an efficient solution for continental-scale wetland sample generation by combining automatic sampling method and visual interpretation.In the first part,the samples of five wetland types can be automatically generated by rule filtering based on multi-source existing datasets.River,lake,and reservoir samples were created using the JRC Global Surface Water,Global River Widths from Landsat and HydroLAKES datasets.Coastal swamp(mangrove)samples were produced by using Global Mangrove Watch dataset.Tidal flat samples were generated using the Global Intertidal Change dataset.In the second part,by combining time series of MODIS NDVI images and existing auxiliary datasets,we first produced potential wetland samples for coarse wetland types(i.e.,vegetated wetland samples and inundated wetland samples).Then,we identified them by visual interpretation based on the Google Earth Engine platform,Google Earth software,and Collect Earth software.We applied our sample method in our study area,and produced continental-scale and detailed-type wetland samples.Results indicated that the total wetland samples in our study area was 150688,among which 141412 points were inland wetland samples,11563 were coastal wetland samples,and 17693 were human-made wetland samples.Among the 13 wetland sub-categories,lake accounted for the largest proportion(39.22％)and primarily distributed the northern and central of study area,whereas lagoon accounted for the smallest proportion(0.19％),mostly scattered in coastal region of the study area.Samples of river,reservoir,inland swamp,and inland marsh also shared a considerable amount,accounting for 16.93％,9.86％,7.16％,and 11.12％of total wetland samples,respectively.River samples were primarily distributed north and south of the study area,and reservoir samples were primarily scattered south of the study area.Meanwhile,inland swamp and inland marsh samples were mostly distributed northwest and south of the study area.This study successfully produced stable and high-quality wetland samples at continental scale.The generated samples shared sufficient quantities and reasonable spatial distribution,which can lay a good foundation for classifier training and accuracy validation.Meanwhile,by combining the multi-source thematic datasets and multiple platform,our designed sample solution can make full use of the existing database and greatly reduce manual workload.It can also create high-quality samples for complex wetland types,such marsh,swamp and floodplain.Overall,the designed sample method in our study was efficient and reliable,which has significance for large-scale wetland mapping.
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