Shuying Zang

A Review of Remote Sensing Image Classification Techniques: the Role of Spatio-contextual Information

Abstract This paper reviewed major remote sensing image classification techniques, including pixel-wise, sub-pixel-wise, and object-based image classification methods, and highlighted the importance of incorporating spatio-contextual information in remote sensing image classification. Further, this paper grouped spatio-contextual analysis techniques into three major categories, including 1) texture extraction, 2) Markov random fields (MRFs) modeling, and 3) image segmentation and object-based image analysis. Finally, this paper argued the necessity of developing geographic information analysis models for spatial-contextual classifications using two case studies.

Potential ecological risk of heavy metals and metalloid in the sediments of Wuyuer River basin, Heilongjiang Province, China.

The spatial distribution, sources, and potential ecological risk of heavy metals and metalloid were evaluated in sediments of the Wuyuer River and its tributaries. Metal and metalloid concentrations and chemical speciation (Cr, Cu, Ni, Pb, Cd, As, Zn and Hg) in 187 surface sediment samples were measured using inductively coupled plasma mass spectrometry. Metals and metalloid in the sediments of the Wuyuer River were greater than the background values of the Songnen Plain, and mean heavy metal concentrations were greater in downstream segments of the river than in upstream segments. Speciation results indicated that Cd was chemically mobile and Cu, Pb, Zn, Cr, and Ni were potentially bioavailable. Mercury and As existed mainly in the residual fraction. Hakanson’s potential risk index indicated that the total potential ecological risk of these elements was moderate in the Wuyuer River. Among the selected metals and metalloid, Hg and Cd were the most potentially toxic and mainly distributed near the cities of Keshan, Yi’an and Fuyü. Because the speciation of Cd in the river sediments is highly bioavailable, the concentrations of Cd should be closely monitored. This research provides managers with information needed to better regulate the environment of the Wuyuer River.

Distribution characteristics and ecological risk assessment of toxic heavy metals and metalloid in surface water of lakes in Daqing Heilongjiang Province, China.

It is necessary to estimate heavy metal concentrations and risk in surface water for understanding the heavy metal contaminations and for sustainable protection of ecosystems and human health. To investigate the anthropogenic contribution of heavy metal accumulation surrounding an industrial city in China, the concentrations of six heavy metals, including mercury (Hg), arsenic (As), chromium (Cr), lead (Pb), copper (Cu), and cadmium (Cd) were examined; from four different regions of Daqing in autumn 2011 and winter 2012. The results showed heavy metals distributed in the industrial area at concentrations relatively higher than those in other three areas, while concentrations in the farming area and the protected area were lower. The heavy metal concentrations of water bodies in all areas, except those for Hg and As, Cu, Pb and Cr were lower than the cutoff values for the Class I water quality that was set as the highest standard to protect the national nature reserves. While Hg and As of lakes in industry region had a higher level than those in the agriculture and landscape water, the lowest allowed. The concentrations of all the heavy metals in winter were higher than in the autumn. Cu had a higher ecological risks level to freshwater organisms. The discharge of urban sewage and industrial wastewater might be a major pollutant source, thus these sources should identified before remediation efforts. Efforts are needed to protect the lakes from pollution and also to reduce environmental health risks. This study and the valuable data will pave the way for future research on these Lakes in Daqing.

Wetland mapping in the Zhalong National Natural Reserve, China, using optical and radar imagery and topographical data

Abstract. Knowledge of the spatial extent of wetlands is important to a series of research questions and applications such as wetland ecosystem functioning, water management, and habitat suitability assessment. This study develops a practical digital mapping technique using an optical image of a Landsat thematic mapper (TM), Envisat advanced synthetic aperture radar (SAR) image, and topographical indices derived from topographic maps. An ensemble classifier based on classification tree procedure [random forests (RFs)] is applied to three different combinations of predictors: (1) TM imagery alone (TM-only model); (2) TM imagery plus ancillary topographical data [TM + digital terrain model (DTM)]; and (3) TM imagery, ancillary topographical data and radar imagery (TM + DTM + SAR model). Accuracy assessment results indicate that the radar and topographical variables reduce classification error of marsh. The kappa coefficients for the land cover classification increases significantly as radar imagery and ancillary topographical data are added. The per-grid cell probabilities of each land-cover types are estimated based on the RFs model making use of all available predictors. A final land-cover map is generated by defining pixels as the land-cover type with the highest probabilities. Compared with a single classification and regression tree and a conventional maximum likelihood classifier, RFs produce the highest overall accuracy (72%) with a kappa coefficient of 0.6474, and marsh wetland accuracies ranging from 81.2% to 83.33%. The current study indicates that multisource data (i.e., optical, radar, and topography) are useful in the characterization of freshwater marshes and their adjacent land-cover types. The approach developed in the current study is automated, relatively easy to implement, and could be applicable in other settings over large extents.

Impact of land use and land cover dynamics on Zhalong wetland reserve ecosystem, Heilongjiang Province, China

In view of the significant hydrological and ecological role of wetlands, an analysis was made on the impact of land use and land cover dynamics on the spatial status of Zhalong wetland, a Ramsar site located downstream of Wuyuer River Basin in Northeast China. The impact assessment analyzed multi-temporal changes in the upstream land use/land cover characteristics of the wetland watershed using remote sensing data of Landsat MSS/TM. The multi-temporal land use/land cover statistics revealed that significant changes have taken place in the Wuyuer River Basin. In response to these upstream land use/cover changes, the marsh landscape in Zhalong wetland has showed changes in spatial extension, landscape pattern, and water quality characteristics. The major impacts have resulted from construction of a reservoir and water diversion engineering that has altered the wetland hydrological conditions and reduced the spatial distribution of the marsh landscape. In addition, inputs of agricultural nutrients, and industrial and human wastes from the upper catchments have resulted in increased signs of eutrophication. This study suggests that effective wetland hydrological restoration measures are needed to avoid further deterioration of this internationally important ecosystem.

Wetland mapping and flood extent monitoring using optical and radar remotely sensed data and ancillary topographical data in the Zhalong National Natural Reserve, China

Information regarding the spatial extent and inundation state in the internationally important Wetlands as designated by Ramsar Convention is important to a series of research questions including wetland ecosystem functioning and services, water management and habitat suitability assessment. This study develops an expedient digital mapping technique using optical remotely sensed imagery of the Landsat Thematic Mapper (TM), ENVISAT ASAR active radar C-band imagery, and topographical indices derived from topographic maps. All data inputs were resampled to a common 30 m resolution grid. An ensemble classifiers based on trees (random forest) procedure was employed to produce a final map of per-grid cell wetland probability map. This study also provides a general approach to delineate the extent of flooding builds upon documented relationships between fields measured inundation state and SAR data response on each vegetation types. The current study indicated that multi-source data (i.e. optical, radar and topography) are useful in the characterization of freshwater marshes and their inundation state. This analysis constitutes a necessary step towards improved herbaceous wetland monitoring and provides ecologists and managers with vital information that is related to ecology and hydrology in a wetland area.

Characteristics of phytoplankton distribution for assessment of water quality in the Zhalong Wetland, China

Phytoplankton community structures of water environmental were monitored from July to August 2010 in Zhalong Wetland in northeast China. Phytoplankton community was obtained from samples taken from 341 stations with different environmental conditions. Physicochemical parameters (chemical oxygen demand, the total nitrogen, the total phosphorus, chlorophyll a (Chl-a)) were measured simultaneously for comparison with phytoplankton samples. A total of 410 phytoplankton taxa belonging to 7 phyla, 9 classes, 20 orders, 29 families, 79 genera were obtained. The mean value of trophic state index (TSI) was 49.87 and ranged from 34.89 to 69.12. The dominant species in the study area included Anabaena azotica, Selenastrum minutum, Chlorella vulgaris, Trachelomonas volvocina, Navicula radiosa, and indicator species suggesting that most of the Zhalong Wetland is mesotrophic. The TSI, phytoplankton density, Shannon–Wiener index, Margalef index also indicated that the water quality was a mesotrophic state in Zhalong Wetland.

Hydrological Regime Monitoring and Mapping of the Zhalong Wetland through Integrating Time Series Radarsat-2 and Landsat Imagery

Zhalong wetland is a globally important breeding habitat for many rare migratory bird species. Prompted by the high demand for temporal and spatial information about the wetland’s hydrological regimes and landscape patterns, eight time series Radarsat-2 images were utilized to detect the flooding characteristics of the Zhalong wetland. Subsequently, a random forest model was built to discriminate wetlands from other land cover types, combining with optical, radar, and hydrological regime data derived from multitemporal synthetic aperture radar (SAR) images. The results showed that hydrological regimes variables, including flooding extent and flooding frequency, derived from multitemporal SAR images, improve the land cover classification accuracy in the natural wetlands distribution area. The permutation importance scores derived from the random forest classifier indicate that normalized difference vegetation index (NDVI) calculated from optical imagery and the flooding frequency derived from multitemporal SAR imagery were found to be the most important variables for land cover mapping. Accuracy testing indicate that the addition of hydrological regime features effectively depressed the omission error rates (from 52.14% to 2.88%) of marsh and the commission error (from 77.34% to 51.27%) of meadow, thereby improving the overall classification accuracy (from 76.49% to 91.73%). The hydrological regimes and land cover monitoring in the typical wetlands are important for eco-hydrological modeling, biodiversity conservation, and regional ecology and water security.

Spatial and temporal variation of the urban impervious surface and its driving forces in the central city of Harbin

Associated with the rapid economic development of China, the level of urbanization is becoming a serious concern. Harbin, the capital city of Heilongjiang Province, China and one of the political, economic, cultural, and transportation centers of the northeastern region of China, has experienced rapid urbanization recently. To examine the spatial patterns of long-term urbanization and explore its driving forces, we employed the impervious surface fraction derived from remote sensing image as a primary indicator. Specifically, urban impervious surface information for the central city of Harbin in 1984, 1993, 2002, and 2010 was extracted from Landsat Thematic Mapper image using a Linear Spectral Mixture Analysis (LMSA). Then, the spatial and temporal variation characteristics and the driving factors of percent impervious surface area (ISA) changes were analyzed throughout this 26-year period (1984 to 2010). Analysis of results suggests that: (1) ISAs in the central city of Harbin constantly increased, particularly from 1993 to 2010, a rapid urbanization period; (2) the gravity center of impervious surface area in the central city was located in Nangang District in 1984, moving southeast from 1984 to 1993, northwest from 1993 to 2002, and continuing toward the southeast from 2002 to 2010; and (3) the urban growth of the central city can be characterized as edge-type growth.

Metal concentrations in homing pigeon lung tissue as a biomonitor of atmospheric pollution

Atmospheric pollution in urban areas is a major worldwide concern with potential adverse impacts on wildlife and humans. Biomonitoring can provide direct evidence of the bioavailability and bioaccumulation of toxic metals in the environment that is not available with mechanical air monitoring. The current study continues our evaluation of the usefulness of homing pigeon lung tissue as a biomonitor of atmospheric pollution. Homing pigeons (1–2, 5–6, and 9–10+ year old (yo)) collected from Guangzhou during 2015 were necropsied and concentrations of cadmium (Cd), lead (Pb), and mercury (Hg) were measured in lung tissue. Lung Cd and Pb concentrations were significantly greater in 9–10+-year-old pigeons compared with those in other age groups, indicating their bioavailability and bioaccumulation. Lung Pb and Cd concentrations measured in 5-yo pigeons collected from Guangzhou during 2015 were significantly lower than concentrations reported in 5-yo homing pigeons collected from Guangzhou during 2011 and correlated with concentrations measured using mechanical air monitoring. In addition to temporal differences, spatial differences in concentrations of Cd, Pb, and Hg reported in ambient air samples and in pigeon lung tissues collected from Beijing and Guangzhou are discussed.