Christopher Small

Estimation of urban vegetation abundance by spectral mixture analysis

The spatio-temporal distribution of vegetation is a fundamental component of the urban environment that can be quantified using multispectral imagery. However, spectral heterogeneity at scales comparable to sensor resolution limits the utility of conventional hard classification methods with multispectral reflectance data in urban areas. Spectral mixture models may provide a physically based solution to the problem of spectral heterogeneity. The objective of this study is to examine the applicability of linear spectral mixture models to the estimation of urban vegetation abundance using Landsat Thematic Mapper (TM) data. The inherent dimensionality of TM imagery of the New York City area suggests that urban reflectance measurements may be described by linear mixing between high albedo, low albedo and vegetative endmembers. A three-component linear mixing model provides stable, consistent estimates of vegetation fraction for both constrained and unconstrained inversions of three different endmember ensembles. Quantitative validation using vegetation abundance measurements derived from high-resolution (2 m) aerial photography shows agreement to within fractional abundances of 0.1 for vegetation fractions greater than 0.2. In contrast to the Normalised Difference Vegetation Index (NDVI), vegetation fraction estimates provide a physically based measure of areal vegetation abundance that may be more easily translated to constraints on physical quantities such as vegetative biomass and evapotranspiration.

The effect of seamount subduction on seismic coupling

It has long been speculated that the subduction of seamounts may have profound mechanical consequences, particularly on the generation of large subduction zone earthquakes. We consider this problem as a special case of the seismic coupling model of Scholz and Campos. The model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling. In the case of coupled arcs, this increased coupling will result in a large increase in the recurrence intervals of earthquakes. Such events will therefore appear rare in the instrumental record, as is observed. For decoupled arcs, the subduction of a large seamount should cause them to become locally coupled. Three of the latter localities have been found with satellite altimetry data, two on the Tonga-Kermadec Trench and one on the Izu-Bonin Trench. The diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate. In each case, the small to moderate plate-interface seismicity typical of decoupled seismic zones is almost completely absent in a 100–200 km arc segment, indicating that the subduction zone is locally locked, as predicted by the model. The occurrence of large interplate earthquakes at both Tonga sites confirms that they are seismically coupled.

A global analysis of urban reflectance

A comparative Spectral Mixture Analysis (SMA) of Landsat 7 Enhanced Thematic Mapper (ETM+) imagery for a collection of 28 urban areas worldwide provides a physical basis for a spectral characterization of urban reflectance properties. These urban areas have similar mixing space topologies and can be represented by three‐component linear mixture models in both scene‐specific and global composite mixing spaces. The results of the analysis indicate that the reflectance of these cities can be accurately described as linear combinations of High Albedo, Dark and Vegetation spectral endmembers within a two‐dimensional mixing space containing over 90% of the variance in the observed reflectance. Only two of the 28 cities had greater than 10% median RMS misfit to the three‐endmember linear model. The relative proportions of these endmembers vary considerably among different cities and within individual cities but in all cases the reflectance of the urban core lies near the dark end of a mixing line between the High Albedo and Dark endmembers. The most consistent spectral characteristic of the urban mosaic is spectral heterogeneity at scales of 10–20 m. In spite of their heterogeneity, built‐up areas do occupy distinct regions of the spectral mixing space. This localization in mixing space allows spectrally mixed pixels in built‐up areas to be discriminated from undeveloped land cover types. This provides a basis for mapping the spatial extent of human settlements using broadband optical satellite imagery collected over the past 30 years.

Observations of ridge‐hotspot interactions in the Southern Ocean

The evolution of ridge-hotspot systems is not well understood. In this investigation, satellite-derived marine gravity data are used in conjunction with underway bathymetric and magnetic anomaly profiles to investigate the nature of ridge-hotspot interaction at four sparsely explored systems in the Southern Ocean. These systems illustrate three different stages of ridge-hotspot interaction in which a migrating spreading center approaches a hotspot (Pacific-Antarctic/Louisville), passes over or is captured by the hotspot (Mid-Atlantic/Shona-Discovery), and ultimately migrates away from the hotspot (Southeast Indian/Kerguelen). All of these systems show some evidence of discrete ridge jumps in the direction of the hotspot as the spreading center attempts to relocate toward the hotspot by asymmetric spreading. Interestingly, these ridge jumps show no evidence of propagating offsets as have been seen on many other ridge-hotspot systems. A simple model predicts that typical plume excess temperatures can weaken the lithosphere sufficiently to promote asymmetric spreading and possibly allow a discrete ridge jump. The presence of previously uncharted, obliquely oriented aseismic ridges and gravity lineations between the ridge and the hotspot supports the notion of asthenospheric flux from the plume to the spreading center both before and after the time when the hotspot is ridge centered. The azimuths of the aseismic ridges cannot be explained by plate kinematics alone; they consistently extend from the ends toward the centers of the adjacent spreading segments suggesting some interaction between plume derived asthenospheric flux and local lithospheric structure. The features discussed here also indicate that the transfer of asthenospheric material from the plume to the spreading center is influenced by the local plate boundary configuration and interaction with transform offsets.

The Nightsat mission concept

Nightsat is a concept for a satellite system capable of global observation of the location, extent and brightness of night‐time lights at a spatial resolution suitable for the delineation of primary features within human settlements. Based on requirements from several fields of scientific inquiry, Nightsat should be capable of producing a complete cloud‐free global map of lights on an annual basis. We have used a combination of high‐resolution field spectra of outdoor lighting, moderate resolution colour photography of cities at night from the International Space Station, and high‐resolution airborne camera imagery acquired at night to define a range of spatial, spectral, and detection limit options for a future Nightsat mission. The primary findings of our study are that Nightsat should collect data from a near‐synchronous orbit in the early evening with 50 to 100 m spatial resolution and have detection limits of 2.5E−8 Watts cm−2sr−1µm−1 or better. Although panchromatic low‐light imaging data would be useful, multispectral low‐light imaging data would provide valuable information on the type or character of lighting; potentially stronger predictors of variables such as ambient population density and economic activity; and valuable information to predict response of other species to artificial night lighting. The Nightsat mission concept is unique in its focus on observing a human activity, in contrast to traditional Earth observing systems that focus on natural systems.

Cities from space: potential applications of remote sensing in urban environmental research and policy

Abstract The rapidly expanding urban areas of the world constitute an environmental challenge for the 21st century that requires both new analytic approaches and new sources of data and information. Management of the urban environment must consider phenomena at three scales, including the physical environment within cities, environmental systems consisting of cities and their regional hinterlands, and the environmental implications of the global network of megacities. Increasing availability of remotely sensed observations and a variety of other geospatial information could facilitate the development of new tools and approaches for understanding the urban environment. These new applications should take advantage of the special characteristics of remotely sensed data, including their broad spatial coverage, their capacity for routine and unobtrusive updating and their ability to provide self-consistent measurements of critical physical properties that would be difficult or expensive to obtain in situ. In many cases, using remote sensing data to measure and monitor urban environmental conditions will be more straightforward than using them for urban planning purposes, where traditional sources of governmental and private sector data are more easily obtained and understood. In developing countries, however, remote sensing may provide fundamental observations of urban growth and environmental conditions that are not available from other sources.

The global distribution of human population and recent volcanism

Abstract This study quantifies the spatial relationship between the global distribution of human population and recent volcanism. Using recently compiled databases of population and Holocene volcanoes, we estimate that almost 9% (455 × 106 people) of the worlds 1990 population lived within 100km of an historically active volcano and 12% within 100km of a volcano believed to have been active during the last 10,000 years. The analysis also indicates that average population density generally decreases with distance from these volcanoes (within 200 km). In tropical areas, the elevation and fertile soils associated with volcanic regions can provide incentives for agrarian populations to settle close to potentially active volcanoes. In Southeast Asia and Central America higher population densities lie in closer proximity to volcanoes than in other volcanic regions. In Japan and Chile, population density tends to increase with distance from volcanoes. The current trends of rapid urbanization and sustained popul...

Multitemporal analysis of urban reflectance

Abstract Spatial and temporal changes in urban reflectance have a strong influence on energy flux through the urban environment. Optical sensors on operational satellites provide self-consistent time series of urban reflectance variations, but quantitative analyses are complicated by spectral heterogeneity at sensor instantaneous field of view (IFOV) scales and by temporal changes in illumination and atmospheric conditions. These complications can be minimized by combining a multitemporal radiometric rectification with a physically based reflectance analysis. Spectral Mixture Analysis (SMA) provides a physically based approach to quantifying spatial and temporal changes in spectrally heterogeneous urban reflectance. Multitemporal analysis of Landsat Thematic Mapper (TM) imagery of the New York metropolitan area suggests that urban reflectance can be described with a three-component linear mixture model spanned by high albedo, low albedo, and vegetation endmembers. The topology of the spectral mixing space indicates that mixing fractions are well constrained for the vegetation endmember and that nonlinear mixing occurs primarily between the high and low albedo endmembers. Selection of pseudoinvariant (PIV) image endmembers allows radiometric rectification of multitemporal imagery to a common set of endmembers, thereby minimizing variations in radiance that are unrelated to changes in surface reflectance. Inversion of the three-component linear mixture model for the New York metro area produces robust, consistent fraction estimates for different combinations of rectifications and inversion constraints. Temporal variation of the presumed invariant endmember sites provides a measure of uncertainty for the endmember fraction estimates. The resulting vegetation fraction estimates agree with high-resolution reference measurements to within 10% for a 1996 midsummer validation and PIV endmember fraction estimates vary by less than 7% over the course of the 1996 growing season. In contrast, intraurban spatial variations in vegetation fraction span several tens of percent, suggesting that the measured changes significantly exceed the uncertainty of the estimates. These results suggest that Landsat TM imagery may be used to monitor seasonal to interannual variations in urban reflectance and vegetation abundance.

Improved estimates of coastal population and exposure to hazards released

Though it is well known that the worlds coastlines are heavily populated, the combined implications of population growth and climate change are still subject to debate. Models of hazard impact, adaptation, and vulnerability stress the importance of understanding both exposure and adaptive capacity of the threatened systems [e.g.,Smit et al., 2001]. Combining geophysical and socio-economic data sets can greatly improve our understanding of exposure at a range of scales from local to global. Here we estimate an upper bound on the global exposure to coastal hazards based on 1990 population distribution. The focus is on exposure to natural hazards, but these estimates also provide an indication of the direct human pressure on the coastal zone. Data from 1990 were used, as this global population distribution was the most robust currently available.

Night on Earth: Mapping decadal changes of anthropogenic night light in Asia

Abstract The defense meteorological satellite program (DMSP) operational linescan system (OLS) sensors have imaged emitted light from Earths surface since the 1970s. Temporal overlap in the missions of 5 OLS sensors allows for intercalibration of the annual composites over the past 19 years ( Elvidge et al., 2009 ). The resulting image time series captures a spatiotemporal signature of the growth and evolution of lighted human settlements and development. We use empirical orthogonal function (EOF) analysis and the temporal feature space to characterize and quantify patterns of temporal change in stable night light brightness and spatial extent since 1992. Temporal EOF analysis provides a statistical basis for representing spatially abundant temporal patterns in the image time series as uncorrelated vectors of brightness as a function of time from 1992 to 2009. The variance partition of the eigenvalue spectrum combined with temporal structure of the EOFs and spatial structure of the PCs provides a basis for distinguishing between deterministic multi-year trends and stochastic year-to-year variance. The low order EOFs and principal components (PC) space together discriminate both earlier (1990s) and later (2000s) increases and decreases in brightness. Inverse transformation of these low order dimensions reduces stochastic variance sufficiently so that tri-temporal composites depict potentially deterministic decadal trends. The most pronounced changes occur in Asia. At critical brightness threshold we find an 18% increase in the number of spatially distinct lights and an 80% increase in lighted area in southern and eastern Asia between 1992 and 2009. During this time both China and India experienced a ∼20% increase in number of lights and a ∼270% increase in lighted area – although the timing of the increase is later in China than in India. Throughout Asia a variety of different patterns of brightness increase are apparent in tri-temporal brightness composites – as well as some conspicuous areas of apparently decreasing background luminance and, in many places, intermittent light suggesting development of infrastructure rather than persistently lighted development. Vicarious validation using higher resolution Landsat imagery verifies multiple phases of urban growth in several cities as well as the consistent presence of low DN ( Small et al., 2011 ).