Spatial localization of closeness and betweenness measures: a self-contradictory but useful form of network analysis

Closeness and betweenness are forms of spatial network analysis grounded in a long-standing tradition of measuring accessibility and flow potential. More recently, these measures have been enhanced by the concept of spatial localization, producing effective models for the prediction of pedestrian and vehicle driver behaviour.

A contradiction arises where the distance metric used to define locality does not match the distance metric used to define shortest paths for closeness and betweenness. A typical case is the use of angular shortest paths within a Euclidean buffer as a pedestrian flow model. Such a model assumes that people make a mode choice based on distance, but a route choice based on least angular change – even when this results in an excessively long ‘problem route’, which conflicts with their criterion for mode choice.

This study examines the prevalence of problem routes and the magnitude of their effect on some pedestrian and vehicle models. We show that while in a weighted analysis, pathological cases could invalidate an entire model, in the models presented the effect of this contradiction is minor. We do this by comparing model predictions to real flow data, using four strategies for handling problem routes: ignore, discard, reroute and strict locality. Strict locality is justified on the grounds of bounded rationality. We find all strategies to give broadly similar results, although the reroute and strict strategies give marginally better simulation accuracy. We also present a discussion of the characteristics of each strategy, and findings on computational efficiency.

We conclude that it is prudent in any computation of localized closeness and betweenness to consider the impact of problem routes; however, they do not necessarily invalidate these forms of analysis, which remain useful.     

Pair annihilation and radio emission from galactic jet sources: the case of Nova Muscae

In the hard X-ray spectra of some X-ray binaries line features at around 500 keV are detected. We interpret these as arising from pair annihilation in relativistic outflows leading to a significant Doppler shift of the frequencies of the lines. We show how this can be used to accurately determine the bulk velocity and orientation to the line of sight of the outflows. Constraints on the energy requirements of such outflows are also derived. Furthermore, we show that a small fraction of pairs escaping the annihilation region may give rise to the radio synchrotron emission observed in some of these objects. We apply these ideas to the hard X-ray and radio observations of Nova Muscae 1991. In this object, the energy requirements seem to rule out a large proton fraction in the outflows.     

Predicting acid neutralizing capacity from landscape classification: application to Galloway,south‐west Scotland

Regional estimates of acid neutralizing capacity (ANC) in stream waters are found using a regression model. The model has landscape classifications based on catchment characteristics as its main independent variables. It also includes continuously varying covariates. Landscape classifications and covariates are selected from a priori scientific understanding of acidification processes. Parameter estimates for the model are found using measurements of ANC in 50 streams in Galloway, south‐west Scotland with a history of acidification. The parameterized model is then used to provide ANC simulations for streams throughout a subregion, assuming conservative mixing of ANC through the flow network. The stream water sampling survey is designed to reduce the variance of parameter estimates. A variance model is suggested for the concentrations, and this is used to simulate the variance of ANC concentrations throughout the subregion. Monte Carlo simulation is used to estimate the distribution of the length of river reach with ANC less than zero. © Crown Copyright 2004. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.     

First experimental studies of large samples of gas-laden amorphous “cometary” ices

In a unique machine, the first of its kind, large (200 cm² × 10 cm) samples of gas-laden amorphous ice were prepared at 80 K and 10⁻⁵ Torr. The sample consisted of a fluffy agglomerate of 200-μm ice grains, similar to what is presumed to be the structure of comet nuclei. The sample was heated from above by IR radiation. The properties studied were gas content in the ice and its emanation from the ice upon warming and bearing on the gas/water vapor ratio observed in cometary comae vs this ratio in cometary nuclei and the effect of internal trapped gas on the thermal conductivity of the ice and the density and mechanical properties of pure ice vs gas-laden ice. These findings might have significance for the interpretation of comet observations, the forthcoming ESA’s Rosetta space mission to Comet 46P/Wirtanen in 2012, and to other comet missions.     

Powerful unconfined turbidity current captured by cabled observatory on the Fraser River delta slope,British Columbia,Canada

Turbidity currents are one of the main sediment transport processes on Earth, yet are notoriously difficult to monitor directly. This article presents the first direct and high bandwidth observation of a turbidity current using a cabled sea floor observatory. On 5 June 2012, a platform on Ocean Networks Canada, located in 107 m of water on the Fraser River delta slope, was displaced downslope and severed from its data cable. The platform weighed ca 1000 kg in water. The event took place during high river discharge, high tides and rapid sediment accumulation on adjacent upslope areas of the sea floor. Data recorded as it tumbled downslope allow a reconstruction of the flow, which is inferred to have been an unconfined turbidity current. Lines of evidence indicate that the flow came in as a bed hugging wedge, and built up to between 1 m and 4 m in height as the head passed through. Comparison with laboratory data suggest that the flow was initially supercritical. While the adjacent slope offset to the north clearly exhibits change over an annual resurvey period, the bathymetry directly at the event location show no resolvable change over a period from seven months before the event to one month after. Sediment cores collected after the event were pervasively biototurbated and they contain no obvious deposit connected with this event. The remarkable aspects of this research follow. The flow was powerful enough to carry a 1 tonne platform and sever a heavily armoured cable. The current occurred on the unchannelized open slope. This powerful event failed to cause discernible seabed elevation change. The flow was triggered by tidal conditions. The event was detected by a purpose‐designed cabled observatory, thus providing high bandwidth data and also alerting researchers in real time to mount follow‐on investigations.     

Monitoring recovery after earthquakes through the integration of remote sensing,GIS, and ground observations: the case of L’Aquila (Italy)

The usefulness of remote sensing (RS), geographical information systems, and ground observations for monitoring changes in urban areas has been demonstrated through many examples over the last two decades. Research has generally focused on the relief phase following a disaster, but we have instead investigated the subsequent phases involving early recovery, recovery, and development. Our aim was to determine to what extent integration of the available tools, techniques, and methods can be used to efficiently monitor the progress of recovery following an earthquake. Changes in buildings within the Italian city of L’Aquila following the 2009 earthquake were identified from Earth observation data and are used as indicators of progress in the recovery process. These changes were identified through (1) visual analysis, (2) automated change detection using a set of decision rules formulated within an object-based image analysis framework, and (3) validation based on a combination of visual and semiautomated interpretations. An accuracy assessment of the automated analysis showed a producer accuracy of 81% (error of omission: 19%) and a user accuracy of 55% (error of commission: 45%). The use of RS made it possible for the identification of changes to be spatially exhaustive, and also to increase the number of categories used for a recovery index. In addition, using RS allowed the area requiring extensive fieldwork (to monitor the progress of the recovery process) to be reduced.     

A novel methodology for identifying environmental exposures using GPS data

Aim: While studies using global positioning systems (GPS) have the potential to refine measures of exposure to the neighbourhood environment in health research, one limitation is that they do not typically identify time spent undertaking journeys in motorised vehicles when contact with the environment is reduced. This paper presents and tests a novel methodology to explore the impact of this concern.

Methods: Using a case study of exposure assessment to food environments, an unsupervised computational algorithm is employed in order to infer two travel modes: motorised and non-motorised, on the basis of which trips were extracted. Additional criteria are imposed in order to improve robustness of the algorithm.

Results: After removing noise in the GPS data and motorised vehicle journeys, 82.43% of the initial GPS points remained. In addition, after comparing a sub-sample of trips classified visually of motorised, non-motorised and mixed mode trips with the algorithm classifications, it was found that there was an agreement of 88%. The measures of exposure to the food environment calculated before and after algorithm classification were strongly correlated.

Conclusion: Identifying non-motorised exposures to the food environment makes little difference to exposure estimates in urban children but might be important for adults or rural populations who spend more time in motorised vehicles.     

The green–blue swing: plasticity of plankton food‐webs in response to coastal oceanographic dynamics

The internal organization of plankton communities plays a key role in biogeochemical cycles and in the functioning of aquatic ecosystems. In this study, the structure of a marine plankton community (including both unicellular and multicellular organisms) was inferred by applying an ecological network approach to species abundances observed weekly at the long‐term ecological research station MareChiara (LTER‐MC) in the Gulf of Naples (Tyrrhenian Sea, Mediterranean Sea) in the summers of 2002–2009. Two distinct conditions, characterized by different combination of salinity and chlorophyll values, alternated at the site: one influenced by coastal waters, herein named ‘green’, and the other reflecting more offshore conditions, named ‘blue’. The green and blue ‘phases’ showed different keystone biological elements: namely, large diatoms and small‐sized flagellates, respectively. Several correlations amongst species belonging to different trophic groups were found in both phases (connectance ~0.30). In the green phase, several links between phytoplankton and mesozooplankton and within the latter were detected, suggesting matter flow from microbes up to carnivorous zooplankton. A microbial‐loop‐like sub‐web, including mixo‐ and heterotrophic dinoflagellates and ciliates, was present in the green phase, but it was relatively more important in the blue phase. The latter observation suggests a more intense cycling of matter at the microbial trophic level in the blue phase. These results show that different modes of ecological organization can emerge from relatively small changes in the composition of aquatic communities coping with environmental variability. This highlights a significant plasticity in the internal structure of plankton webs, which should be taken into account in predictions of the potential effects of climatic oscillations on aquatic ecosystems and biogeochemical cycles therein.