Changjoo Kim

Mass balance of higher‐elevation parts of the Greenland ice sheet

Satellite radar and aircraft laser altimeter data and a volume budget comparison of total snow accumulation with total ice discharge give three independent estimates of the recent mass balance of parts of the Greenland ice sheet above ∼2000 m elevation. Results show the entire region, on average, to be in balance to within 10 mm yr−1, with very low rates of regional thickness change (dH/dt) in the northeast but high rates with large spatial variability in the south. Only the volume budget estimates show significant thinning in the northwest, but thinning in this area is also inferred from local measurements of ice vertical velocities down boreholes. South of latitude 68°N, there has been rapid thickening west of the ice divide, with equally rapid thinning in the southeast, but with large differences between the three estimates of dH/dt throughout this region. The radar data apply to the period 1978–1988, the laser data are for 1993/1994–1998/1999, and the volume budget calculations represent conditions over at least the last few decades. Consequently, many of the differences between results could be caused by temporal changes, particularly in snow accumulation rates, that occurred since the 1970s. However, taken with other information, our results suggest long-term thickening in the southwest and possibly quite recent onset of rapid thinning in the southeast.

Strategic route extension in transit networks

This study proposes a methodology through which transportation analysts and policy makers can use spatial optimization to support strategic planning, with the goal of extending existing service networks. Based on modeling objectives common to many service industries, an approach is developed for integrating geographic information systems (GIS) and spatial optimization modeling in order to extend an existing transit system through prioritizing route and stop additions. Development of a strategic methodology such as this is vital for agencies interested in extending transit networks to accommodate urban growth and development. This is especially true in public transit applications, such as bus route planning, as the future of bus-based public transportation depends on the success of route expansion and modification. The developed approach is applied to the transit system in Columbus, Ohio.

Modeling interregional commodity flows with incorporating network autocorrelation in spatial interaction models: An application of the US interstate commodity flows

Abstract Spatial interaction models are frequently used to predict and explain interregional commodity flows. Studies suggest that the effects of spatial structure significantly influence spatial interaction models, often resulting in model misspecification. Competing destinations and intervening opportunities have been used to mitigate this issue. Some recent studies also show that the effects of spatial structure can be successfully modeled by incorporating network autocorrelation among flow data. The purpose of this paper is to investigate the existence of network autocorrelation among commodity origin–destination flow data and its effect on model estimation in spatial interaction models. This approach is demonstrated using commodity origin–destination flow data for 111 regions of the United States from the 2002 Commodity Flow Survey. The results empirically show how network autocorrelation affects modeling interregional flows and can be successfully captured in spatial autoregressive model specifications.

Internet Reliability with Realistic Peering

Peering between network providers is modeled as a set of interconnections at hub nodes. The paper employs probabilistic methods to determine the likelihood of a path between nodes, under a variety of simulated disruptions or failures of parts of a network. Reliability is measured as the probability of withstanding failure of network components such as nodes or linkages. A reliability envelope is devised in order to define a range of impacts—with the recognition that the damage to subsets of the network can vary from a relatively benign effect to a much more critical or lethal impact. Thus, we describe a range of scenarios from best to worst, within the same level of infrastructure loss. With the use of simulation data for the United States, results show the critical role of major hubs in sustaining communication among cities. In the paper we highlight especially reliable nodes (for example, Chicago and Atlanta) and those that are most susceptible (for example, Birmingham, Alabama, and Nashville, Tennessee). Failure in major hub cities such as Chicago, Dallas, and San Francisco would significantly influence adjacent areas and could cause severe degradation for the entire network with a loss of resiliency to below tolerable levels. An important finding is that various patterns in the reliability envelope can be influenced by factors such as network structures, hub structures, and geographical locations.

Producing an Indigenous Knowledge Web GIS for Arctic Alaska Communities: Challenges, Successes, and Lessons Learned

A traditional knowledge “Inupiaq Web GIS”, based on a five-year study and containing observations and environmental knowledge of Inupiat communities indigenous to Arctic Alaska, was incorporated into a Web-based platform. The website, “Arctic Cultural Cartography,” was created to be an open portal through which the password-protected “Inupiaq Web GIS” could be accessed. We discuss the process of developing the web GIS including the incorporation of user-friendly features such as links to interactive maps, video clips of interviews, discussion boards, and the integration of popular web interfaces such as Facebook. We also discuss short- and long-term goals for the further development of the GIS, its potential as a sustainable, participatory online database for sharing pertinent ecological knowledge, and challenges in achieving optimal community involvement given constraints imposed by remote locations with limited bandwidth.

Characterizing bicycle collisions by neighborhood in a large Midwestern city.

Introduction. Local environmental factors provide important contributions to bicycle safety. The purpose of this study was to characterize bicycle collisions by neighborhood in Cincinnati, Ohio. Background. The majority of prior bicycle safety research has focused on helmet use, especially among youth. Studies that have considered the neighborhood have centered on the built environment and its facilitation of bicycling (e.g., connectivity of roads and road conditions). Other broad conditions may be associated with injury beyond the use of protective equipment and the physical environment. Method. This study sought to determine spatial clustering, local patterning, temporal differences (time of day and season of year), and significant neighborhood-level predictors of bicycle collisions. Bicycle collision data were obtained from the Cincinnati, Ohio Police Department. Conclusions. This study showed that collisions occur at higher rates in the south-central and southwest neighborhoods of Cincinnati, Ohio. There were seasonal and time-of-day differences with respect to collision rates with summer and afternoon being the most common collision times. Neighborhood ethnicity, population density and presence of public transportation were all significant predictors of bicycle collisions. These findings will be disseminated to local city authorities and bicycle advocacy groups.

Network Reliability and Resilience of Rapid Transit Systems

The recent increase in demand and transportation security highlights the importance of the public transit system in the United States. This study explores how potential failures on nodal disruptions affect transit system flows and examines the change in the reliability of transit systems with a case study of the Greater Metropolitan Area of Washington, DC. For methodology, we employ network reliability and system flow loss and assess the criticality of stations under a variety of simulated nodal disruptions. We evaluate network resilience by identifying the best and worst geographical impact scenarios on networks.

Methods for Space-Time Analysis and Modeling: An Overview

With increasing availability of spatio-temporal data and the democratization of Geographical Information Systems (GIS), there has been a demand for novel statistical and visualization techniques which can explicitly integrate space and time. The paper discusses the nature of spatio-temporal data, the integration of time within GIS and the flourishing availability of spatial and temporal-explicit data over the Internet. The paper attempts to answer the fundamental question on how these large datasets can be analyzed in space and time to reveal critical patterns. The authors further elaborate on how spatial autocorrelation techniques are extended to deal with time, for point, linear, and areal features, and the impact of parameter selection, such as critical distance and time threshold to build adjacency matrices. The authors also discuss issues of space-time modeling for optimization problems.

The role of peer effects and the built environment on individual travel behavior

While urban planners and transportation geographers have long emphasized the importance of social influences on individual travel behavior, many challenges remain to bridge the gap between complex conceptual frameworks and operational behavioral models. Improving the ability of models to forecast activity-travel behavior can provide greater insights into urban planning issues. This paper proposes a new model framework by evaluating how individual travel behavior is influenced by inter- and intra-household interactions. The built environment, land-use mix, and social interactions influence household member choices among different transport modes. We propose a spatial multivariate Tobit specification that allows each individual to face a set of potential destinations and transport modes and takes into consideration the travel behavior of other household members and nearby neighbors. Using the Greater Cincinnati Household Travel Survey, we analyzed more than 37,000 trips made by 1968 individuals located in Hamilton County in Cincinnati, Ohio. Results reveal that social influences and the built environment have a strong impact on the willingness to walk and to cycle.

Spatial-Temporal Analysis of Recent Air Passenger Flows

Airline mergers and acquisitions (MAs) are on the rise across the globe and have been a growing trend in the U.S. aviation industry in the last few years. MAs are taking several factors into consideration, such as cost efficiency, competition, and geographic coverage. For airlines, these transactions can eliminate overlapping routes and help reduce competition, leading airlines to achieve higher operating margins. For travelers, MAs often lead to lesser flight frequency, higher airfares or longer travel miles. To explore spatial-temporal variations from airline and passenger perspectives, this paper focuses on 55 major airports in the 50 largest cities between 2000 and 2010. The detailed results of passenger flow patterns suggest that some airports have more spatial imbalance than others in terms of passenger travel distances. Further, the findings indicate that the MAs have different effects on passenger flows and traveled distances, and the effect is complexly related to the airport’s spatial status.