Showing papers by "David Wong published in 2007"

A surface-based approach to measuring spatial segregation

Abstract: Quantitative indices of residential segregation have been with us for half a century, but suffer significant limitations. While useful for comparison among regions, summary indices fail to reveal spatial aspects of segregation. Such measures generally consider only the population mix within zones, not between them. Zone boundaries are treated as impenetrable barriers to interaction between population subgroups, so that measurement of segregation is constrained by the zoning system, which bears no necessary relation to interaction among population subgroups. A segregation measurement approach less constrained by the chosen zoning system, which enables visualization of segregation levels at the local scale and accounts for the spatial dimension of segregation, is required. We propose a kernel density estimation approach to model spatial aspects of segregation. This provides an explicitly geographical framework for modeling and visualizing local spatial segregation. The density estimation approach lends itself to development of an index of spatial segregation with the advantage of functional compatibility with the most widely used index of segregation (the dissimilarity index D). We provide a short review of the literature on measuring segregation, briefly describe the kernel density estimation method, and illustrate how the method can be used for measuring segregation. Examples using a simulated landscape and two empirical cases in Washington, DC and Philadelphia, PA are presented.

Modeling population density across major US cities: a polycentric spatial regression approach

Abstract: A common approach to modeling population density gradients across a city is to adjust the specification of a selected set of mathematical functions to achieve the best fit to an urban place’s empirical density values. In this paper, we employ a spatial regression approach that takes into account the spatial autocorrelation latent in urban population density. We also use a Minkowskian distance metric instead of Euclidean or network distance to better describe spatial separation. We apply our formulation to the 20 largest metropolitan areas in the US according to the 2000 census, using block group level data. The general model furnishes good descriptions for both monocentric and polycentric cities.

Vulnerability assessment of rainfall-induced debris flows in Taiwan

Abstract: A GIS-based decision support system, which incorporates local topographic and rainfall effects on debris flow vulnerability is developed. Rainfall at a scale compatible with the digital elevation model resolution is obtained using a neural network with a wind-induced topographic effect and rainfall derived from satellite rain estimates and an adaptive inverse distance weight method (WTNN). The technique is tested using data collected during the passage of typhoon Tori-Ji on July 2001 over central Taiwan. Numerous debris flows triggered by the typhoon were used as control for the study. Our results show that the WTNN technique outperforms other interpolation techniques including adaptive inversed distance weight (AIDW), simple kriging (SK), co-kriging, and multiple linear regression using gauge, and topographic parameters. Multiple remotely-sensed, fuzzy-based debris-flow susceptibility parameters are used to describe the characteristics of watersheds. Non-linear, multi-variant regressions using the WTNN derived rainfall and topography factors are derived using self-organizing maps (SOM) for the debris flow vulnerability assessment. An index of vulnerability representing the degrees of hazard is implemented in a GIS-based decision support system by which a decision maker can assess debris flow vulnerability.

Introduction - Segregation and neighborhood change: Where are we after more than a half-century of formal analysis

Abstract: Beginning with the work of University of Chicago sociologists in the 1920s, social scientists have been interested in explaining the uneven group distributions of individuals and households as well as the dynamics driving these distributions by using a human ecological approach (McKenzie, 1924; Park et al., 1925; Park, 1936). By the 1950s, this approach began to take a sharp turn toward formal quantitative analysis, a turn that marked the beginning of segregation analysis. What was diminished by this change was the attention paid to both the spatial dimensions of local unevenness and the dynamic processes of neighborhood change. The first formal measure of segregation probably was introduced by Bell (1954), yet Duncan and Duncan’s (1955) dissimilarity index D was used far more widely to summarize residential segregation patterns for entire metropolitan areas. There are some disciplinary variations; for example, economists favor using the Gini index (Silber, 1989), which also has a long history, when measuring income segregation. But the study of segregation has become truly interdisciplinary, expanding from its sociological roots to attract the attention of economists (Galster, 1988; Bayer et al., 2004), geographers (Clark, 1986; Wong, 1993), and urban policy analysts and planners (Jargowsky, 1996; Dawkins, 2004).

Integrating computational fluid dynamics (CFD) models with GIS: an evaluation on data conversion formats

Abstract: Computational fluid dynamics (CFD) models are powerful computational tools to simulate urban-landscape scale atmospheric dispersion events. They are proven to be very useful for security management and emergency response. Essential inputs to CFD models include landscape characteristics, which are often captured by various GIS data layers. While it is logical to couple GIS and CFD models to take advantage of available GIS data and the visualization and cartographic rendering capabilities of GIS, the integration of the two tools have been minimal. In this paper, we took the first step to evaluate the use of GIS data in CFD modeling. Specifically, we explore how efficient is to use GIS data in CFD models and how sensitive the CFD results are to different GIS data formats. Using campus topography and building data, and the FEFLO-URBAN CFD model, we performed atmospheric release simulations using topographic data in contour and raster formats. We found that using raster format was quite efficient and contour data required significant effort. Though the simulation outputs from the two data formats were not identical, their overall outcomes were similar and did not post alarming discrepancies. We concluded that using GIS data have tremendous potential for CFD modeling.

Introduction-Further Innovations in Segregation and Neighborhood Change Research

Abstract: This special issue is the second in a two-part series emphasizing recent innovations in the measurement and modeling of segregation and neighborhood change. The segregation/neighborhood change literature has blossomed over the past century from empirical descriptions of the social ecology of Chicago neighborhoods to intermetropolitan comparisons of segregation dynamics over time. During the last two decades, scholars have also begun to incorporate innovations from the field of spatial statistics into segregation metrics, a development that has substantially improved our understanding of the different spatial “dimensions” of segregation (Massey and Denton, 1988). Another important development since 1990 has been the enhanced interdisciplinary and geographic scope of segregation studies. From its early beginnings in the Chicago School of sociology, segregation is now being studied by scholars from a variety of disciplines, including geography, economics, education, and urban planning. Recent international comparisons have provided new insights on the role of globalization and national policies in shaping patterns of segregation and neighborhood change at both the neighborhood and metropolitan level. The three articles in this special issue elaborate on these themes, examining the influence of metropolitan land-use patterns on segregation, the contribution of mixed-race households to estimated levels of segregation, and the influence of spatio-perceptual mental models on segregated patterns of urban space utilization. 1 We are grateful to Bill Clark of UCLA, who encouraged us to launch this special-issue project after we organized two special sessions on segregation measurement at the Association of American Geographers Annual Meeting in 2006. Several articles accepted in this and a previous special issue of Urban Geography were originally presented as papers in the two special sessions on methodology; these articles have addressed the theory of segregation, segregation as a cause or outcome of other social processes, and related issues. 2

Treatment of exposed explants with donor sclera and amniotic membrane.

Abstract: Dear Editor: Exposure of scleral explants is not an uncommon postoperative complication [2]. It can lead to acute or acute-onchronic infection that is resistant to antibiotic treatment [3]. The symptoms are chronic discharge and discomfort. Early removal of the explant leads to resolution of the infection and symptoms in most patients. Whilst it is safe to do so in the majority of patients, recurrent re-detachment can occur in between 4 and 47% of patients following explant removal [10]. Particularly at risk are patients with a history of multiple detachment procedures, those with residual vitreoretinal traction, patients with proliferative vitreoretinopathy, those with multiple small post-oral breaks and those in whom the retinal break was not identified at the time of retinal surgery [7]. In these patients, retaining the explant in situ may be the safer option. Simply re-suturing the conjunctiva leads to recurrent breakdown of the wound. We report on the use of sclera and amniotic membrane (AM) to deal with exposed surgical explants. A 68-year-old woman (Case 1) was referred with discomfort in the left eye. Five years ago she had two retinal detachment operations involving pars plana vitrectomies and at her last operation a 276 encircling explant was applied. Her recent symptom was due to a breakdown of the conjunctiva in the inferonasal quadrant (INQ) where the explant was exposed. We applied a donor scleral patch graft to cover and secure the explant. There was a large epithelial defect and the surrounding conjunctiva and Tenon could not be sufficiently mobilised. We used AM and sutured it over the scleral graft using 10/0 Vicryl. At the end of surgery a lid suture was applied and the eye was kept shut for 1 week. At the 12-month follow-up the scleral explant was well covered and the retina was attached. A 36-year-old woman (Case 2) was referred with discomfort in the right eye. Six years ago the patient was treated for a retinal detachment with scleral buckling procedure plus a segmental 276 circumferential explant in the superotemporal quadrant (STQ) as well as a 40-band encirclement. On examination, the 276 scleral explant was exposed in STQ (Fig. 1a) and a Watzke sleeve was found to be prominent over the explant. We performed a scleral graft to cover the explant, replaced the Watzke sleeve with a 5/0 mattress suture and mobilised the conjunctiva and Tenon to cover the scleral graft. A tarsorrhaphy suture was applied for 1 week (Fig. 1b). At the 12-month follow-up the area was well healed (Fig. 1c). Neither of the patients had clinical evidence of infection (significant conjunctival hyperaemia or purulent conjunctival discharge) and both had culture-negative preoperative conjunctival swabs. Postoperatively, both patients were given topical preservative-free chloramphenicol 0.5% and prednisolone 0.5% four times a day for 4 weeks. Neither of the patients were receiving sub-conjunctival antibiotics or steroids at the time of surgery. Biomaterial such as temporalis fascia, fascia lata and preserved pericardium have been used to cover exposed explants [1, 10]. There are also several reports of the use of donor sclera [5, 9]. Irrespective of the material used, recurrent wound dehiscence and breakdown occurred in a Graefe`s Arch Clin Exp Ophthalmol (2007) 245:915–916 DOI 10.1007/s00417-006-0502-4