Papers by Isabelle Thomas
Governing Cities on the Move
Este articulo presenta los primeros resultados de una encuesta sobre la ocupacion de los geografo... more Este articulo presenta los primeros resultados de una encuesta sobre la ocupacion de los geografos y las geografas llevada a cabo por el Comite Nacional de Geografia Belga. Dicha encuesta muestra un aumento de la proporcion de hombres entre los estudiantes de geografia y un acentuado incremento de los empleos fuera de la educacion. Junto a los recientes desarrollos en la disciplina, estas tendencias generan nuevas presiones en el curriculo de Geografia.
Revue d’Économie Régionale & Urbaine, 2015
« L'espace n'est pas économiquement neutre. Son introduction n'apporte pas des raffinements de dé... more « L'espace n'est pas économiquement neutre. Son introduction n'apporte pas des raffinements de détail : elle change tout » (Ponsard, 1988). De fait, si le rôle de l'espace et des territoires dans la structuration des échanges et des interactions économiques constitue l'objet d'étude des géographes et des auteurs de sciences régionales depuis toujours, ce rôle a été assez peu considéré en économie jusqu'à l'article séminal de Krugman (1991). L'une des explications avancées est celle des contraintes de modélisation (Thisse, 1997) : les externalités, les rendements croissants et la concurrence stratégique sont en effet incontournables dès lors qu'on souhaite parler d'espace de façon pertinente.
Journal of Geographical Systems, 2000
Distances between demand points and potential sites for implementing facilities are essential inp... more Distances between demand points and potential sites for implementing facilities are essential inputs to location-allocation models. Computing actual road distances for a given problem can be quite burdensome since it involves digitalizing a network, while approximating these distances by l pnorms, using for instance a geographical information system, is much easier. We may then wonder how sensitive the solutions of a location-allocation model are to the choice of a particular metric. In this paper, simulations are performed on a lattice of 225 points using the k-median problem. Systematic changes in p and in the orientation of the orthogonal reference axes are used. Results suggest that the solutions of the k-median are rather insensitive to the speci®cation of the l p-norm.
Journal of Geographical Systems
Urban density is central to urban research and planning and can be defined in numerous ways. Most... more Urban density is central to urban research and planning and can be defined in numerous ways. Most measures of urban density however are biased by arbitrary chosen spatial units at their denominator and ignore the relative location of elementary urban objects within those units. We solve these two problems by proposing a new graph-based density index which we apply to the case of buildings in Belgium. The method includes two main steps. First, a graph-based spatial descending hierarchical clustering (SDHC) delineates clusters of buildings with homogeneous inter-building distances. A Moran scatterplot and a maximum Cook's distance are used to prune the minimum spanning tree at each iteration of the SDHC. Second, within each cluster, the ratio of the number of buildings to the sum of inter-building distances is calculated. This density of buildings is thus defined independently of the definition of any basic spatial unit and preserves the built-up topology, i.e. the relative position of buildings. The method is parsimonious in parameters and can easily be transferred to other punctual objects or extended to account for additional attributes.
Physica A: Statistical Mechanics and its Applications, 2019
HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific r... more HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Belgeo, 2000
For this article, we are greatly indebted to M. Loopmans, who was of great help for translating p... more For this article, we are greatly indebted to M. Loopmans, who was of great help for translating parts of the text in English.
Revue d’Économie Régionale & Urbaine, 2002
hemière version 15 juillet 2001, version révisée 30 avril2ffi2. RERU 2002,V,pp. 7 29 -7 60 * Les ... more hemière version 15 juillet 2001, version révisée 30 avril2ffi2. RERU 2002,V,pp. 7 29 -7 60 * Les chiffres entre parenthèses renvoient aux notes en fin d'article.
Nous partons du fait que tout objet spatial i est caractérisé par des coordonnées en x (latitude)... more Nous partons du fait que tout objet spatial i est caractérisé par des coordonnées en x (latitude), en y (longitude) ainsi qu'en z (traditionnellement l'altitude et que nous appellerons dans cet article troisième dimension). La discussion proposée ici s'articule autour de trois aspects : (1) les problèmes issus du passage de la troisième dimension (le volume de l'ellipsoïde terrestre) à la deuxième dimension (le plan de la carte) et les nombreuses erreurs et déformations qui en résultent seront d'abord évoqués. Ensuite, (2) quelques principes élémentaires de cartographie thématique sont rappelés afin de montrer l'importance de la nature de z dans la production d'une image cartographique, et enfin, (3) nous terminons par la discussion du problème de la représentation de cette troisième dimension par les techniques modernes de visualisation, issues des progrès de disciplines proches mais différentes de la cartographie. Note : Cet article reprend l'essentiel de la leçon publique présentée le 26 mai 2000 en vue de l'obtention du titre d'Agrégée de l'Enseignement Supérieur. Conformément au règlement de l'Université catholique de Louvain, l'intitulé de la leçon a été imposé par le jury lors de la soutenance de la thèse principale (publiée : Thomas, 2002). Le titre initial de cette leçon était « La troisième dimension en cartographie ». Le texte proposé ici constitue un rappel des bases de cartographie et de nombreuses interrogations pour l'avenir. Il ne constitue en aucun cas un état des lieux exhaustif ni un apport sur un aspect pointu d'un problème particulier de cartographie. Le texte présenté ci-dessous constitue une version légèrement modifiée d'un article publié .
An urban microeconomic model of households evolving in a 2D cellular automata allows to simulate ... more An urban microeconomic model of households evolving in a 2D cellular automata allows to simulate the growth of a metropolitan area where land is devoted to housing, road network and agricultural/green areas. This system is self-organised: based on individualistic decisions of economic agents who compete on the land market, the model generates a metropolitan area with houses, roads, and agriculture. Several simulation are performed. The results show strong similarities with physical Dieletric breackdown models (DBM). In particular, phase transitions in the urban morphology occur when a control parameter reaches critical values. Population density in our model and the electric potential in DBM play similar roles, which can explain these resemblances.
Social Science & Medicine, 2011
In this paper, we develop and test a method for examining the influence of national level context... more In this paper, we develop and test a method for examining the influence of national level contextual influences on population health. Acknowledging calls for the use of experimental study designs to explore contextual influences on health, we develop a study design in which sets of local areas from Britain and Belgium became akin to two 'treatment' groups; one exposed to British society and culture and the other exposed to Belgian society and culture. The areas are matched on the basis of showing very strong similarities in economic, demographic and historical characteristics. Data describing these characteristics are obtained from national census data. A principal component analysis of these variables permits areas in Britain and Belgium with similar scores on the resulting components to be matched into pairs. A sequence of logistic regression models identifies between-country difference in the risk of reporting poor health. Our final model compares the risk of reporting poor health among Belgians and people from Britain living in similar local contexts, adjusting for any residual differences in individual level characteristics. We compare results from this new method with those from more conventional approaches. All approaches show that residence in Britain is associated with a substantial and significantly higher risk of reporting poor health for both men and women, after adjustment for both individual and local contextual influences. We then critically reflect on our method and on the context-composition framework for research into area variation in health. We conclude that whilst our approach succeeded in applying the idea of comparable groups with different exposures to an observational, international comparison, it also brought associated questions about external validity and the extent to which a sample of matched areas captures a 'national' context.
Journal of Geographical Systems, 2014
In rapidly growing urban areas, it is deemed vital to expand (or contract) an existing network of... more In rapidly growing urban areas, it is deemed vital to expand (or contract) an existing network of public facilities to meet anticipated changes in the level of demand. We present a multi-period capacitated median model for school network facility location planning that minimizes transportation costs, while functional costs are subject to a budget constraint. The proposed Vintage Flexible Capacitated Location Problem (ViFCLP) has the flexibility to account for a minimum schoolage closing requirement, while the maximum capacity of each school can be adjusted by the addition of modular units. Non-closest assignments are controlled by the introduction of a parameter penalizing excess travel. The applicability of the ViFCLP is illustrated on a large US school system (Charlotte-Mecklenburg, North Carolina) where high school demand is expected to grow faster with distance to the city center. Higher school capacities and greater penalty on travel impedance parameter reduce the number of non-closest assignments. The proposed model is beneficial to policy makers seeking to improve the provision and efficiency of public services over a multi-period planning horizon.
Geographical Analysis, 2010
The impact of transportation networks on the location of human activities is a surprisingly negle... more The impact of transportation networks on the location of human activities is a surprisingly neglected topic in economic geography. Using the simple plant location problem, this paper investigates such an impact in the case of a few idealized networks. It is seen that a grid network tends to foster a dispersed pattern of activities, while the center of a radial network acts as an attractor. The case of two economies characterized by different network configurations that form a custom union is then analyzed. It is shown that the structural properties of the networks still hold, though some locations are pulled toward the common border. This suggests that no such relocation should be expected within the European Union if the state members endorse similar fiscal and social policies after the formation of the single market.
Transportation Networks and the Optimal Location of Human Activities: A Numerical Geography Approach. By Isabelle Thomas
Economic Geography, 2009
Reviews the book 'Transportation Networks and the Optimal Location of Human Activities: A Num... more Reviews the book 'Transportation Networks and the Optimal Location of Human Activities: A Numerical Geography Approach,' by Isabelle Thomas.
Cahiers de géographie du Québec, 1996
Cet article a pour objectif de démontrer, par comparaison, que la structure spatiale des accident... more Cet article a pour objectif de démontrer, par comparaison, que la structure spatiale des accidents de la route est indépendante du pays dans lequel le phénomène est mesuré et que seul le facteur d'échelle constitue une différence. Les entités spatiales choisies pour cette démonstration sont le Québec et la Belgique. La structure spatiale des accidents de la route est modélisée et comparée sur la base de trois variables à expliquer et de quatre variables potentiellement explicatives, définissant 52 districts belges et 97 municipalités régionales de comté (MRC) et communautés urbaines (CU) québécoises pour l'année 1991. Les résultats indiquent certaines similitudes quant à la structure spatiale de la densité d'accidents, des taux d'accidents et des taux de mortalité et une différence relative à la structure spatiale de la motorisation. Au Québec comme en Belgique, la densité de population se révèle un bon prédicteur de la densité d'accidents corporels et des taux d...
This paper aims at showing how far the shape of a studied area influences the results of optimal ... more This paper aims at showing how far the shape of a studied area influences the results of optimal location-allocation models. Simulations are performed on rectangular toynetworks with an equal number of vertices but with different length/width ratios. The case of merging two such networks into a common market is also considered. We limit our experience to the Simple Plant Location Problem (SPLP) which captures the fundamental trade-off of economic geography between accessibility and economies-of-scales. Results are analysed in terms of locations, allocations and costs. On the average, we confirm that regions that are elongated require a greater number of facilities that those with a compact shape; this effect however depends upon the way a region is merged into a common market (type of border; relative position). The results help at understanding how far an area (country/region) has larger development problems than others just because of its shape and/or of the way this area is linked within a common market (elongation of the country and length of the common border).
Accident Analysis & Prevention, 2012
The purpose of this study is to gain insight into bicycle accidents. Bicycle accident data and we... more The purpose of this study is to gain insight into bicycle accidents. Bicycle accident data and weekly exposure data were prospectively collected for one year to calculate the incidence rate (IR) of bicycle accidents. An accident was included if it occurred during utilitarian cycling, resulting in an acute injury with corporal damage. If an accident occurred, a detailed questionnaire was filled out to collect detailed information about its circumstances and consequences. A sample of 1087 regular (≥2 cycling trips to work a week) adult (40 ± 10 years) cyclists was analyzed. Over the 1-year follow-up period, 20,107 weeks were covered, accumulating 1,474,978 cycled kilometers. Sixty-two participants were involved in 70 bicycle accidents, of which 68 were classified as 'minor'. The overall IR for the 70 accidents was 0.324 per 1000 trips (95% CI 0.248-0.400), 0.896 per 1000 h (95% CI 0.686-1.106) and 0.047 per 1000 km (95% CI 0.036-0.059) of exposure. Brussels-capital region is the region with the highest IR (0.086; 95% CI 0.054-0.118), with a significantly (P < 0.05) higher IR compared to Flanders (0.037; 95% CI 0.025-0.050). Injuries were mainly caused by 'slipping' (35%) or 'collision with a car' (19%). The accidents caused abrasions (42%) and bruises (27%) to the lower (45%) and upper limbs (41%). Police, hospital emergency department or insurance companies were involved in only 7%, 10% and 30% of the cases, respectively. It is noteworthy that 37% of the participants indicated that they could have avoided the accident. In order to decrease the number of accidents, measures should be taken to keep cycling surfaces clean and decrease the number of obstacles on bicycle infrastructure. Roads and intersections need to be built so that the collisions between cars and bicycles are decreased to a minimum. Car drivers and cyclists should pay more attention towards each other. Underreporting of minor bicycle accidents in Belgium is confirmed, and is higher than expected. Reliable accident statistics, taking into account exposure, are needed to decide which road safety measures are the most effective. The 'safety in numbers' principle is also applicable for minor bicycle accidents.
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Papers by Isabelle Thomas