Probabilité d'apparition d'un phénomène parasitaire et choix de modèles de régression logistique

ACTNNational audienceEpidemiological processes are now using spatial statistics and modelling tools. The main objective of most health risks studies consists in identifying potential contamination sources and factors capable of explaining their localization. Health data often prove binary (typically presence/absence) and specific methods such as binary logistic regression have to be used. This method's output consists in a probability for the pathogen of interest. A posterior classification of each sample is then conducted using a probability threshold. The method used to maximize this threshold is called the ROC curve which consists in giving a representation of the behaviour of the model and then to choose the optimal value to discriminate between negative and positive predictions. Using a point data epidemiological base, several models were generated and tested. Landscape indices have been derived in the environment of the points at three scale levels. Probability values allocated to each sample were then spatially represented, giving an insight on the expected geographical dispersion of contaminated samples. Variables identified in the models were then used to establish hypothesis as to which landscape factors might play a role in epidemiological processes. The outlining of potential high risk areas is a result of first importance in the geography of health.Les processus épidémiologiques sont de plus en plus fréquemment abordés à l'aide des outils de la statistique spatiale et de la modélisation. Ces travaux ont généralement pour but de mettre en évidence des foyers de contamination et d'identifier des variables pouvant expliquer leur présence. Le caractère binaire des données sanitaires nécessite la mise en oeuvre de méthodes spécifiques comme la régression logistique binaire, qui permet d'attribuer à chaque échantillon une probabilité de présence de l'agent pathogène d'intérêt. La classification postérieure de chaque échantillon nécessite le choix d'un seuil de probabilité pour définir le caractère à risque ou sans risque des observations. La méthode ROC permet de définir ce seuil et de comprendre précisément les conséquences de ce choix. À partir d'une base de données épidémiologique ponctuelle, plusieurs modèles ont été générés et testés. Des indices paysagers ont été dérivés dans l'environnement des points à trois échelles d'analyse. Les probabilités attribuées à chaque échantillon par les modèles ont été représentées spatialement ce qui donne un outil d'interprétation de la répartition attendue des échantillons contaminés. Les variables identifiées à ces trois niveaux d'échelle ont conduit à ébaucher des hypothèses quant aux facteurs paysagers qui entrent en jeu dans les processus épidémiologiques. L'apparition de foyers de risque potentiel constitue un premier résultat

Les nuées dynamiques floues appliquées au développement dune typologie paysagère

International audienceUn programme d'épidémiosurveillance actuellement en cours vise à évaluer la présence du parasite Echinococcus multilocularis en France. Contrairement aux hypothèses écologiques initiales, le parasite et ses hôtes ne semblent pas privilégier certains modes d'occupation du sol plus que d'autres. L'objectif de ce travail consiste à identifier dans un premier temps les contextes paysagers au sein desquels la présence parasitaire est confirmée puis, dans un second temps, à mettre en évidence un gradient dans les prévalences spécifiques à chacun de ces contextes. Les nuées dynamiques floues ont permis d'optimiser et de critiquer la typologie paysagère établie. La hiérarchisation de cette typologie en fonction des prévalences constatées sur le terrain a permis de donner un éclairage nouveau sur le risque de présence du parasite et sur les facteurs favorisant sa présence

Analysis of the intensity of agrarian exploitation by spatial analysis of ancient field systems preserved by forest cover

As part of the framework for the ArchaeDyn project, this work aims to understand spatial discontinuities in ancient land use through the analysis of ancient agrarian structures preserved by forest cover in Lorraine and Burgundy, northeast France. The first step consisted of examining the intensity of landuse (the density of linear features, stone piles, and the mean length of features) and its structuring (degree of closure, shape and surface area of field parcels) indicators. The second step consisted of spatially analysing these parameters in order to identify homogeneous zones. These zones will then be compared to environmental and human factors in an attempt to understand how these field systems were established

Confidence maps: a tool to evaluate archaeological data'srelevance in spatial analysis

Inventory data used in archaeology is often incomplete and heterogeneous. In the framework of the ArchaeDyn program, a method has been proposed to evaluate heterogeneity in archaeological inventories. The purpose of this work is to create a validation tool to interpret the results. This tool is called a “confidence map” and is produced by combining representation and reliability maps. The first step consists of generating representation maps to describe the clustering of archaeological items. The second step is based on reliability maps. Data providers are asked to define and outline the level of reliability of their data. Then the representation and reliability layers are combined using map algebra. The resulting maps allow for the comparison and analysis of data confidence

Intérêt de l’utilisation de la photogrammétrie SfM (Structure from Motion) pour le suivi des processus nivo-glaciaires. Application au bassin du glacier Austre Lovén, Spitsberg

National audienceL’Arctique est reconnu comme un ex¬cellent indicateur des évolutions clima¬tiques contemporaines : sa sensibilité aux moindres sollicitations climatiques en fait une zone d’étude idéale. L’étude des pro¬cessus et des dynamiques glaciaires revêt, dans ce contexte, un intérêt majeur afin de mieux comprendre les impacts du climat sur la récession glaciaire observée durant cette dernière décade

Assessing the relevance of digital elevation models to evaluate glacier mass balance : application to Austre Lovénbreen (Spitsbergen, 79 ° N)

International audienceThe volume variation of a glacier is the actual indicator of long term and short term evolution of the glacier behaviour. In order to assess the volume evolution of the Austre Lovénbreen (79 • N) over the last 47 years, we used multiple historical datasets, complemented with our high density GPS tracks acquired in 2007 and 2010. The improved altitude resolution of recent measurement techniques, including phase corrected GPS and LiDAR, reduces the time interval between datasets used for volume subtraction in order to compute the mass balance. We estimate the sub-metre elevation accuracy of most recent measurement techniques to be sufficient to record ice thickness evolutions occurring over a 3 year duration at polar latitudes. The systematic discrepancy between ablation stake measurements and DEM analysis, widely reported in the literature as well as in the current study, yields new questions concerning the similarity and relationship between these two measurement methods. The use of Digital Elevation Model (DEM) has been an attractive alternative measurement technique to estimate glacier area and volume evolution over time with respect to the classical in situ measurement techniques based on ablation stakes. With the availability of historical datasets, whether from ground based maps, aerial photography or satellite data acquisition, such a glacier volume estimate strategy allows for the extension of the analysis duration beyond the current research programmes. Furthermore, these methods do provide a continuous spatial coverage defined by its cell size whereas interpolations based on a limited number of stakes display large spatial uncertainties. In this document, we focus on estimating the altitude accuracy of various datasets acquired between 1962 and 2010, using various techniques ranging from topographic maps to dual frequency skidoo-tracked GPS receivers and the classical aerial and satellite photogrammetric techniques

Where does a glacier end ? GPR measurements to identify the limits between the slopes and the real glacier area. Application to the Austre Lovénbreen, Spitsbergen -- 79°N

International audienceGlacier limits are usually mapped according to a spatial discrimination based on color of remote sensing images or aerial photography. What appears like ice (white or light colored areas) at the end of the ablation period (end of summer) corresponds to the glacier, while what appears as rock (dark areas) is identified as the slope. This kind of visual discretization seems to be insufficient in the case of small arctic glaciers. Indeed, the slopes have been described as very unstable parts of glacial basins. Debris are generated by the inclination of the slopes, and reach the glacier surface. Thus, the visible limit does not correspond to the ice extension: a significant amount of ice is potentially covered by rock debris, enlarging the actual glacier surface with respect to the observed area. Hence, we apply Ground Penetrating Radar (GPR) measurements for mapping, beyond the central parts of the glacier, the steep slopes of the Austre Lovénbreen (Spitsbergen, 79°N). The aim is to assess the discrepancy between the limits extracted from remote sensing methods -- aerial photography, satellite images and derived digital elevation models -- and the GPR data which exhibit significant ice thickness at locations considered outside the glacier itself. The ice is observed to extend typically from 25 to 30 meters, and up to 100~meters, under the slopes. These measurements allow for a new determination of the rock/ice interface location following criteria beyond the visual and morphological characteristics seen from the surface, as obtained by remote sensing techniques or in-situ observations

Approche multivariée des facteurs influençant le bilan de masse d’un glacier arctique

National audienceL’Arctique est la zone géographique où la modification du climat au cours du XXIieme siècle sera la plus forte de la planète. La totalité de la cryosphère (banquise, glaciers, pergélisol, etc.) subira de profondes altérations qui se traduiront spatialement par des évolutions majeures. Les glaciers sont de bons indicateurs de ces changements, aussi bien à une échelle locale que globale, car leur dynamique est très sensible au climat. Cette étude porte sur un glacierdu Spitsberg où il a été observé que des conditions climatiques et nivologiques contrastées d’une année sur l’autre avaient des conséquences multiples, et parfois inattendues, sur les bilans de masse

Confidence maps: a tool to evaluate archaeological data's relevance in spatial analysis

L'article est disponible en ligne sur Virtual Library Propylaeum : http://archiv.ub.uni-heidelberg.de/propylaeumdok/volltexte/2010/551/pdf/08_05_ostir_et_al_confidence.pdfInternational audienc

High density coverage investigation of The Austre LovénBreen (Svalbard) using Ground Penetrating Radar

COMInternational audienceA three week field survey over April 2010 allowed for the acquisition of 120 Ground Penetrating Radar (GPR) profiles, adding to a 40 km long walk across an Arctic glacier. The profiles were acquired using a Mal°a equipment with 100 MHz antennas, walking slowly enough to record a 2.224 s trace every 30 cm on the average. Some acquisitions were repeated with 50 MHz or 200 MHz antenna to improve data quality. The GPR was coupled to a GPS system to position traces. Each profile has been manually edited using standard GPR data processing, to pick the reflection arrival time from the ice-bedrock interface. Traveltimes were converted to ice thickness using a velocity of 0.17 m/ns. Dual-frequency GPS mapping and snow coverage thickness were acquired during the same survey. Using interpolation methods, we derived the underlying bedrock topography and evaluated the ice volume