Assessing Quality of Spatial Models Using the Structural Similarity Index and Posterior Predictive Checks

Model assessment is one of the most important aspects of statistical analysis. In geographical analysis, models represent spatial processes, where variability in mapped output results from uncertainty in parameter estimates. Slight spatial misalignments can cause inflated error scores when comparing maps of observed and predicted variables using traditional error metrics at the level of individual spatial units. We conceptualize spatial model assessment as a continuous value map comparison problem and employ methods from image analysis to score model outputs. The structural similarity index, a measure that attempts to replicate the human visual system using a local region approach, is used as an exploratory map comparison statistic. The measure is implemented within a Bayesian spatial modeling framework as a discrepancy measure in a posterior predictive check of model fit. Results are reported for simulation studies representing a variety of spatial processes in a spatial and space–time context. A case study of rainfall mapping in Sri Lanka demonstrates the proposed methodology applied to assessment of Bayesian kriging interpolations. Both simulation studies as well as the case study demonstrate that the approach reveals hidden spatial structure not uncovered by traditional methods. The spatially sensitive assessment methodology provides a diagnostic tool to support spatial modeling and analysis. La evaluación de modelos es uno de los aspectos más importantes de análisis estadístico. En el análisis geográfico, los modelos representan procesos espaciales en los que la variabilidad en los outputs es el resultado de la incertidumbre en los parámetros estimados. Leves desajustes espaciales pueden inflar los valores de error en la comparación entre los mapas de las observaciones y los mapas de las predicciones de las variables si es que se usan medidas tradicionales de medición de error al nivel de unidades espaciales individuales. Los autores conceptualizan la evaluación de modelos espaciales como un problema de comparación mapas de valor continuo y emplea métodos de análisis de imágenes para cuantificar los resultados del modelo. Se utiliza el índice de similitud estructural (SSIM), una medida que intenta replicar el sistema visual humano utilizando un enfoque de región local, como técnica de exploratoria comparación estadística de mapas. El índice es implementado dentro de un marco de modelización espacial bayesiano como medida de discrepancia en la comprobación posterior de predicción del desempeño del modelo. Los resultados se presentan para varios casos de simulación que representan una serie de procesos espaciales en un contexto espacio‐temporal y espacial. Un estudio de caso de mapeo de lluvias en Sri Lanka sirve como demostración de la metodología propuesta y su aplicación a la evaluación de las interpolaciones tipo krigeage (kriging) bayesianas. Tanto los estudios de simulación, así como el estudio de caso demuestran que el enfoque propuesto revela la estructura espacial oculta no evidenciada por métodos tradicionales. La metodología de evaluación espacialmente sensible que se presenta en este artículo proporciona una herramienta de diagnóstico para apoyar la elaboración de modelos y análisis espacial. 模型评估是统计分析中最为重要的内容之一。在地理分析中用模型表达空间过程,参数估计的不确定性会导致地图输出结果的可变性。当采用传统误差指标度量,在个体空间单位水平上进行观测和预测变量的地图比较时,微小的空间错位就可能导致误差的倍增。为此,本文通过将空间模型评估指标概念化为一个连续值图比较问题,并利用图像分析方法来评定模型输出。一种尝试以局域方法仿制人类视觉系统的度量指标——结构相似指数(SSIM),被用作为探索性地图的比较统计量。在贝叶斯空间模型框架下实现其量算,并将其作为一个偏差度量应用于模型拟合的后预测校验。仿真研究的结果显示出空间及时空环境下多类空间过程。以斯里兰卡降雨过程图为案例,展示了上述方法对贝叶斯克里格插值的准确性评估。仿真研究与实证结果均证明本文提出的方法可揭示以往传统方法掩盖的空间结构特征,空间敏感性评价为本研究的空间建模和分析提供了一个诊断工具。     

Topographies of citizenship: Purhépechan Mexican women claiming political subjectivities

A variety of politics are waged through recourse to the language of ‘citizenship’ and ‘democracy’: from George W. Bush's selling of free trade for the Americas by invoking freedom and democracy, to the calls for citizenship and equality by popular movements throughout Latin America and other regions. This article links these paradoxical and transnational constructions of ‘citizenship’ to the daily economic and political struggles of indigenous women in rural Mexico. A transnational and what Cindi Katz calls a ‘topographical’ analysis of local processes deepens and complicates our understanding of local changes as they articulate with global dynamics, and it transforms how we conceptualize the global. Drawing on an ethnography of local gendered political transformation in Cherán, Mexico, I map processes visible locally onto spatialities of power and meaning across scales, weaving together various symbolic and material processes—the intentional actions and negotiations of individual women; the history of Cherán as a place and community; neoliberal economic globalization; and the effects of profoundly gendered and racialized nationalisms—in order to produce a situated knowledge of global citizenship politics. This approach highlights how women in Cherán, situated within global political economic relations and the symbolic horizons of ‘modernity’, transform the meaning and practice of citizenship and political subjectivity.     

From limestone caves to concrete graves: 3000 years of mortuary practice in the Palauan archipelago,Micronesia

Human skeletal remains have been discovered from a variety of contexts in the Palauan archipelago of western Micronesia. These include caves, rockshelters, earthen mounds, stone platforms, midden burials, crypts, sarcophagi, and historic period gravesites. Recent excavation of a prehistoric cemetery in a rockshelter on Orrak Island dating from ca 1000 BC–AD 200, combined with nearly contemporaneous surface finds in caves on both Orrak and other nearby islands, shed light on the earliest known burial practices in Palau. Interment in limestone caves and rockshelters was then replaced in succession by burial in earthwork terraces, beneath stone platforms, in middens, within limestone slab crypts and at least one known stone sarcophagus, and finally in Western or Asian‐style gravesites with headstones. Here we present the first major synthesis of mortuary patterns in Palau from the earliest periods of known settlement (ca. 1000 BC) to modern times. Understanding how these burial practices change over time provides valuable insight into changing sociocultural practices within Palauan society, including how contact with outsiders during the historical period drastically altered traditional mortuary behaviours. Copyright © 2007 John Wiley & Sons, Ltd.     

Patronage or Participation? Community‐based Natural Resource Management Reform in Sub‐Saharan Africa

This article examines the institutional factors that account for the outcome of efforts to decentralize control over natural resources to local communities. It focuses on the political nature of institutional processes associated with decentralization in sub‐Saharan Africa through a comparative analysis of wildlife management reforms in seven east and southern African countries. Institutional reforms are largely dependent on state authorities' patronage interests, which in turn are shaped by the relative economic value of wildlife, the degree of central control over commercial utilization, and the accountability of governance institutions. Our findings have a range of practical implications for the design of CBNRM initiatives and institutional reform strategies.     

Correlation between modern plant δN values and activity areas of Medieval Norse farms

This further nitrogen isotopic study of Greenlandic Norse archaeological sites provides new evidence of the affect of past human activity on modern plant δ¹⁵N values. Grass samples were taken from a number of spatially defined features at four Norse farms and one site interpreted to be a seasonally occupied herding station (sæter). At the farms, un-naturally high δ¹⁵N values were found for plants from each sampled feature, including those from the likely infields and a churchyard. At the sæter no strong isotopic effect was expected and none was found. Comparing the data from the farms we found that the average plant δ¹⁵N values were similar for features that were functionally alike. We conclude from these observations, that the unusually high plant δ¹⁵N values reflect the strong conservation of the isotopic composition of the nitrogenous materials deposited in the past, and the magnitude of the values reflects the amount of this material admixed in the soil. The results confirm previous exploratory studies of this isotopic effect and further demonstrate its potential value as an archaeological tool.