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Modeling the Uncertainty of Slope and Aspect Estimates Derived from Spatial Databases 总被引:1,自引:0,他引:1
Estimates of slope and aspect are commonly made from digital elevation models (DEMs), and are subject to the uncertainty present in such models. We show that errors in slope and aspect depend on the spatial structure of DEM errors. We propose a general-purpose model of DEM errors in which a spatially auto-regressive random field is added as a disturbance term to elevations. In addition, we propose a general procedure for propagating such errors through GIS operations. In the absence of explicit information on the spatial structure of DEM errors, we demonstrate the potential utility of a worst-case analysis. A series of simulations are used to make general observations about the nature and severity of slope and aspect errors. 相似文献
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Most travel models and theories assume the existence of habitual behavior, yet few empirical studies have examined the question of how much repetition or variability characterizes daily travel. We first review the arguments for expecting both repetition and variability in urban travel-activity patterns and then conduct an empirical analysis of repetition and variability in travel over a five-week period. The results show that, whereas many behaviors that make up the daily pattern are highly repetitious (they occur with high frequency over the 35 days), the similarity between daily travel patterns on different days in an individual's longitudinal record is quite low. Even the day that is the most typical of the individual's travel is a poor indicator of that person's overall travel pattern. Moreover, we found that each person generates several archetypical daily patterns over 35 days, and this finding has important implications for data collection and modeling in studies of urban travel. 相似文献
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The distribution of higher education leadership in state legislatures is examined. It is based on questionnaire and interview information from 285 legislators who exercise influence on education policy in the fifty states, and one-third of whom give special attention to issues of higher education. These higher education leaders differ from education leaders generally in terms of their experience, their location within the legislature, and their effort and relationships. What explains higher education leadership as much as anything else, however, is the nature of the environment in which legislators find themselves. In focusing on eleven states, where higher education is of considerable salience, it is possible to discern how environment combines with other factors to produce such leadership. 相似文献
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