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1.
Abstract

We present a model for describing the visibility of a polyhedral terrain from a fixed viewpoint, based on a collection of nested horizons. We briefly introduce the concepts of mathematical and digital terrain models, and some background notions for visibility problems on terrains. Then, we define horizons on a polyhedral terrain, and introduce a visibility model, that we call the horizon map. We present a construction algorithm and a data structure for encoding the horizon map, and show how it can be used for solving point visibility queries with respect to a fixed viewpoint.  相似文献   

2.
While inventive activity is necessary for industrial economies to progress, little is known about the types of enterprises inventing in Canada. The present study examines a typology of Canadian inventive enterprises at the national, subnational, and urban levels and the types of inventions developed in these areas from 1975 to 1989. Canadian patent and industrial directory information shows that Canada's largest enterprises are the most inventive, followed by small and then medium-sized enterprises. Although Canadian enterprises used teams of inventors to develop a considerably larger share of their inventions by 1989, they became discernably more inventive in only one industry after 1981 while concentrating on many process inventions between 1975 and 1989. Geographically, Canada's two largest cities and its core region were adversely affected by the declining inventiveness of this nation's large and foreign-owned enterprises during the 1980s. With respect to Canada's periphery, many of the characteristics typical of Canadian inventive enterprises were not readily apparent in this region, which had a more specialized inventive base than did the core.  相似文献   

3.
Moving objects produce trajectories, which are typically observed in a finite sample of time‐stamped locations. Between sample points, we are uncertain about the moving objects's location. When we assume extra information about an object, for instance, a (possibly location‐dependent) speed limit, we can use space–time prisms to model the uncertainty of an object's location.

Until now, space–time prisms have been studied for unconstrained movement in the 2D plane. In this paper, we study space–time prisms for objects that are constrained to travel on a road network. Movement on a road network can be viewed as essentially one‐dimensional. We describe the geometry of a space–time prism on a road network and give an algorithm to compute and visualize space–time prisms. For experiments and illustration, we have implemented this algorithm in MATHEMATICA.

Furthermore, we study the alibi query, which asks whether two moving objects could have possibly met or not. This comes down to deciding if the chains of space–time prisms produced by these moving objects intersect. We give an efficient algorithm to answer the alibi query for moving objects on a road network. This algorithm also determines where and when two moving objects may have met.  相似文献   

4.

While inventive activity is necessary for industrial economies to progress, little is known about the types of enterprises inventing in Canada. The present study examines a typology of Canadian inventive enterprises at the national, subnational, and urban levels and the types of inventions developed in these areas from 1975 to 1989. Canadian patent and industrial directory information shows that Canada's largest enterprises are the most inventive, followed by small and then medium-sized enterprises. Although Canadian enterprises used teams of inventors to develop a considerably larger share of their inventions by 1989, they became discernably more inventive in only one industry after 1981 while concentrating on many process inventions between 1975 and 1989. Geographically, Canada's two largest cities and its core region were adversely affected by the declining inventiveness of this nation's large and foreign-owned enterprises during the 1980s. With respect to Canada's periphery, many of the characteristics typical of Canadian inventive enterprises were not readily apparent in this region, which had a more specialized inventive base than did the core.  相似文献   

5.
《Urban geography》2013,34(2):263-300
Negative spatial autocorrelation (NSA), the tendency for dissimilar neighboring values to cluster on a map, may go undetected in statistical analyses of immature Anopheles gambiae s.l., a leading malaria mosquito vector in Sub-Saharan Africa. Unquantified NSA generated from an inverse variance-covariance matrix may generate misspecifications in an An. gambiae s.l. habitat model. In this research, we used an eigenfunction decomposition algorithm based on a modified geographic connectivity matrix to compute the Moran's I statistic, to uncover hidden NSA in a dataset of georeferenced An. gambiae s.l. habitat explanatory predictor variables spatiotemporally sampled in Malindi and Kisumu, Kenya. The Moran's I statistic was decomposed into orthogonal synthetic map patterns. Global tests revealed that |zMC|s generated were less than 1.11 for the presence of latent autocorrelation. The algorithm captured NSA in the An. gambiae s.l. habitat data by quantifying all non-normal random variables, space-time heterogeneity, and distributional properties of the spatial filters.  相似文献   

6.
Lake Nicaragua, the largest lake in Central America, is a promising site for paleolimnological study of past climate change, tectonic and volcanic activity, and pre-Columbian agriculture in the region. It is near the northern limit of the Intertropical Convergence Zone (ITCZ), which brings the rainy season to the tropics, so effects of decreasing precipitation due to southern migration of the ITCZ through the Holocene should be observable. Because fault zones and an active volcano lie within the lake, the long-term impact of tectonic and volcanic activity can also be examined. Finally, the fertile volcanic soils near the lake may have encouraged early agriculture. We analyzed diatoms, biogenic silica (BSi), total organic carbon (TOC), water content, volcanic glass, and magnetic susceptibility in a sediment core from Lake Nicaragua with eleven accelerator mass spectroscopy radiocarbon dates, spanning ~5,700 years. Sediment accumulation rates decreased from the bottom to the top of the core, indicating a general drying trend through the Holocene. An increase in eutrophic diatom abundance suggests that pre-Columbian agriculture impacted the lake as early as ~5,400 cal yr BP. Above a horizon of coarser grains deposited sometime between ~5,200 and 1,600 cal yr BP, planktonic diatoms increased and remained dominant to the top of the core, indicating that water depth permanently increased. Although magnetic susceptibility peaked and water content dipped at the coarse horizon, volcanic glass fragments did not increase, suggesting that the coarse horizon and subsequent increase in water depth were caused by tectonic rather than by volcanic activity. Decreased accumulation rates of BSi and TOC indicate that water became clearer when depth increased.  相似文献   

7.
The Kyoto Protocol's Clean Development Mechanism (CDM) is a carbon credit trading scheme intended to reduce anthropogenic greenhouse gas emissions and promote ‘sustainable development’. Hundreds of CDM‐sponsored hydroelectric dams have been constructed in southwest China's Yunnan Province, where carbon finance contributes substantial financial incentives to hydropower expansion. This article investigates whether riparian Handai farmers settled near the Madushan hydropower plant on the Chinese section of Red River have experienced positive outcomes from this project's participation in the CDM. I assess how Handai individuals' access to core livelihood assets has been modified following dam completion and probe how the CDM reconfigures scalar relations among the various stakeholders involved in hydropower governance in Yunnan. Though the CDM facilitates hydropower expansion, it fails to produce development that is more sustainable than ‘business as usual’ from a local perspective. Rather, the CDM consolidates hydropower governance in the same way as it unfolded in Yunnan before the province became an active participant in this scheme. The CDM also facilitates a national development campaign fostering specific socio‐economic modernization patterns in China's western provinces.  相似文献   

8.
In the summer of 1917 severe floods inundated the city of Tientsin for several months. As far as one could see from the roofs of high buildings, the plain to the west was a vast expanse of water with numerous islands — villages on mounds — and junks sailing between them, and the sun set in a horizon of water as if it were the ocean itself. For several weeks the Tientsin inhabitants had noticed that the water on the plain stood unusually high, but it used to be inundated more or less every third or fourth summer, so no one cared particularly. The Hai Ho — the Sea River — Tientsin's navigable access to the sea — was high and flowed with an exceptionally strong current through the city. But this was needed to clean out the mud from the river's bottom, and shipping people expected good navigation conditions the next few years.  相似文献   

9.
Delhi, the worlds' second most populous city, has experienced rapid, planned and unplanned expansion at the cost of its green cover in recent decades. In this study, we use satellite images from 1986, 1999 and 2010 to map changes in urban and green cover, assess the fragmentation of green spaces, and identify the drivers of change. We find that urban patterns of development have shaped the distribution and fragmentation of green spaces, with the city center containing more green spaces with less fragmentation compared to intermediate areas and the peri-urban periphery. Yet, the city core has also experienced the greatest degree of vegetation clearing and fragmentation over time due to infrastructural expansion, while the peri-urban periphery has shown an increase in vegetation and a decrease in fragmentation due to recent compensatory plantation in these peripheral areas. Forests, archaeological sites, and military and academic campuses have played a major role in protecting green cover and limiting fragmentation in the core and intermediate areas of the city. This research helps in advancing our understanding of the pattern–process relationship between urbanization and land cover change/fragmentation in India's largest city.  相似文献   

10.

Spatial barriers to employment limit women's job opportunities, but their effects differ among racial/ethnic minority groups. This study evaluates the degree of spatial mismatch for minority women and men by comparing the commuting times of African American, Latino, and white workers in the New York metropolitan region. Using Public Use Microdata for 1980 and 1990, we perform a partial decomposition analysis to assess the role of spatial mismatch in lengthening commuting times for minority workers. The results show that African American men and women living in the center of the region have poorer spatial access to employment than their white counterparts. In the suburbs, African American women and Latinas suffer no spatial mismatch; rather, their longer commuting times reflect greater reliance on mass transit. Comparison with 1980 findings reveals little change in spatial mismatch over time despite significant economic and social restructuring in the 1980s. Spatial barriers still limit employment prospects for the majority of minority women living at the core of the region.  相似文献   

11.
《Urban geography》2013,34(5):406-427
This paper examines how computer services (CS) employment is spatially distributed among United States metropolitan counties in 174 metropolitan statistical areas in 1982 and 1993. The analysis assesses whether multiple counties within major MSAs have significant portions of a region's CS employment, and examines the stability of CS employment as a proportion of core and noncore county employment. CS employment is concentrated in core counties, and mean core county employment shares exceed mean noncore shares. More than 60% of MSA data processing (DP) employment is in core counties. The more counties in an MSA, the more relative CS dispersion to noncore counties. The more noncore counties, the smaller the average share of CS employment. Mean noncore CS shares are low and changed little from 1982 to 1993. The pattern is core county CS control. Programming employment has dispersed to noncore counties, but in small shares. Mean DP concentration in core counties exceeds that of programming and maintenance employment. Many noncore counties still have no DP or maintenance employment at all. [Key words: computer services (CS), metropolitan development, centralization, urban employment.]  相似文献   

12.
Abstract: During the past year, the media, public and officialdom have focused on Sir Edmund Hillary, his achievements, and what they mean to New Zealand. In this commentary, we reflect on how they relate to human geography. Although we acknowledge the obvious tensions that exist between adventuring and the contemporary concerns of the discipline, we also illustrate how Hillary's life and actions resonate with many of the discipline's current hopes, aims and challenges. Specifically, we discuss thematic overlaps in the fields of geopolitics and national identities, colonial histories and resistances, as well as the emerging public geography. We posit that ‘Hillary's geography’ is closer to human geography than is realized or at least acknowledged by geographers.  相似文献   

13.
Abstract

This article explores Houston's two Chinatowns within the context of the city's recent urban reinvestment and revitalization initiatives. It provides a history of the Chinatowns and examines their changing condition during the years that Houston was promoting ethnic diversity and urban revival. The analysis is framed within the growth machine perspective, as initially explored in Houston by Joe Feagin, and later by Jan Lin and Igor Vojnovic. Houston's late-20th-century urban redevelopment initiatives occurred during a period in history when Houston's Chinese entrepreneurs lost their influence over the local growth machine. The eventual outcome was the erasure of Houston's downtown Chinatown and the displacement of its residents from their lucrative urban core location. The disappearance of Old Chinatown and other urban ethnic neighborhoods—including Little Saigon and Freedmen's Town—from the urban core all took place as the Houston media and public were celebrating diversity, pluralism, and ethnicity in the city.  相似文献   

14.
In this commentary, we wish to draw from Kathryn's experience in Askole to complicate the argument we developed in favor of an autoethnographic sensibility in the earlier article in this issue ( Butz and Besio 2004 ). Just as we used David's first‐person singular voice in much of that article, we use Kathryn's voice here to reflect the central influence of her research circumstances on the points that we make. We will return to the first‐person plural voice in the commentary's conclusion where we attempt to synthesize the lessons of our two sets of research experiences.  相似文献   

15.
A Pedagogic Framework to Link GIS to the Intellectual Core of Geography   总被引：4，自引：3，他引：1  
《The Journal of geography》2012,111(6):578-591
Abstract

This paper aims to develop a new pedagogic framework for teaching GIS at the college and university level using Berry's geographic matrix. By synthesizing different schools of thought, this paper argues that GIS education essentially involves two aspects—how to teach about GIS and how to teach with GIS. Berry's geographic matrix can be used to tie these dual aspects of GIS education together neatly. As an abstract representation of geographical phenomena, the geographic matrix embeds all three entities of GIS—location, attribute, time—and thus can help GIS instructors teach about GIS. As a synthesis of geographical approaches, the geographic matrix can assist GIS instructors teach with GIS. This paper demonstrates that GIS is actually an implementation of Berry's geographic matrix. Furthermore, the 10 approaches to geographical analysis, originally proposed by Berry for the geographic matrix, can be executed routinely in a GIS environment. By incorporating Berry's geographic matrix into GIS education, teachers can enable students to surpass technical issues and to appreciate the conceptual and functional linkages between GIS and geography's intellectual core.  相似文献   

16.
《Urban geography》2013,34(6):550-578
Because of their intensive need for face-to-face contacts, producer services have, historically, been found at the core of the central business district (CBD). However, it has been suggested that advances in information technologies could lead to the erosion of the CBD's economic base, rendering face-to-face contacts obsolete and enabling producer services to suburbanize. Although a considerable amount of empirical work has been done on the suburbanization of these activities in North America, the same is not true of France. In this paper, we adopt an original methodology to study the role played by face-to-face contacts in the spatial distribution of producer services in the Île-de-France region between 1978 and 1997. Our findings confirm that producer services did indeed suburbanize during the study period. Nonetheless, this suburbanization was multicentric, rather than scattered, suggesting that face-to-face contacts remain an important factor in the location of such services.  相似文献   

17.
Substantial changes in a core idea of geography, integration, have occurred since Alexander von Humboldt published Kosmos (1845-1862). These changes are part of a larger shift in Western civilization to mechanistic reasoning. This shift led to the strengthening of system-based analysis, central to the development of geographic information systems (GIS). The duality of holism and the systems approach has led to an apparent contradiction in geography. R. Hartshorne in The Nature of Geography described this contradiction, but as did Alfred Hettner and Emil Wisotzki before, moved to partial systems as the core concept of geographic integration. Hartshorne's concept of vertical integration is the antecedent for the ubiquitous GIS layer model. The reduction of geographic relationships and processes to mechanistic components (layers) aids the systematic approach, but may lessen geographic understanding of a place's interrelationships. Although the partiality of the system approach was already acknowledged by Finch and Hartshorne in the 1930s, the tension between holistic and system approaches in geography remains. Holism and system-based approaches are indeed complementary methods for developing geographic understanding. Using holistic approaches to understand geographic phenomena, before we teleologically (following a purpose) analyze phenomena as a system, extends GIS to include broader interrelationships of geography in specific locations.  相似文献   

18.
A city can be topologically represented as a connectivity graph, consisting of nodes representing individual spaces and links if the corresponding spaces are intersected. It turns out in the space syntax literature that some defined topological metrics can capture human movement rates in individual spaces. In other words, the topological metrics are significantly correlated to human movement rates, and individual spaces can be ranked by the metrics for predicting human movement. However, this correlation has never been well justified. In this paper, we study the same issue by applying the weighted PageRank algorithm to the connectivity graph or space–space topology for ranking the individual spaces, and find surprisingly that: (1) the PageRank scores are better correlated to human movement rates than the space syntax metrics, and (2) the underlying space–space topology demonstrates small world and scale free properties. The findings provide a novel justification as to why space syntax, or topological analysis in general, can be used to predict human movement. We further conjecture that this kind of analysis is no more than predicting a drunkard's walking on a small world and scale free network.  相似文献   

19.
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20.
Models of urban land use are often used in the policy analysis process to forecast long-range impacts of various land use policies or urban infrastructure projects. For models that have been deemed suitable for multiple implementations, a critical, initial test of the model framework is to examine the model's transferability to other geographies. The purpose of this paper is to present a transferability study of the Large Zone Economic Module (LZEM) of the SE3M model of land use and transportation. The SE3M was initially implemented in the US territory of Guam. The islands of Puerto Rico and Oahu were selected as case studies for this model, transferability evaluation. LZEM is implemented with three base years (2000, 2002 and 2007) for these case studies and then validated to 2010 as horizon year for which validation data was available in the Puerto Rico and Oahu implementations. This paper presents evidence that LZEM is reasonably accurate when applied to other geographic regions and that if specific economic parameters are identified, the entire regional economy and the associated population and employment distribution can be spatially modeled within the LZEM framework.  相似文献   

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