摘要:

Spatial relations are important in numerous domains, such as Spatial Query Languages, Image and Multimedia Databases, Reasoning and Geographic Applications. This paper is concerned with the retrieval of topological and direction relations using spatial data structures based on Minimum Bounding Rectangles. We describe topological and direction relations between region objects and we study the spatial information that Minimum Bounding Rectangles convey about the actual objects they enclose. Then we apply the results in R-trees and their variations, R-trees and R*-trees, in order to minimize the number of disk accesses for queries involving topological and direction relations. We also investigate queries that express complex conditions in the form of disjunctions and conjunctions, and discuss possible extensions.

ISSN:1365-8816    

DOI:10.1080/136588197242428

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Many problems of practical interest involve line-of-sight on a topographic surface. Some such problems can be successfully studied on the basis of the mutual visibility among a finite number of representative points. Such visibility problems can be formalized and resolved as graph problems. In this paper, we show that graph algorithms can be useful to find efficient solutions for discrete visibility problems in several cases. On the basis of results from the theory of complexity, we give some practical rules to apply such an approach. We further investigate the solution of some relevant visibility problems under this perspective.

ISSN:1365-8816    

DOI:10.1080/136588197242437

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This paper presents a spatial access method based on the B+ -tree structure. This new structure is called SB+ -tree, and should allow commercial databases an access method for spatial objects without major changes, since most commercial databases already support B+ -tree as an access method for text data. SB+ -tree is a hybrid of the existing spatial access methods. For each axis of the space, a set of indexing points is generated, where an indexing point is created whenever a new minimum bounding rectilinear rectangle (or MBR) begins or ends. The indexing points are then used to create an SB+ -tree. The number of SB+ -trees generated is dependent upon the number of dimensions of the approximation of the object, and not on the number of relations. We have developed an algorithm which uses SB+ -tree for performing spatial join between two spatial relations. Through simulation, we show that the performance of spatial join using SB+ -tree is more efficient than that of R+ -tree.

ISSN:1365-8816    

DOI:10.1080/136588197242446

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

To derive efficient predictions from a learning set provided by a geographical information system (GIS) needs to take into account errors that occur because of the poor quality of source maps and error propagation through GIS procedures. Using classification tree analysis, our objective was to build an approach which could take these errors into account, and so provide robust prediction rules for mapping natural systems. Our application addresses soil unit predictions from topographical and geological data. The learning set is taken from a sample area located in a French Mediterranean valley in which a large scale soil survey was conducted. We propose a rule for only conserving, among the splits of an initially large classification tree, the ones that provide a significant gain of purity (i.e. precision of soil predictions). This rule needs an estimate of the standard error on the purity index. This is achieved by evaluating the errors in the learning set, and then studying their propagation through the classification tree. The severity of the rule can be adjusted according to a risk threshold defined by the user. Our results indicate that the new rule has a strong influence on classification tree size. Furthermore, comparison with actual soil maps indicates that this rule provides more robust predictions than a classical pruning technique used in classification tree analysis. The soil predictions we obtain provide a medium scale soil map.

ISSN:1365-8816    

DOI:10.1080/136588197242455

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

This paper examines the impact of DEM resolution on the accuracy of terrain representation and of the gradient determined. The DEMs of three terrain types with diverse complexities were constructed from digitizing contours and then kriging. The accuracy (RMSE) of the DEMs was regressed against contour density (D) and DEM resolution (S) at six resolution levels. It is found that RMSE may be expressed as (7.274 1.666S)D/1000 with an R value of 0.9659. The representation accuracy decreases moderately at an intermediate resolution, but sharply at coarse resolutions for all three terrain types. Resolution reduction profoundly affects the gradient determined from the DEM. While exerting little influence on mean gradient, resolution significantly affects the standard deviation of gradient, especially for a simple terrain. The results obtained in this study may be used to determine the DEM resolution that is appropriate to the accuracy requirements of a particular user.

ISSN:1365-8816    

DOI:10.1080/136588197242464

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor