摘要:

Although many GIS data models are available, a declarative, operational, well-defined, implementation-independent, and object-oriented language is lacking. Based on the theory of many sorted algebra, this work presents a family of geometric data models. Some geographical data models of urban information systems are illustrated using homomorphism. According to the results, the preferred characteristics of mixing declarative and operational statements, multiple representations, tight interdependency among objects, and integration of vector and raster based systems can be achieved through this mechanism.

ISSN:1365-8816    

DOI:10.1080/136588198241491

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The automation of the map design process through map generalisation continues to be a challenging area of research. It is acknowledged that a diverse range of techniques are applied during the process of map generalisation and these have been mirrored by the creation of a range of algorithms that mimic these discrete operations (such as typification, aggregation, selection). This paper discusses in detail one such algorithm that resolves conflict among objects through displacement. Perhaps more critical than the algorithm itself, is the stage prior to the application of displacement (identification, modelling), and the phase after application (the evaluation). It is argued that these two stages are absolutely critical to the successful design of automated systems. The paper begins with a review of other approaches to displacement and then describes a methodology that encompasses detection, resolution through displacement, and evaluation. This methodology has been implemented in Stratege, an object oriented expert system devoted to contextual generalisation. Details of the implementation are given, and results are illustrated using 'real' geographical data. The results are evaluated and the applicability of the entire methodology is discussed in the broader context of other map generalisation algorithms, based on the explicit representation of constraints.

ISSN:1365-8816    

DOI:10.1080/136588198241509

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Regular grid sampling structures in the plane are a common spatial framework for many studies. Constructing grids with desirable properties such as equality of area and shape is more difficult on a sphere. We studied the distortion characteristics of recursive partitions of the surface of the globe starting with the octahedron and icosahedron polyhedral models. We used five different methods for mapping from the polyhedral model to the surface of the sphere: the Gnomonic projection, Fuller's Dymaxion projection, Snyder's equal area polyhedral projection, direct spherical subdivision, and a recursive polyhedral projection. We increased partition density using both a 4-fold and a 9-fold ratio at each level of recursive subdivision by subdividing to the 8th level with the 4-fold density ratio (65 536 cells per polyhedral face) and to the fifth level with the 9-fold density ratio (59 049 cells per polyhedral face). We measured the area and perimeter of each cell at each level of recursion for each method on each model using each density ratio. From these basic measurements we calculated the range and standard deviation of the area measurement, and the mean, range, and standard deviation of a compactness measurement defined as the ratio of (the ratio of the perimeter to the area of the cell) to (the ratio of the perimeter to the area of a spherical circle with the same area). We looked at these basic measurements and their statistics using graphs of variation with recursion level, sums of squares analyses of variation, histograms of the distributions, maps of the spatial variation, and correlograms. The Snyder projection performed best in area distortion and the Gnomonic projection performed best in compactness distortion. The Fuller projection and the Sphere method had moderate distortion in both area and compactness relative to the worst methods. There was little difference in distortion performance between partitions using the 4-fold density ratio and those using the 9-fold density ratio. Partitions based on the icosahedron performed better for all statistics than those based on the octahedron.

ISSN:1365-8816    

DOI:10.1080/136588198241518

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Digital elevation models (DEMs) given by spheroidal trapezoidal grids are more appropriate for large regional, sub-continental, continental and global geological and soil studies than square-spaced DEMs. Here we develop a method for derivation of topographic variables, specifically horizontal (k) and vertical h (k) landsurface curvatures, from spheroidal trapezoidal-spaced DEMs. First, we v derive equations for calculation of partial derivatives of elevation with DEMs of this sort. Second, we produce formulae for estimation of the method accuracy in terms of root mean square errors of partial derivatives of elevation, as well as k h and k (m and m respectively). We design the method for the case that the v kh k v Earth's shape can be ignored, that is, for DEM grid sizes of no more than 225 km. We test the method by the example of fault recognition using a DEM of a part of Central Eurasia. A comparative analysis of test results and factual geological data demonstrates that the method actually works in regions marked by complicated topographic and tectonic conditions. Upon increasing DEM grid size, one can produce generalised maps of k and k. Spatial distributions of m and m h v kh k v depend directly on the distribution of elevation RMSE. Areas with high values of m are marked by low values of m, and vice versa, areas with high values kh k v of m are marked by low values of m. Data on m and m should be utilised k v kh kh k v to control and improve applications of k and k to geological studies. The method h v developed opens up new avenues for carrying out some 'conventional' raster operations directly on geographical co-ordinates.

ISSN:1365-8816    

DOI:10.1080/136588198241527

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

3-D terrain representation plays an important role in a number of terrain database applications. Hierarchical Triangulated Irregular Networks (TINs) provide a variable-resolution terrain representation that is based on a nested triangulation of the terrain. This paper compares and analyzes existing hierarchical triangulation techniques. The comparative analysis takes into account how aesthetically appealing and accurate the resulting terrain representation is. Parameters, such as adjacency, slivers, and streaks, are used to provide a measure on how aesthetically appealing the terrain representation is. Slivers occur when the triangulation produces thin and slivery triangles. Streaks appear when there are too many triangulations done at a given vertex. Simple mathematical expressions are derived for these parameters, thereby providing a fairer and a more easily duplicated comparison. In addition to meeting the adjacency requirement, an aesthetically pleasant hierarchical TINs generation algorithm is expected to reduce both slivers and streaks while maintaining accuracy. A comparative analysis of a number of existing approaches shows that a variant of a method originally proposed by Scarlatos exhibits better overall performance.

ISSN:1365-8816    

DOI:10.1080/136588198241536

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

This paper explores the quantitative relation between the reliability of slope aspect, gradient, and form mapped from a gridded DEM and the sampling interval (SI) of elevations. Grid DEMs initially interpolated from digitised contours at 10 m were sampled to five other resolution levels. The topographic variables mapped at these SIs were compared with those at 10 m. It is found that the reliability of mapped slope aspect and form is not significantly affected by SI. By comparison, the reliability of slope gradient is more susceptible to SI, especially if it is derived from a gently rolling terrain. Around 90% of the variation in the mapped slope aspect and gradient are accounted for by the inaccuracy of DEMs. A lower percentage exists for slope form. The stability of the mapped topographic variables can be reliably predicted from SI and terrain complexity.

ISSN:1365-8816    

DOI:10.1080/136588198241545

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The concept of computing least-cost paths has been proposed and discussed for a few decades but only in simplified form due to the limited computational resources in the past. With the advancement of computer technology in speed and data storage, it is now possible to implement least-cost path algorithms with realistic conditions. In this paper, we present our implementations of least-cost paths by integrating viewshed information computed from digital elevation models. Our implementations and analyses include four possible types of paths. They are scenic paths, strategic paths, hidden paths, and withdrawn paths. While possible applications of these least-cost paths include planning of civil engineering, military and environmental planning, other extensions can be formulated without much difficulty.

ISSN:1365-8816    

DOI:10.1080/136588198241554

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

ISSN:1365-8816    

DOI:10.1080/136588198241563

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor