Algorithm and implementation uncertainty in viewshed analysis

Abstract

In most documentation of geographical information systems (GIS) it is very rare to find details of the algorithms used in the software, but alternative formulations of the same process may derive different results. In this research several alternatives in the design of viewshed algorithms are explored. Three major features of viewshed algorithms are examined: how elevations in the digital elevation model are inferred, how viewpoint and target are represented, and the mathematical formulation of the comparison. It is found that the second of these produces the greatest variability in the viewable area (up to 50 per cent over the mean viewable area), while the last gives the least. The same test data are run in a number of different GIS implementations of the viewshed operation, and smaller, but still considerable, variability in the viewable area is observed. The study highlights three issues: the need for standards and/or empirical benchmark datasets for GIS functions; the desirability of publication of algorithms used in GIS operations; and the fallacy of the binary representation of a complex GIS product such as the viewshed.     

Integrating geographical information systems and multiple criteria decision-making methods

Abstract

Many spatial decision-making problems, such as site selection or land use allocation require the decision-maker to consider the impacts of choice-alternatives along multiple dimensions in order to choose the best alternative. The decision-making process, involving policy priorities, trade-offs, and uncertainties, can be aided by Multiple Criteria Decision making (MCDM) methods. This paper presents a framework for integrating geographical information systems (GIS) and MCDM methods. In this framework the MCDM methods are classified and matched with choice heuristics used by the decision-makers in the presence of competing alternatives and multiple evaluation criteria. Two strategies for integrating GIS with MCDM are proposed. The first strategy suggests linking GIS and MCDM techniques using a file exchange mechanism. The second strategy suggests integrating GIS and MCDM functions using a common database. The paper presents the implementation of the first strategy using PC-ARC/INFO, a file exchange module, and four different MCDM computer programs.     

Utilizing discriminant function analysis with a geographical information system to model ecological succession spatially

Abstract

A raster/quadtree geographical information system (GIS) was established For a 932 ha natural area in central Missouri. The GIS contained map layers depicting vegetative cover from 1939 to 1982, soils, topographic aspect and distance from a forest seed source. Discriminant function analysis was used to quantify and describe ecological succession on the area during this period. The calibration of discriminant functions is discussed, as are the statistical and spatial validations of the model. It was concluded that this approach provides a useful technique to examine long-term temporal phenomena in a spatial context.     

A geographical information system for some Mediterranean benthic communities

Abstract

This paper describes a regional geographical information system (GIS) for some Mediterranean benthic communities. The area covered by the GIS lies between the cities of La Ciotat and Giens in southeast France. The distinctive characteristics of this GIS compared with others usually described in the literature, are that all its layers describe the same theme but as seen at different moments with different scales and techniques used by different oceanographers. A method was devised to synthesize, on a pixel basis, the content of all these layers. Each pixel within each layer is weighted with a function relating to the year of survey, the sampling technique and the scale of the original map corresponding to that layer. The synthesis map is composed of the highest weighted values found in the set of layers. Also at each pixel, conflicts between the contents of layers are quantified and mapped.     

A spatial data model design for feature-based geographical information systems

Abstract

In state-of-the-art GIS, geographical features are represented as geometric objects with associated topological relations and classification attributes. Semantic relations and intrinsic interrelations of the features themselves are generally neglected. In this paper, a feature-based model that enhances the representation of geographical features is described. Features, as the fundamental depiction of geographical phenomena, encompass both real world entities and digital representation. A feature-based object incorporates both topological relations among geometric elements and non-topological (semantic) relations among features. The development of an object-oriented prototype feature-based GIS that supports relations between feature attributes and feature classes is described. Object-oriented concepts such as class inheritance and polymorphism facilitate the development of feature-based GTS.     

Quadtree storage of vector data†

Abstract

Vector data storage has various advantages in a cartographic or geographical information system (GIS) environment, but lacks internal spatial relationships between individual features. Quadtree structures have been extensively used to store and access raster data. This paper shows how quadtree methods may be adapted for use in spatially indexing vector data. It demonstrates that a vector quadtree stored in floating point representation overcomes the classical problem with raster quadtrees of data approximation. Examples of vector quadtrees applied to realistic size data sets are given     

Environmental decision support system project: an exploration of alternative architectures for geographical information systems

Abstract

Recent changes in information technology offer the opportunity to explore alternative architectures for geographical information systems (GIS) which might better support advanced applications. This paper describes the architecture and implementation of the environmental decision support system (EDSS), a prototype GIS tool kit. The architecture is based on a simple yet powerful systems model using only data collections, views and operations as the basic entity types. The design of the user interface, data management and data analysis within the model are outlined, with particular emphasis on the advanced facilities for which implementation is simplified by the architecture. A prototype applications system, BANKSIA, is also described.     

A spatial analytical perspective on geographical information systems

Abstract

The field of geographical information systems (GIS) is reviewed from the viewpoint of spatial analysis which is the key component of the familiar four-part model of input, storage, analysis and output Input is constrained by the limits of manual methods and problems of ambiguity in scanning. The potential for developments in output is seen to be limited to the query mode of GIS operation, and to depend on abandoning the cartographic model. Discussion of storage methods is organized around the raster versus vector debate and the need to represent two spatial dimensions in one. A taxonomy of GIS spatial analysis operations is presented together with a generic data model. Prospects for implementation are discussed and seen to depend on appropriate scales of organization in national and international academic research.     

Developing and testing of an error propagation model for GIS overlay operations

Abstract

This study examines the propagation of thematic error through GIS overlay operations. Existing error propagation models for these operations are shown to yield results that are inconsistent with actual levels of propagation error. An alternate model is described that yields more consistent results. This model is based on the frequency of errors of omission and commission in input data. Model output can be used to compute a variety of error indices for data derived from different overlay operations.     

Constructing Interfaces between (and within) Geographical Information Systems

Abstract

An interface is a software package used, alone or in conjunction with other interfaces, to transfer data from one geographical information system to another, or between different representations within a single GIS. The aim in this paper is to extend the traditional view of an interface to include interfaces that use communication networks to move data between (and within) GIS. We suggest that, to exchange data across a network, the best solution may be to construct a set of communicating interfaces, each able to send data to, and receive data from, other communicating interfaces. We describe a working set of communicating interfaces that transfer data between a number of GIS data representations.     

Geographic Information Technologies and the History of a Shoreline: The Case of Chapter 91 in New Bedford,Massachusetts

Abstract

This article discusses the use of geographic information technologies to assist researchers in the exploration of historical databases. The case study is a pilot project in which we used geographic information systems (GIS) and global positioning systems (GPS) to study the history of the Public Waterfront Act (the “Chapter 91” program), a policy designed to balance private property rights, public interest, and environmental protection in the Massachusetts tidelands. The issues discussed range from the role of GIS in society and its limitations as a representational tool to the ability of current GIS to deal with historical data and to manage temporal attributes.

     

Teaching generic GIS using commercial software

Abstract

The debate between training and education in GIS in a university or college setting and whether or not commercial systems should be used in such institutions may be far from resolution, but an indication of the level of interest shown in hands-on, computer-based GIS education using a commercial system is the appearance and growth of the Institute for GIS in Education. The Curriculum Development Toolkit produced by the Institute has involved academic input and evaluation, and contains both generic GIS background information and system-specific computer-based learning materials, used in over 200 academic institutions world-wide.     

The economic evaluation of implementing a GIS

Abstract

Implementation of geographical information systems (GIS) involves a substantial commitment of resources by the sponsoring agency. Economic evaluation of the proposed GIS is an important step in the implementation process. This article discusses the traditional benefit cost analysis and suggests alternative approaches which may prove useful in situations where the benefit cost model may not be appropriate. A case study of an actual implementation of a GIS which illustrates the approaches discussed is also presented.     

Fundamental operations in computer-assisted map analysis

Abstract

Geographical information systems (GIS) provide capabilities for the mapping, management and analysis of cartographic information. Unlike most other disciplines, GIS technology was born from specialized applications. A comprehensive theory relating the various techniques used in these applications is only now emerging. By organizing the set of analytic methods into a mathematical structure, a generalized framework for cartographic modelling is developed. Within this framework, users logically order primitive operators on map variables in a manner analogous to traditional algebra and statistics. This paper describes the fundamental classes of operations used in computer-assisted map analysis. Several of the procedures are demonstrated using a fourth-generation computer language for personal computers.     

Editorial

Abstract

Project GeoSim is a multidisciplinary effort by members of Virginia Tech's Departments of Geography and Computer Science, College of Education, and Learning Resources Center to develop computer-aided education (CAE) software for introductory geography and related classes. GeoSim laboratory exercises draw on the five fundamental themes of geography for their subject matter. The programs emphasize interactive learning combining the information capabilities of Geographic Information Systems (GIS) with the techniques of computer simulation. The result is a series of geographic explorations that will make available some of the most exciting aspects of geography to a potential audience of 425,000 introductory geography students per year.