J. Mark Ware

Conflict Reduction in Map Generalization Using Iterative Improvement

Map data are usually derived from a source that is based on a particular scale of representation and hence are subject to a particular degree of map generalization. Attempts to display data at scales smaller than the source can result in spatial conflict, whereby map symbols become too close or overlap. Several map generalization operators may be applied to resolve the problem, including displacement. In this paper we address the problem of displacing multiple map objects in order to resolve graphic conflict. Each of n objects is assigned k candidate positions into which it can possibly move, resulting in a total of kn map realizations. The assumption is that some of these realizations will contain a reduced level of conflict. Generating and evaluating all realizations is however not practical, even for relatively small values of n and k. We present two iterative improvement algorithms, which limit the number of realizations processed. The first algorithm adopts a steepest gradient descent approach; the second uses simulated annealing. They are tested on a number of data sets and while both are successful in reducing conflict while limiting the number of realizations that are examined, the simulated annealing approach is superior with regard to the degree of conflict reduction. The approach adopted is regarded as generic, in the context of map generalization, in that it appears possible in principle to employ several map generalization operators combined with more sophisticated evaluation functions.

Map generalization in the Web age

Geographic information represents our understanding of the association of phenomena with their location on and near the Earth’s surface. When presented in map form, an essential aspect of that information is that it may be adapted in semantic abstraction and level of geometric detail according to the purpose of the map and the extent of the Earth that is being considered at any one time. Representations of small areas in detail result in so-called large-scale maps, while representations of large regions in lesser detail are referred to as small-scale maps. Traditionally, cartographers have performed the task of adapting the content and the level of detail of a map to suit its scale and purpose, and this process is called map generalization. The widespread use of geographic information in computers in the context of geographical information systems (GIS) has brought with it the demand for automation of map generalization. Ideally, when accessing geographic information in a GIS, it would be possible to be able to modify the level of generalization of the available geo-data entirely automatically. At present, no such automatic facilities are readily available in commercial GIS. Some commercial GIS do provide mapgeneralization tools that the user may apply in a somewhat ad hoc manner to selected geometric data. These tools appear, however, to be targeted at cartographic specialists wanting to derive a generalized dataset from a more detailed one. There are some notable examples of the way in which automated map-generalization tools have been exploited for purposes of professional map production. For example, the Institut Géographique National of France have used GALBE (this issue) to generalize the entire French road network to produce 1 : 250 000 scale maps from their BDCarto database (approximately 1 :50 000). More recently, the demand for automated map generalization, which has been longstanding in the context of conventional GIS, has been reinforced by the prevalence of geographical information access on the Internet. There are several types of public access map-based Web sites that allow a user to zoom in and out of a particular region, but at present this is usually based on stepping between independent pre-generalized datasets which may differ markedly in their degree of generalization. It would be desirable to be able to change the level of detail on such systems in a smooth and progressive manner rather than the quantum-leap changes that often characterize the current approach. Hand-held computers and mobile phones with small screens can now support the display of small maps, but the size

Automated map generalization with multiple operators: a simulated annealing approach

This paper explores the use of the stochastic optimization technique of simulated annealing for map generalization. An algorithm is presented that performs operations of displacement, size exaggeration, deletion and size reduction of multiple map objects in order to resolve graphic conflict resulting from map scale reduction. It adopts a trial position approach in which each of n discrete polygonal objects is assigned k candidate trial positions that represent the original, displaced, size exaggerated, deleted and size reduced states of the object. This gives rise to a possible kn distinct map configurations; the expectation is that some of these configurations will contain reduced levels of graphic conflict. Finding the configuration with least conflict by means of an exhaustive search is, however, not practical for realistic values of n and k. We show that evaluation of a subset of the configurations, using simulated annealing, can result in effective resolution of graphic conflict.

Modelling and prediction of GPS availability with digital photogrammetry and LiDAR

This paper describes an automated method for predicting the number of satellites visible to a GPS receiver, at any point on the Earths surface at any time. Intervisibility analysis between a GPS receiver and each potentially visible GPS satellite is performed using a number of different surface models and satellite orbit calculations. The developed software can work with various ephemeris data, and will compute satellite visibility in real time. Real‐time satellite availability prediction is very useful for mobile applications such as in‐car navigation systems, personal navigations systems and LBS. The implementation of the method is described and the results are reported.

Automated Production of Schematic Maps for Mobile Applications

The advent of high-end miniature technology, together with the increasing availability of large scale digital geographic data products, has created a demand for techniques and methodologies that assist in the automated generation of maps specifically tailored to mobile GIS applications. This paper concerns itself with the problem of automatic generation of schematic maps. Schematic maps are diagrammatic representations based on linear abstractions of networks. In the context of mobile mapping they are seen as being a particularly useful means of displaying transportation networks. This paper describes an algorithm that automates the production of schematic maps. The algorithm makes use of the simulated annealing optimisation technique. An implementation of the algorithm is also presented, together with experimental results.

Matching and aligning features in overlayed coverages

1. ABSTRACT The problems caused by locational error when overlaying spatial data from different sources have been recognised for some time, and much research has been directed towards finding solutions. In this paper we present a solution in the form of an algorithm that seeks to match and align semantically equivalent features prior to overlay. It is assumed that, because of locational error, semantically equivalent features will not always be geometrically equivalent. The technique has been developed to assist in the detection of change between multi-date vector-defined data sets. Initial results, obtained by applying our algorithm to land cover data, are presented. 1.1

Automated schematization for web service applications

For the purposes of this paper, a schematic map is a diagrammatic representation based on linear abstractions of networks. With the advent of technologies for web-based delivery of geospatial services it is essential to develop map generalization applications tailored for the same. This paper is concerned with the problem of producing automated schematic maps for web map applications. The paper looks at how previous solutions to the spatial conflict reduction can be adapted and applied to production of automated schematic maps for web services.

Prediction of GPS multipath effect using LiDAR digital surface models and building footprints

This paper aims to investigate how 1m LiDAR data and 2D building footprints can be used to predict GPS multipath effects in urban areas. A ray tracing model is implemented in order to model reflected and diffracted GPS signals. Some preliminary results are presented and explained in detail.

A genetic algorithm approach to cartographic map generalisation

Rendering map data at scales smaller than their source can give rise to map displays exhibiting graphic conflict, such that objects are either too small to be seen or too close to each other to be distinguishable. Furthermore, scale reduction will often require important features to be exaggerated in size, sometimes leading to overlapping features. Cartographic map generalisation is the process by which any graphic conflict that arises during scaling is resolved. In this paper, we show how a Genetic Algorithm (GA) approach was used to resolve spatial conflict between objects after scaling, achieving near optimal solutions within practical time constraints.

Multiscale Terrain and Topographic Modelling with the Implicit TIN

The Multiscale Implicit Triangulated Irregular Network (TIN) provides a storage and access scheme for generating triangulated terrain models that adapt their content and level of detail to the requirements of the user. The scheme combines storage of data representing the terrain surface, and two and three dimensional terrain features, with a retrieval and triangulation procedure that generates a constrained Delaunay triangulation at run time. The feature content and level of detail may be specified by the user, thereby providing a flexible facility that adapts to the requirements of a wide range of applications, whether global or local, exploratory or precise. This paper provides an overview of the scheme and illustrates its application for a variety of queries requiring multiscale representations.