Topological querying of multiple map layers

Abstract

This paper first recaptures why multiple map layers are required in Geographic Information Systems. The two main motivations are: flexibility in data modeling, and efficient processing of data. In order to make the map layer discussions clearer, we introduce two different types of map layers: a structure layer, and a thematic layer. Though the concept of a structure layer is defined in à general sense, to illustrate its practicability the organization of data in a structure layer is initially represented according to the formal data structure for single-valued vector maps as proposed by Molenaar. In order to develop a data model for a multi-layered system, the concept of structure layers, specified for the fds, is extended for multi-valued vector maps. It turns out that the data can be modeled in various ways. After that the topic of topological querying of multiple map layers is introduced with a few examples. Map overlay plays a central role in this process. But map overlay is a computationally expensive operation, and therefore several alternative optimization techniques are described for answering the queries efficiently. An important goal of the described multiple map layer query language is that it is a realistic approach. That is, the resulting implementation can be used in an interactive environment with real data sets: with at least several megabytes of geographic data. This is reflected by the case study presented in this paper.