Integration of web-based gis
and online analytical processing
Keywords: Spatial OLAP, Web-based GIS, Data Mining
Abstract
Nowadays, huge volume of geo-referenced data has been available to geographers and other scientists. This data-rich environment is considerably different from the data-poor environment when Geographic Information System (GIS) originated. Furthermore, although Web-based GIS improved the accessibility, simple query and retrieval function of Web-based GIS cannot meet the need of geographers and other scientists. On-Line Analytical Processing (OLAP) occurs to facilitate the decision-maker to gain insight from large treasury of data, instead of drowning in the sea of unmeaning data. We believe that OLAP should be integrated into GIS so that we can make the most of the geographic data.
In this paper, we would give a very brief introduction to OLAP, Web-based GIS and related techniques first. Then the system architecture for integrating OLAP and Web-based GIS is proposed. Subsequently, spatial OLAP server is examined. We take a prototype system, which is under development, targeting on facilitating users' access to OLAP function through Internet, as an example. A conclusion is given at last.
With the advance of Web technology, Web-based GIS has already become one of the focuses of GIS research. Many Web-based GIS systems have been built around the world. Even some commercial software packages are now available which make the construction of Web-based GIS systems easier than ever (ESRI, 2000a, 2000b). However, most Web-based GIS systems have limited analysis functions, let alone mining knowledge interactively from the precious geo-referenced data. Web-based GIS with OLAP style makes geo-referenced data more understandable to users, which leads to detection of implicit but valuable patterns and associations possible.
Furthermore, huge volume of data is now available. Traditional GIS is only a good repository of geographical information, rather than helping scientist to gain insight from these data. Spatial OLAP and other data mining technique can meet this end. Spatial OLAP help scientist observe these geo-referenced data from different perspective and various levels of concept hierarchy.
Introduction
Data mining and knowledge discovery in databases is relatively a new field of study. It can be understood as the discovery of interesting, implicit, and previously unknown knowledge from large databases (Frawley, 1991). Spatial data mining is a promising field of data mining, which intends to extract implicit knowledge from spatial database. Some researches have been conducted in this field with good outcome. (Ester, 1997; Kroperski, 1996; Lu, 1993). Issues covered in these researches include spatial generalization, spatial clustering, spatial association detection and spatial classification.
OLAP is an indispensable part of spatial data mining, focusing on the end user's analytical requirements and computation process necessary to fulfill them. Just like OLAP, the popular operations of spatial OLAP are slicing and dicing, pivoting, roll-up and drill-down. Obviously, these interesting functions cannot be found in current Web-based GIS systems. We believe that OLAP should be integrated into Web-based GIS so that the capability of web-based GIS system will be improved greatly.
Architecture of Integrated System
The architecture of the integrated system is depicted in figure 1 in a brief way. This is a multitier structure. Concisely, it consists of three tiers. Presentation Tier publishes data through Internet and gathers requests from users. Service Tier processes request and generates response while Data Management Tier is the repository of geographical information.
As a Web-based system, users can access geographical information through Internet using popular Internet browser. Web Server communicates with Internet Map Server. Internet Map Server retrieves geographical information from Spatial Database and transform into appropriate format before sending the data to a Web Server. Some complicated requests which cannot be handled by Internet Map Server, such as drilling down and rolling up, are handed to Spatial OLAP Server for processing. Spatial OLAP Server processes data retrieved from Spatial Database, combining with hierarchy information and maybe materialized data, to get the results and the then
transfers to Internet Map Server for publishing through Internet.
Figure 1. Architecture of Integrated System
The general architecture seems easy to understand. Even some commercial packages are available for some of the services mentioned above which can accelerate system construction. Microsoft IIS, ArcIMS and ArcInfo are good examples for these services respectively. However, Spatial OLAP Server deserves more attention. We would focuses on it in next section.
