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Distributed Computing Architecture based on Geo Services; A Loosely Coupled Method for Linking GIS and Environmental Models

Gholamreza Fallahi Surveying and Geomatic Eng. Faculty – K.N. Toosi University of Technology Tehran, Iran GIS Dept. – National Cartographic Center, Azadi Sq., Meraj St. Tehran, Iran
fallahi@ncc.neda.net.ir

Abstract
Most of the environmental problems do have an obvious spatial dimension. In this regard Geographic Information Systems (GIS) can be widely used in various environmental disciplines for solving environmental problems. Therefore linking GIS and environmental models is becoming common and interest in merging the technologies. Currently different methods are used for linking which are based on loosely or tightly coupled relationship between monolithic GIS and environmental models or integration of one of them into another or tightly coupled relationship between GIS and environmental models in a distributed computing architecture. These methods as isolated islands are no longer appropriate for heterogeneous environmental disciplines due to the fact that they are still discipline specific. They can not support a good communication in an interdisciplinary effort for solving environmental problems due to their lack of interoperability. The motivations for adopting new methods are derived from the essential needs of environmental modelers and promoting interoperability between GIS and modelers of environmental disciplines.

This paper will discuss about current methods of linking GIS and environmental models their advantages and drawbacks. Then it is discussed about distributed computing technology based on web services as well as the role of interoperability in communication. Standard protocols for deploying, discovering and invoking geo services are explained. Geo services can be classified into geo data services and geo data service due to the fact that geo data services are tightly coupled to geo data. We pay attention to geo data services and requirements for producing a map as a picture, answering basic queries about the content of the map, and telling other programs what maps it can produce and which of those can be queried further. Then, the architectural view for a web map server based on OGC implementation specification which practically implemented is presented.

1. Introduction
The environmental models do have an obvious spatio-temporal nature. They are occurred in a part of spatial region like surface and subsurface water flow, soil erosion, impact assessment in the event of a chemical and/or oil spill or urban extend. Thus environmental modelers encourage using GIS for describing the models of how environment changes (e.g., models of erosion, flooding, vegetation growth and changes, urbanization).

Currently different approaches have been used to link GIS with environmental models. According to Goodchild these approaches can be classified to full integrate (embedding), loose coupling and close or tight coupling [Goodchild, 2000].

A variety of environmental disciplines have entered the modeling arena and a broad range of users belong to environmental disciplines are involved to exchange spatial data and operations. Thus close coupling and full integration approaches which lead to stand alone systems in which spatial data and operations are tightly coupled with environmental models, are not able to cover the needs of these broad ranges of user. These tightly coupled approaches are inflexible and cannot inherently take advantage of physical network and internet. These approaches suffer from the lack of interoperability.

Due to the popular use of the Internet and the dramatic progress of communications and telecommunications technology, the paradigm of linking GIS and environmental models is shifting into distributed computing technology with independently provided, specialized, interoperable geo services.

This paper presents the implementation of a web map server which provides the visualization of the physical fields for environmental modeler based on OGC (Open Geospatial Consortium) implementation specification. The structure of the paper is as follow:

In section 2 the related works about linking GIS and environmental models are briefly discussed. This section is followed by pointing to scientist's conceptualization from environment. In section 3 we pay attention to new distributed GIS architecture which is based on web services, due to the fact that service oriented architecture is able to perform communication between experts and provide knowledge sharing between them. This section continues with explaining the role of interoperability in communication. In the next section the web service discovery and the protocols which are developed for deploying, discovering and invoking web services are introduced. In section 5, classification of geo services will be explained. We pay attention to geo data services and requirements for producing a map as a picture, answering basic queries about the content of the map, and telling other programs what maps it can produce and which of those can be queried further. In the next section, the development of an http server which can be used as a Web Map Services based on OGC implementation specification in order to prepare requirements for geo data services will be explained. This paper comes to the end with conclusion.

2. Background
There are two distinct conceptualization about the natural system called the object and field conceptualization [Couclelis, 1992; Smith and Mark, 2003].

Many scientists as environmental modelers conceptualize the world as fields. The field conceptualization assumes environment as continues space and involves apprehending reality in terms of distributions of properties (attributes) such as temperature, population density, pH of the soil, soil type or tree-coverage.

In the environmental modeling, when we model the environment, we concentrate on a set of states which describe the conditions of natural system under study [Casti, 1989]. Each state can be observed and measured in each location and according to the measurement a value can be associated to it and represented by a number. These numbers which can be classified into ration, interval, ordinal and nominal scale may describe soil type, land use type, elevation, distance of each location from a phenomenon, noise levels from an airport, radiation levels from a nuclear plant and so on. A wide range of geo operations can be performed on field data. These geo operations perform a mapping between new and old property values of physical fields.

Most of research works related to linking GIS and environmental models use one of the full integration, loose coupling and tight coupling methods in order to use geo operations in their environmental models. [e.g. Sydelko, et al, 2000, Fedra, 1996, Djokic, 1996]. The highest level of integration is an embedded system, one in which GIS and modeling functions are interwoven elements of a software system (or full integration). [Goodchild, 2000].