Ling Bian
Department of Geography
State University of New York
Buffalo, NY 14261-0023

The most recent accomplishment of OpenGIS will have a significant impact on a broad range of disciplines. Environmental modeling requires geographic data and geoprocessing functions; thus it is an inseparable part of the interoperability mission. Integra ting environmental models and GIS involve issues that are well beyond the concerns of interface standards, and should be first addressed at a conceptual level.

Current Integration Approaches

Many integration frameworks have been proposed or implemented in the past few years (Chou and Ding, 1992; Nyerges, 1993; Abel et al., 1994). These efforts have alleviated the difficulties encountered almost daily in using GIS for environmental modeling. This endeavor also sees its limitations. First, these proposals have always involved multiple options, from the lower level, simple data transfer to higher level, complex coupling. There has not been a more focused solution because higher levels of integr ation are always associated with higher development burden. The newly developed OpenGIS interfaces provide transparent access to heterogeneous GIS data sets. This development will partly free users from the integration labor and make it affordable to choo se more desired, higher level of integration. This effect will help narrow the "multiple choice" down to more focused solutions.

A second problem still remains that, even for a higher level integration, the aforementioned integration strategies are limited to "per-model" solutions. An integration system, often including a user interface and a shared database, developed for one mod el is not portable to another. This problem stems from the fact that many environmental models are closed, monolithic systems. Most of them have no capability to communicate readily with either GIS or other environmental models. The diversity of these mod els makes it impractical to develop specifications for every GIS-model and model-model interface. This has been a long-standing problem in an era when multiple data sets and multiple models are required to solve environmental problems. Evidently environme ntal models must "open up" in order to achieve a full integration with GIS and between models themselves.

Moreover, one of the high level integration options calls for implementing environmental models in GIS or vise versa. The former is more often seen for the benefit that the models can use GIS data models and languages directly. Rewriting mathematical equ ations in AML or map algebra type of languages is a typical attempt. This option can achieve reasonable results for a limited type of models, such as simple empirical models that use black box approaches or simple physical models whose parameters are temp orally invariant and spatially homogeneous. For a majority of physically based models used in hydrology, atmospheric science, and increasingly in ecology, executing mathematical equations directly by GIS languages is an impractical choice because GIS lang uages cannot perform at the level of computer programming languages traditionally used for such tasks. Similarly, conducting spatial functions in environmental models is equally inefficient. The ideal integration needs to be worked in a middle ground.

Working in a Middle Ground

It is necessary to understand GIS and environmental models at a conceptual level before technical solutions are sought. GIS and environmental models differ in their representations of the world. GIS focuses on descriptions of space and relationship betwe en spatial features. Environmental models aim at descriptions of dynamic processes of phenomena. This space-process difference determines the distinction in abstract models and languages used by GIS and the models (Maidment 1996). While environmental mode ls use mathematical languages to model the dynamic aspect of the world, GIS languages are designed primarily for spatial operations. Reflected in integration practice, the role of GIS in physically based process modeling has not been much beyond "front en ds" (pre-processing spatial data to prepare model input) and "back ends" (visualize model output spatially). This difference should be well respected and kept. Instead of forcing one into the other, the two representation models should be linked in a comm on framework.

Object orientation may provide such a framework that links the space- and process-oriented abstract models (Raper and Livingstone, 1995). The design and implementation of object systems outlined in Cook and Daniels (1994) are adopted by OpenGIS specifica tions. They can be extended to set the framework of linking GIS and environmental models. The fact that environmental processes occur in geographic space helps establish an essential model of the link. At the specification level, mathematical operations a re executed on spatial fields or features. The spatial fields or features may be defined as objects and they possess properties (e.g., geometry-topology, location-time, or non-spatial attributes). These objects can exert or receive operations, spatial or process-based. Events execute the operations and trigger the state change for the objects. While this framework defines the nature of the link between GIS and the models, more questions arise at the implementation level.

If both the spatial and process representations can be appropriately implemented as spatial objects, spatial operations, and process operations, they would likely or should be implemented in different languages most efficient for the implementation. The linkage between them should be able to interface the difference. Kemp (1993, 1997a, 1997b) elaborated an interfacing strategy that went a step further. It provides intelligent match between the spatial and process representations. The interfacing syntax c an be implemented in computer programming languages so that the process models can directly call for the appropriate data models and spatial operations. The development of OpenGIS specifications helps realize this strategy that the process models can acce ss directly the standardized GIS data and operation components. However, this is still a one-way solution. On the other side of the interface, environmental models need to be opened.

Opening Environmental Models

Opening environmental models should aim at  communicating not only between GIS and models but also between models themselves. The development of OpenGIS specifications sets a precedent for how this could be achieved. Developing componentware in a distrib uted computing environment seems to be an idea approach and consistent with the development in computing industry. Long before the success of OpenGIS, there had been many calls for developing standard module libraries and data exchange formats for integra ting GIS and environmental models (Moore et al., 1993; Kemp, 1993; 1997b; Leavesley et al., 1996). These calls were from both GIS and modeling communities. The standard module libraries can be developed for either spatial operations or process operations.
 
Developing componentware is feasible and appropriate for opening environmental models. The dynamic processes of the physical world contain a series of specific processes through time. The mathematical models that represent these physical processes normal ly consist of a series of algorithms corresponding to the specific processes. Some of the specific processes are common to different models. For example, evaporation process may be a common component shared by atmosphere, surface hydrology, and soil moist ure models. Leavesley et al. (1996) developed a module library with standard module structure so that users can select and link modules for a particular modeling purpose. Although their work was not language-, platform-, or GIS independent, the strategy c an be used to implement componentware for environmental models.

The components should be compatible to GIS and between themselves, and they should be reusable, extendible, and retrievable from a distributed environment. Object orientation may be the most appropriate implementation approach (as opposed to developing t he conceptual framework mentioned previously). Environmental models can be implemented in terms of objects and operations (although these may not be an exact one-to-one mapping of variables and algorithms used in the process models). This implementation a llows development of operation libraries. In the libraries, the operation components are reusable and extendible whenever necessary to meet specific modeling needs. Object-oriented design is the most appropriate approach known for attaining these goals (M eyer, 1987). Furthermore, the environmental components should be retrievable in a distributed environment. Object oriented design allows cataloging the component types and attaching "metadata" to the types so that users can identify and locate appropriate  components.

Opening environmental models is not an easy undertaking. It requires research at several different levels, from establishing conceptual framework to technical implementations. The institutional challenge may be greater than the technical ones. OpenGIS specifications are made possible by the efforts of private sectors. Environmental models, especially those in hydrology and atmosphere sciences, were developed or endorsed by federal government agencies. A full collaboration from modeling community is the p remise for any further progress.

Abel, D.J., and Kilby, P.J. (1994) The systems integration problem. International Journal of Geographical Information Systems, 8(1):1-12.

Chou, H.-C., and Ding, Y. (1992) Methodology of integrating spatial analysis/modeling and GIS. Proceedings, 5th International Symposium on Spatial Data handling, Charleston, South Carolina, 514-523.

Cook, S. and J. Daniels, 1994. Designing Object Systems, Object-Oriented Modeling with Syntropy. Prentice Hall, New York, pp389.

Kemp, K.K., 1993. Environmental modeling with GIS: a strategy for dealing with spatial continuity. Technical Report, 93-3, National Center for Geographic Information and Analysis, Santa Barbara.

Kemp, K.K., 1997a. Fields as a framework for integrating GIS and environmental process models. Part 1: representing spatial continuity. Transactions in GIS, 1(3): 219-234.

Kemp, K.K., 1997b. Fields as a framework for integrating GIS and environmental process models. Part 2: specifying field variables, Transactions in GIS, 1(3): 235-246.

Leavesley, G.H., P.I. Restrepo, L.G. Stannard, L.A. Frankoski, and A.M. Sautins, 1996. In GIS and Environmental Modeling: Progress and Research Issues, Goodchild, M.F., L.T. Steyaert, and B.O. Parks, C. Johnston, D. Maidment, and S. Glendinning (eds.), GIS World, Inc. Fort Collins, Colorado, 155-158.

Maidment, D.R., 1996. Environmental modeling with GIS. In GIS and Environmental Modeling: Progress and Research Issues, Goodchild, M.F., L.T. Steyaert, and B.O. Parks, C. Johnston, D. Maidment, and S. Glendinning (eds.), GIS World, Inc. Fort Collins, Colorado, 315-323.

Meyer, B., 1987. Reusability: the case for object-oriented design. IEEE Software.

Moore, I., A.K. Turner, L.P. Wilson, S.K. Jenson, and L.E. Band, 1993. GIS and land-surface-subsurface process modeling. In Environmental Modeling with GIS, Goodchild, M.F., B.O. Parks, and L.T. Steyaert (eds.), Oxford University Press, New York, 196-230.

Nyerges, T. (1993) Understanding the scope of GIS: its relationship to environmental modeling. In Environmental Modeling with GIS, Goodchild, M. F., B. O. Parks, and L. T. Steyaert (eds.), Oxford University Press, New York, 75-93.

Raper, J., and D. Livingstone, 1995. Development of a geomorphological spatial model using object-oriented design. International Journal of Geographical Information Systems, 9(4):359-383.