Combining a System Dynamics Model with a GIS:
An Illustrative Example of the Mono Basin
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Appendix I of Modeling the Environment describes four projects in which researchers have combined a dynamic model with a geographic information system (GIS). The previous researchers have argued that the combination of methods is more powerfull than each method used alone. They believe that:
  • Adding a GIS to a dynamic model helps us study the spatial implictions of management alternatives.
  • Adding a dynamic model to a GIS adds a dynamic perspective that is normally missing in a GIS.

This page summarizes a combination of a GIS with the system dynamics model of water flows in the Mono Basin. The combined system was developed by Hallie Anthony (1998). She reported her results for readers who are thinking about the challenges of building such an integrated systyem.

Anthony began with the Stella model of Mono Lake in chapter 4. She built this model in Powersim to take advantage of Powersim's dynamic data exchange (DDE). The DDE facilitates the export of data from Powersim models to other applications (such as a GIS program).

The GIS Program
The term GIS refers to a computer mapping system that can record, store, analyze and manipulate geographic information (Aronoff 1993). The user typically provides input information as data layers, and the GIS software allows overlays to create "coverages." The coverages can then be queried for useful information.

Anthony selected PC ARC/INFO, a verctor based system, as the platform for the GIS. This program is valued for its realistic representation of landscape features (i.e., shorlines), for the ability to build topology and for its graphics interface. Theobald and Gross (1994) note that ARC/INFO is the predominant GIS platform for land use planning.


Illustrative Display of a GIS of the Mono Basin from Anthony (1998, p. 32).
Contour lines are drawn to a resolution of 1 meter. This map portrays the lake at 1943 meters (6374.7 feet).
The lake is shaded in grey, the land in black. The black shading in the middle of the lake is Paoha Island.
The land bridge to Negit Island is visible in this display just to the northwest of Paoha Island.

The Data
A GIS was not available for the Mono Basin in 1998, so Anthony turned to data from the United States Geological Survey (USGS). The USGS provides digital elevation data (sampled every 30 meters) in quadrangles, and six quadrangles were required to cover the Mono Basin. The data were imported into ARC/INFO on a UNIX station, merged, and clipped. (A clip coverage eliminates unnecessary data, such as elevations above the historical high stand of the lake.)

The data was point data with (x,y) coordinates and their associated elevations. Anthony used linear interpolation to create a TIN, a Triangulated Irregular Network. She then converted the TIN to a lattice. (The lattice contour produces sudden elevation changes due to the abrupt TIN facets.) She used a low pass filter to smooth jagged features. The final step was to convert the filtered lattice to a polygon coverage classified for elevation range. The final dataplane is the coverage used to map the lake surface from the lake elevation calculated in the Powersim model of Mono Lake.

An Integrated System
To build an integrated system, Anthony used DDE in Powersim, VBA macros in Excel and SML (Simple Macro Language) macros in PC ARC/INFO. The initial step is to export Powersim elevation via DDE to Excel. VBA macros in Excel were used to create symbol files needed by PC/ARC INFO to create lake surfaces. Each polygon in the final elevation coverage will have an associated symbol, indicating water or land. The SML macros were then used to map the surfaces at the elevations found in the Powersim model. The ARCPLOT subprogram was used to draw the elevation contours. (The resolution is one meter, the same as the resolution in the orginal USGS data.) The polygons were then shaded to represent land or water.

The Final Product
The final product is a collection of maps associated with each of the elevations emerging from the Powersim model. The map shown on this page corresponds to an elevation of 1943 meters above sea level. This is 6374.7 feet, an elevation at which the land bridge to Negit Island is formed (as shown in photo 4.1 in Modeling the Environment and explained on the Key Levels page.)

Anthony's maps were created and updated, one after another, so that the receding lake shore is visually mapped at 25 year intervals in the Powersim simulation. She recommends that her system be improved for interactive studies through the pause interval. A user could change a key policy variable (such as water export) during the pause, and the integrated system would respond with a new map at the next pause interval.