Google Earth as a tool in 2-D hydrodynamic modeling

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Abstract

A method for coupling virtual globes with geophysical hydrodynamic models is presented. Virtual globes such as GoogleTM Earth can be used as a visualization tool to help users create and enter input data. The authors discuss techniques for representing linear and areal geographical objects with KML (Keyhole Markup Language) files generated using computer codes (scripts). Although virtual globes offer very limited tools for data input, some data of categorical or vector type can be entered by users, and then transformed into inputs for the hydrodynamic program by using appropriate scripts. An application with the AnuGA hydrodynamic model was used as an illustration of the method. Firstly, users draw polygons on the Google Earth screen. These features are then saved in a KML file which is read using a script file written in the Lua programming language. After the hydrodynamic simulation has been performed, another script file is used to convert the resulting output text file to a KML file for visualization, where the depths of inundation are represented by the color of discrete point icons. The visualization of a wind speed vector field was also included as a supplementary example.

Introduction

Every geoscience model contains geographical information. Visualization and interactive data manipulation are therefore essential tools for simulating the dynamics of the natural systems. Unfortunately, these tasks are not fully implemented in many scientific computer programs. In the field of geophysical hydrodynamic modeling, many open source, community-based models rely on plain text input files, including AnuGA (Geoscience Australia and ANU, 2010) for tsunami risk modeling, SWAN (Holthuijsen, 2007) for ocean and coastal wave computation, XBeach (Roelvink et al., 2009) for simulating nearshore circulation and sea bed changes. Other programs utilize GIS interfaces, e.g. ROMS (Shchepetkin and McWilliams, 2005) with the SeaGrid MatLab scripts and a plotting package, or CHIMP (Crouch et al., 2008) with a graphical user interface built on the Fast Light Toolkit (Spitzak et al., 2007). In such cases, the implementation is specific for the software and requires elaborate development of the corresponding the computer codes. The authors suggest integrating the hydrodynamic model with computerized geographical systems, such as virtual globes, to incorporate flexibility and to leverage on the widely used platform of Google Earth.

A virtual globe is software that renders images of the Earth surface on a graphical geoid, allowing users to interactively pan, zoom and rotate the images. Several virtual globes have emerged, such as Microsoft® Virtual Earth, NASA World Wind, GoogleTM Earth, Earth3D, and Marble. Most are free for users to download and experiment with. In this paper Google Earth is chosen as the working platform. However, the principle here can be applied to other virtual globes as well, since all current virtual globes have similar layouts and support the KML geographical description language (to be described in Section 2.2).

Typical commercial hydrodynamic software display spatial data on their own screen windows. For example, in the MIKE software suite (DHI, 2006), users are provided with a friendly data input environment where data are entered through dialogs within a “wizard” (step-by-step) window system. Although such proprietary softwares offer sufficient functionalities for data manipulation and visualization (panning, zooming, spatial queries, drawing and overlaying of different layers), they also have potential shortcomings including the following:

  • The visualization window cannot incorporate the curvature of the Earth surface.

  • The format of graphical representation is specific to the software. Moreover, if the software is proprietary (as in most cases), any data extraction and format conversion are restricted.

  • For researchers who develop software, building the GIS GUI (Graphical User Interface) part is more elaborate than writing and maintaining the computational codes.

  • The ground information often changes (e.g. the decline in forest coverage) but this variation may not have been reflected in the map of the study area, because the land database system has not been brought up to date. In such a case, updating the information to the GIS database usually requires substantial efforts on the part of the user.

These shortcomings can be overcome using a virtual globe coupled with a modeling system. The basis of this coupling technique is presented in Section 2. In Section 3, the authors demonstrated the feasibility of the approach using the AnuGA hydrodynamic model. We confine to the discussion of visualization on a 2-D plane, although virtual globes such as Google Earth can support 3-D representation. In addition to the example with AnuGA model, an example to visualize wind speed vector field is presented in Section 2.5. Results and practical issues arising from the integration between the simulation model and the virtual globe are presented.

Section snippets

The role of virtual globes in modeling

The virtual globe plays the role of both a data input and a visualization tool. As a data input tool, the virtual globe renders the view on the study domain and allows users to specify and extract spatial entities to be used in the model. After the model/simulation has been executed and output file produced in appropriate text format, a computer program is used to convert the content of this file into a format that is compatible to the virtual globe. Fig. 1 shows the flow of the processes

Hypothetical tsunami run-up in Cairns

We used the aforementioned techniques to couple Google Earth with AnuGA—a hydrodynamic model originally developed for quantifying tsunami risk on coastal areas. The AnuGA program calculates shallow water flow and inundation on a triangular mesh, and has its own 3-D viewer based on the OpenScenceGraph library.1 The viewer can be used to produce animation of fluid flow, but it was not designed to produce 2-D maps of physical quantities. Google Earth on

Discussion

The data input process using Google Earth is not straightforward, as the modeler must always adhere to the format of the input file. One difficulty is that Google Earth facilitates minimal property data input through a pop-up dialog window. We can improve the input process by specifying in that property dialog the parameters of spatial objects using the KEY=value expression. Script languages such as Lua has good character string parsing feature, either through Lua's string pattern matching

Conclusion

The authors presented a method to couple the virtual globes with general hydrodynamic models. The AnuGA program was used as an illustrative example. Our examples demonstrate that the Google Earth virtual globe has adequate capabilities to visualize spatial data for hydrodynamic models. However, its capability in providing interactive data input is still restricted.

Acknowledgments

This work is supported by the DHI-NTU Centre, Nanyang Environment and Water Research Institute, Singapore. The AnuGA source code, documentation and data for test cases are developed by Australian National University and Geoscience Australia, and kindly released under the GNU General Public License. Lua programming language is released by PUC-Rio under MIT license. Python programming language is developed by Python Software Foundation and released under the PSF License.

References (20)

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