Elsevier

Computers & Geosciences

Volume 37, Issue 9, September 2011, Pages 1295-1302
Computers & Geosciences

Visualizing dynamic geosciences phenomena using an octree-based view-dependent LOD strategy within virtual globes

https://doi.org/10.1016/j.cageo.2011.04.003Get rights and content

Abstract

Geoscientists build dynamic models to simulate various natural phenomena for a better understanding of our planet. Interactive visualizations of these geoscience models and their outputs through virtual globes on the Internet can help the public understand the dynamic phenomena related to the Earth more intuitively. However, challenges arise when the volume of four-dimensional data (4D), 3D in space plus time, is huge for rendering. Datasets loaded from geographically distributed data servers require synchronization between ingesting and rendering data. Also the visualization capability of display clients varies significantly in such an online visualization environment; some may not have high-end graphic cards. To enhance the efficiency of visualizing dynamic volumetric data in virtual globes, this paper proposes a systematic framework, in which an octree-based multiresolution data structure is implemented to organize time series 3D geospatial data to be used in virtual globe environments. This framework includes a view-dependent continuous level of detail (LOD) strategy formulated as a synchronized part of the virtual globe rendering process. Through the octree-based data retrieval process, the LOD strategy enables the rendering of the 4D simulation at a consistent and acceptable frame rate. To demonstrate the capabilities of this framework, data of a simulated dust storm event are rendered in World Wind, an open source virtual globe. The rendering performances with and without the octree-based LOD strategy are compared. The experimental results show that using the proposed data structure and processing strategy significantly enhances the visualization performance when rendering dynamic geospatial phenomena in virtual globes.

Introduction

The “Digital Earth” concept was first proposed by the former U.S. Vice President Albert A. Gore, Jr., in 1998. The concept refers to “a multiresolution, three-dimensional representation” of the Earth, through which scientists and the general public can develop a better understanding of various physical phenomena on the planet (Gore, 1998). To support the development of the Digital Earth, different geovisualization platforms, such as World Wind by NASA, have been developed based on the virtual globe concept. Researchers utilize these virtual globes to visualize a broad range of geospatial datasets to support spatial analysis, knowledge mining, and decision making (e.g., Butler, 2006, Webley et al., 2009, Wu et al., 2010). The graphical representations of these datasets and the tools provided in these visualization platforms help users discover patterns in the data, formulate hypotheses to be tested, and develop a better understanding of the processes under investigation.

While visualization techniques have the potential to reveal patterns and uncover knowledge hidden behind the complex and/or large volume of geospatial data, the use of these techniques is encountered with some impediments. Gahegan (1999) identifies several typical challenges in geovisualization such as representing natural phenomena in a multidimensional space and organizing geospatial data efficiently to improve visualization performance despite increasing data volume. New visualization techniques and schema are constantly needed to handle the increasing volume of geospatial data.

More technical challenges arise when providing multidimensional visualization capabilities over the Internet. In the Internet environment, datasets from different sources are processed in the same visualization environment. Synchronization of retrieval, processing (e.g., reprojection), and rendering in a collaborative environment is critical to performance. The capabilities of the clients, especially their hardware configurations, may vary. Techniques such as hardware acceleration may not be applicable to all clients. In addition, visualization performance is constrained by the implementation environment and visualization platform adopted. Visualization platforms developed in Java are usually not as efficient as those in C/C++ because of the limitations of the Java virtual machine.

The current study addresses some of the challenges in visualizing dynamic geosciences phenomena represented by complex data in an Internet environment. This study has three main objectives: (1) to develop a systematic framework to visualize multidimensional data in an Internet-based virtual globe platform; (2) to develop a data structure to accommodate dynamic volumetric visualization; an octree-based efficient data structure is adopted to tackle this data complexity issue; and (3) to address the visualization performance issue using a level of detail (LOD) strategy. This method is expected to support and enhance the visualization of a large volume of dynamic volumetric geospatial data in virtual globe environments.

The remainder of the paper is organized as follows. Section 2 provides a focused review of previous studies on visualizing 3D/4D geospatial data in an Internet environment. Section 3 describes our approach to organize and render dynamic volumetric data in virtual globes. Section 4 demonstrates the proposed approach using simulated dust storm data. The performances of visualization with and without our proposed strategy are experimented and compared. Section 5 concludes and discusses future directions.

Section snippets

Related work

Due to the advancement of data collection technologies, data in 3D with and without the temporal dimension have become abundantly available in the areas of geosciences and environmental science (Engel-Cox et al., 2004, Kelley et al., 2004). Computational and numerical models also simulate the dynamics of many physical phenomena in a 3D space (e.g., Shen et al., 2006, for dynamic geosciences phenomena). While novel visualization platforms such as virtual globes could be regarded as a logical

Data

In this paper, volumetric data simulating geosciences phenomena are expressed in 3D matrices. In such a structured matrix, each element represents the attribute value at the linear-encoded location. For example,P={pijk},wherei=0,1,,I;j=0,1,,J;k=0,1,,K.

In Eq. (1), pijk is the attribute value of the phenomenon in the 3D space at (xi, yj, zk). The attribute may represent, for instance, the air pressure at a certain height over a location. The terms xi, yj, and zk are defined as follows: xi=x0+iΔ

An illustrative example

To demonstrate the proposed 3D/4D visualization framework, we use data simulating a dust storm event occurring in the southwestern United States in 2008. The data stored in NetCDF format are hourly dust particle densities generated by the Nonhydrostatic Mesoscale dust Model (NMM-dust) (Xie et al., 2010). The simulation replicates a storm event from 12:01 am on January 7, 2008 to 11:59 pm on January 8, 2008, producing data for the 48-h period. The geographic coverage of the data is from 25.56°N

Conclusions and future work

The objective of this paper is to propose a systematic framework for rendering large-scale geosciences phenomena in virtual globes. A multiresolution octree data organization method with a view-dependent continuous LOD rendering strategy is implemented. The original volumetric dataset is organized into an octree structure so that the efficiency to access 3D/4D geospatial data is improved. Exploiting the structure and organization of this data model, the view-dependent LOD was configured to

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