Computers & Geosciences 90 (2016) 34–40 Contents lists available at ScienceDirect Computers & Geosciences journal homepage: www.elsevier.com/locate/cageo Visualization and dissemination of global crustal models on virtual globes Liang-feng Zhu n, Xin Pan, Jian-zhong Sun Key Laboratory of GIS for Ministry of Education, East China Normal University, Shanghai 200241, China article info abstract Article history: Global crustal models, such as CRUST 5.1 and its descendants, are very useful in a broad range of Received 9 September 2015 geoscience applications. The current method for representing the existing global crustal models relies Received in revised form heavily on dedicated computer programs to read and work with those models. Therefore, it is not suited 29 January 2016 to visualize and disseminate global crustal information to non-geological users. This shortcoming is Accepted 30 January 2016 becoming obvious as more and more people from both academic and non-academic institutions are Available online 17 February 2016 interested in understanding the structure and composition of the crust. There is a pressing need to Keywords: provide a modern, universal and user-friendly method to represent and visualize the existing global Crust crustal models. In this paper, we present a systematic framework to easily visualize and disseminate the Global crustal model global crustal structure on virtual globes. Based on crustal information exported from the existing global Virtual globe crustal models, we first create a variety of KML-formatted crustal models with different levels of detail KML 3D visualization (LODs). And then the KML-formatted models can be loaded into a virtual globe for 3D visualization and Google Earth model dissemination. A Keyhole Markup Language (KML) generator (Crust2KML) is developed to auto- matically convert crustal information obtained from the CRUST 1.0 model into KML-formatted global crustal models, and a web application (VisualCrust) is designed to disseminate and visualize those models over the Internet. The presented framework and associated implementations can be conveniently ex- ported to other applications to support visualizing and analyzing the Earth's internal structure on both regional and global scales in a 3D virtual-globe environment. & 2016 Elsevier Ltd. All rights reserved. 1. Introduction models, users must delve into data formats specific to given mis- sions, and develop dedicated computer programs or systems for As the Earth's outermost solid shell, the crust comprises a cri- visualizing and analyzing the structure of the crust. For example, tical zone that participates in and controls processes of Earth's CRUST 5.1 and its descendants provide Fortran computer codes deep interior as well as in the atmosphere (Laske, 2014). On the and xyz-formatted model files for scientific users (Mooney et al., global scale, the crust possesses a complex structure with diverse 1998a; Laske et al., 2000; Laske et al., 2013). This representation compositions. Variations in the thickness and composition of in- has serious limitations because it is only feasible for academic dividual sublayers within the crust significantly influence the investigators, who are typically located at universities or research spatial variation of Earth's magnetic and gravitational fields. Over institutions, to conduct professional geological and geophysical research (Laske, 2014). With increased attention paid to the Earth's the years, a number of global crustal models with various levels of deep interior, more and more people, including atmospheric sci- detail, such as 3SMAC (Nataf and Ricard, 1996), CRUST 5.1 (Mooney entists, educators, policy-makers and even the general public, are et al., 1998a; Mooney et al., 1998b), CRUST 2.0 (Bassin et al., 2000; getting interested in the 3D structure and composition of the Laske et al., 2000), CRUST 1.0 (Laske et al., 2013) and LITHO1.0 Earth's crust. The current method for representing the structure of (Pasyanos et al., 2014), have been presented to depict structural the crust becomes insufficient as it is difficult to respond to the features and property parameters of the Earth's crust. These demands drawn from these users. There is a pressing need to models are generally in the forms of computer codes and corre- provide a more modern, universal and user-friendly method to sponding model files. In order to read and work with these represent and visualize the existing global crustal models. The emergence of virtual globes provides an innovative op- n portunity for geoscientists to represent, disseminate and visualize Corresponding author. E-mail addresses: [email protected] (L.-f. Zhu), geospatial information, including global crustal information. In the [email protected] (X. Pan), [email protected] (J.-z. Sun). last 10 years, a number of sophisticated and powerful online http://dx.doi.org/10.1016/j.cageo.2016.01.015 0098-3004/& 2016 Elsevier Ltd. All rights reserved. L.-f. Zhu et al. / Computers & Geosciences 90 (2016) 34–40 35 virtual globes, typified by Google Earth and NASA's World Wind, 2. Global crustal model CRUST 1.0 have been developed and applied to transform our capability to visualize and hypothesize in three dimensions (Butler, 2006). As In July 2013, CRUST 1.0, a global crustal model at a 1° Â 1° re- digital models of the entire planet, virtual globes not only offer solution, was first formally released by Laske et al. (2013).Asan users the capability to image, analyze, synthesize and interpret updated version of CRUST 5.1 (a global crustal model at a 5° Â 5° geospatial objects on different spatial scales, but also can be re- resolution) and CRUST 2.0 (at a 2° Â 2° resolution), CRUST 1.0 in- garded as reliable platforms for exploring, discovering, analyzing, corporated a wealth of newly available data on global surface to- exchanging and sharing geospatial information at regional or pography, seafloor bathymetry, seismic refraction, as well as the global scales (Butler, 2006; Tiede and Lang, 2010; Bailey and Chen, thickness data of ice, sediment and the crust. Therefore, it is the 2011; Goodchild et al., 2012; Yu and Gong, 2012; Liang et al., 2014; most detailed global crustal model of the moment, and might be Mahdavi-Amiri et al., 2015; Tian et al., 2016). In recent years, a widely embraced by Earth scientists in the foreseeable future. number of research teams have invested considerable effort in As shown in Fig. 1, CRUST 1.0 consists of 64800 1° Â 1° cells modeling and visualizing geospatial objects on virtual globes, arranged in a fixed latitude-longitude grid (Laske et al., 2013). In especially Google Earth. With joint efforts contributed by Earth each cell, the crust is described vertically by eight geophysically scientists and virtual globe developers, a number of techniques identified sublayers: (1) water, (2) ice, (3) upper sediments, have been proposed and applied to address the needs of com- (4) middle sediments, (5) lower sediments, (6) upper crust, municating and visualizing subsurface information in 3D virtual- (7) middle crust, and (8) lower crust. In order to make the model globe platforms (Yamagishi et al., 2010, 2011; De Paor and Pinan- as complete as possible, water and ice are included in the CRUST Llamas, 2006; De Paor and Whitmeyer, 2011; De Paor et al., 2011; 1.0 model as the first two sublayers (Mooney et al., 1998b; Laske Postpischl et al., 2011; Mochales and Blenkinsop, 2014; Zhu et al., et al., 2013). For each sublayer, the boundary depth and physical 2014a, 2014c; Lewis and Hampton, 2015). However, the existing properties, including density ρ, compressional wave velocity Vp research only concerned the representation and visualization of and shear wave velocity Vs, are specified to depict the variation of subsurface models within local or regional areas. There is currently the crustal thickness and associated properties. no readily available method for representing the global crustal structure on virtual globes. Therefore, it is a clear need to develop a universal method for the representation, dissemination and vi- 3. Overall framework sualization of the global crustal structure on virtual globes. In this paper, we explore the representation techniques and asso- All current major virtual globes, including Google Earth and other ciated implementation methods for visualizing the global crustal comparable online Earth browsers, provide users with powerful and structure on virtual globes. Our ultimate goal is to establish a sys- flexible rendering tools for visualizing geospatial objects in a 3D virtual tematic framework, within which the location and variation of the environment, as they all support the OpenGIS KML Encoding Standard Earth's crust may be represented, visualized and disseminated on (OGC KML) (Wilson, 2008; Wernecke, 2009). OGC KML is a popular, virtual globes over the Internet. In order to fulfill this goal, we adopt pervasive and international standard for expressing and displaying the recently released Global Crustal Model CRUST 1.0 as the research geospatial objects within Internet-based 2D maps and 3D virtual object, and choose the Google Earth virtual globe as a platform for globes. For the convenience of representing geospatial objects, KML visualizing and distributing the global crustal information. This paper provides a series of geometry elements (such as oPoint4, first summarizes the essentials of the CRUST 1.0 model, and then puts oPolygon4 and oModel4) and feature elements (such as forward a general framework for the representation, visualization and oPlacemark4 and oGroundOverlay4)todescribe“what” is em- dissemination of the existing global crustal models. Subsequently, key bedded in the “where” and “when” of digital globes. Users of virtual steps for performing the proposed framework are illustrated in great globes only need to describe geospatial objects in accordance with the detail. The implementation program and its web application (〈http:// KML Encoding Standard, and then the existing virtual globes can www.visualearth.org/globalcrust10/crust10web/visualcrust10.html〉) identify, visualize and disseminate these objects automatically (Ballagh are finally described.