Earth Science Informatics (2018) 11:591–603 https://doi.org/10.1007/s12145-018-0350-x

RESEARCH ARTICLE

A virtual globe-based integration and visualization framework for aboveground and underground 3D spatial objects

Qiyu Chen1,2 & Gang Liu1,2 & Xiaogang Ma3 & Zhong Yao1 & Yiping Tian1,2 & Hongling Wang1

Received: 25 June 2017 /Accepted: 23 May 2018 /Published online: 31 May 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract The construction of a large-scale integrated information system has been a hot issue in the field of geoinformatics. It aims to integrate aboveground and underground spatial information and objects in a unified visual environment. Virtual globe, as the most commonly used technology in the construction of Digital Earth, can provide a platform and framework for the integration and visualization of worldwide spatial objects and models. However, the existing works mainly focused on terrains and above- ground spatial entities, and there is still little research on the integration and visualization of large-scale underground geological models and entities in a virtual globe. In this work, the data organizations of aboveground and underground 3D spatial objects were analyzed in detail according to the technical characteristics of the virtual globe. Improved strategies were proposed to achieve the integrated visualization of aboveground and underground 3D spatial objects in a virtual globe-based spherical coordinate. In this process, the terrain surface based on Triangulated Irregular Network (TIN) was used as an intermediate layer to unify the spatial coordinate system. An improved scene cutting approach was used to overcome the challenge that underground geological structures cannot be integrated and visualized with aboveground spatial entities, terrains and landforms. Finally, we developed a virtual globe-based prototype system using OpenSceneGraph (OSG) and osgEarth as the 3D visualization engine. The aboveground and underground spatial models of Fuzhou, a coastal city of eastern China, were applied in this system to verify the validity of the strategies proposed in this paper. In addition, the efficiency of this system in terms of scheduling and visualizing was tested by using the massive models of Fuzhou.

Keywords Virtual globe . Integration and visualization . 3D spatial objects . Aboveground and underground . Scene clipping

Introduction information system has become a core issue in the field of geoinformatics, which aims to integrate aboveground and un- With the development of the concepts of Digital City (Ishida derground spatial information into a unified visual environ- and Isbister 2000), Digital Earth (Gore 1999) and Glass Earth ment. As one of the most common tools in the construction of (Carr et al. 1999), the construction of a large-scale integrated Digital Earth, the virtual globe can be served as a platform and

Communicated by: H. A. Babaie

* Gang Liu Hongling Wang [email protected] [email protected]

Qiyu Chen [email protected] 1 School of Computer Science, China University of Geosciences, Wuhan 430074, China Xiaogang Ma [email protected] 2 Hubei Key Laboratory of Intelligent Geo-Information Processing, Zhong Yao China University of Geosciences, Wuhan 430074, China [email protected] Yiping Tian 3 Department of Computer Science, University of Idaho, 875 [email protected] Perimeter Drive MS 1010, Moscow ID 83844-1010, USA 592 Earth Sci Inform (2018) 11:591–603 framework for the integration and visualization of worldwide etc.), terrain (mountains, rivers, etc.) and underground objects spatial objects and models (Gore 1999; Craglia et al. 2012;De (geological structures, underground facilities, boreholes, etc.) Paor and Whitmeyer 2011). For the spatial information visu- (Wu et al. 2005a). Over the past 20 years, scholars have pro- alization, the virtual globe is not only a substitute for tradition- posed more than 20 kinds of spatial data models (Wu 2004) al 2D maps, but also a catalyst for the emerging architecture around the visualization of geospatial entities. With the devel- and data organization of 3D information software platforms opment of a variety of software and their application, a large (Butler 2006; Cozzi and Ring 2011;Gongetal.2010). It is of number of data formats were produced, and the data organi- great importance for both theory and practice to research the zation of each format is different (Zhu and Wang 2013;Liang related theories and key technologies of a virtual globe-based et al. 2015;Kuhnetal.2010; Langelier et al. 2005; Christen 3D visualization system which aims at integrating massive and Nebiker 2011; Li and Li 1998). And in different data aboveground and underground spatial entities. formats, the data model for the same spatial entities may also Virtual globes, the most important software development be different. Moreover, for the different application require- tools for the Digital Earth, have been widely applied in spatial ments, spatial models are usually built in different coordinate information (Cozzi and Ring 2011;Wuetal.2010). Among systems. For geographic information, we tend to choose a the many virtual globe-based systems, some of them have standard geographic coordinate system, but individual CAD- more complex functions, such as organizing and scheduling based 3D models are often in a relative coordinate system (Wu digital terrain models, 3D visualization of landscape models, et al. 2005a). For the abovementioned challenges in integrat- and integrating vector and raster data (Cozzi and Ring 2011; ing and visualizing spatial objects, many scholars have studied Bernardin et al. 2011; Yu and Gong 2012; Ai and Livingston these problems and proposed some effective approaches and 2009). However, it is very rare that a virtual globe-based sys- strategies. 3D ellipsoid subdivision of earth system was pro- tem is able to integrate and visualize large-scale underground posed to achieve the organization, management, expression spatial objects. At present, the more popular virtual globe- and analysis of space-air-ground, underground and underwa- based systems includes Google Earth1,NASAWorld ter integrated data (Zhou et al. 2009;Pruss2014). In order to Wind2 , Microsoft Virtual Earth3 ,ArcGIS better express and simulate the morphological characteristics Explorer4,osgEarth5, GeoGlobe6 and so on. and their changes of the surface and the space above and OpenSceneGraph (OSG) is an open source 3D graphics below the Earth’s surface, such as crustal plate movement application programming interface, and is used by application (Yu et al. 2012), atmospheric circulation (Li and Xiao 2010), developers in fields such as visual simulation, computer ocean circulation (Karssenberg and Jong 2005), near-earth games, virtual reality, scientific visualization and modeling space environment (Stemmer et al. 2006) and so on, scholars (Martz 2007). osgEarth is an extension of OSG in the field have extended the earth-grid system from the spherical surface of virtual globes. It provides an XML-based file to point to to the entire sphere, and derived the three-dimensional spher- remote sensing data, digital elevation models, raster and vec- ical grid system (Zhou et al. 2009;Luetal.2013). In principle, tor data, and 3D models used in a virtual earth (Zhu and Wang any spatial entities, whether they are geographic objects on the 2013; Liang et al. 2015). 3D scenes can fast schedule large- surface, aboveground buildings underground models or even scale data with different levels to achieve real-time rendering airborne objects, can be integrated into this spherical grid sys- and visualizing in osgEarth, and are also able to receive a tem. Its main target is to address the problem of which data variety of common data formats. So we choose OSG and standards are not uniform from different departments, fields, osgEarth as the rendering engine for the virtual globe-based and software (Mahdavi-Amiri et al. 2016). These proposals prototype system presented in this paper. have outlined the prospects of integrating spatial entities, how- 3D geospatial modeling depicts the geometrical morphol- ever, a large number of aboveground and underground model ogies of a number of spatial entities in a 3D sense through data have been accumulated, and it is still a challenge to inte- different data models (Ai and Livingston 2009; Kessler et al. grate them into a uniform virtual earth system. 2009; Stadler and Kolbe 2007; Döllner and Hagedorn 2007; Kolbe and Gröger (2004) designed a set of spatial models Isikdag et al. 2008). And then each individual spatial entity containing geometric topologies to express urban 3D geospatial can be combined using a certain integrating strategy to objects using GML, and addressed the contact relationship achieve the presentation for the whole spatial objects. Spatial between the terrain surface and aboveground objects due to entities include aboveground objects (houses, trees, bridges, the Delaunary triangulation. Wu (2004)proposedastrategy combining Triangulated Irregular Network (TIN) and 1 Google Earth: http://www.google.com/earth/ Generalized Tri-Prism (GTP) to integrate spatial objects. In this 2 NASA World Wind: http://worldwind.arc.nasa.gov 3 3D scene, all the spatial entities are described using an essential Microsoft Virtual Earth: http://map.live.com element, triangle face. This strategy is to use Delaunay triangu- 4 ArcGIS Explorer: http://resource.esri.com/arcgisexplorer 5 osgEarth: http://osgearth.org/ lation to solve the connecting problem between the terrain 6 GeoGlobe: http://www.geostar.com.cn/Eng/Software/GeoGolobe.shtml surface and aboveground objects. Li and Li (1998) proposed Earth Sci Inform (2018) 11:591–603 593

Table 1 Categories of the geospatial entities (Changed from Aboveground entities Infrastructures, buildings, trees Bridges, signal towers, apartments, Wu et al. 2005a) high-rise buildings, trees, etc.

Topographic features Terrain, geomorphology Mountains, hills, rivers, roads, etc. Underground entities Geological bodies, natural space, Strata, boreholes, caves, subway, mine artificial facilities tunnels, underground pipes, etc.

a 3D solid model to integrate vector data and raster according to into three levels in the vertical direction: aboveground entities, analyzing and studying the type of 3D geospatial entities. topographic features, and underground entities. Table 1 briefly From the abovementioned, the current virtual globe-based classifies these spatial entities according to the three levels. systems are focused on the integrated visualization of the ter- Over the past 20 years, scholars have proposed more than rain and aboveground landscapes. There are no mature or 20 kinds of spatial data models according to the geometrical reliable methods to manage and visualize the large-scale morphologies and the topological relationships of geospatial geospatial objects, especially for the integration and visuali- entities. As shown in Table 2,Wu(2004) divided these data zation of underground 3D geological models (Chen and Liu models into three categories by analyzing the basic elements 2015;Wuetal.2005b; Jones et al. 2009;Chenetal.2017, of these data models. 2018) in a virtual globe-based environment. Under the ad- In general, the existing facial and volumetric models have vance of Digital Earth, the virtual globe-based integrated in- been more nature and achieved good results. However, most formation system is and continues to be an important research of the existing data models are difficult to meet the integrated topic in geoinformatics. visualization for geospatial data. These various data models In this work, we analyzed the types and data organizations also bring great challenges to the integration of aboveground of 3D geospatial entities, and presented integration and con- and underground spatial entities. version strategies of different data formats and multi- coordinate systems in a virtual globe-based environment. The challenge that underground geological models are not Integration strategy for aboveground able to display in a virtual globe-based scene was also over- and underground objects come by using a new scene cutting strategy. Finally, a virtual globe-based prototype system was developed on the basis of In a virtual globe environment, the scene is very complex OSG and osgEarth. since it involves a variety of spatial entities. So a general solution is urgently needed to express various entities in a unified environment. A modular solution is usually used to simplify such complex problem so that it can be solved step by Methodology step. Different entities may be constructed by a same data model, and the different data models can also be used to pro- Data organization of 3D geospatial entities duce the same type of entities. Obviously, various data sources and spatial coordinate systems bring difficulties to the integra- Spatial entities refer to a variety of objects with geometrical tion of entity objects. In addition, due to the different data morphologies in a real geographical space. By using the ter- models, it is necessary to adjust and fuse the spatial contact rain surface as an interface, geospatial entities can be divided relationships when integrating the aboveground entities,

Table 2 Categories of 3D spatial data models (According to Wu 2004)

Facial model Volumetric model Mixed model

Regular volume Irregular volume

Irregular triangular network (TIN) CSG-tree Tetrahedral network (TEN) TIN-CSG mixed Grid Vo xe l Pyramid TIN-Octree mixed Boundary representation (B-Rep) Octree Tri-prism (TP) Wire Framework-Block mixed Non-uniform rational B-splines (NURBS) Needle Geocellular Octree-TEN mixed Wire-Frame Regular block Irregular block Serial sections Solid Sections-TIN mixed 3D voronoi volume Multi-DEMs Generalized tri-prism (GTP) 594 Earth Sci Inform (2018) 11:591–603

Fig. 1 Modular integration for the Source Data Modularization Verious Formats various data types Parsing module Parsing module Remote Sensing 3ds format For For (RS) data 3D models RS images 3ds format

Parsing module Parsing module obj format DEM data For For 3D models DEM data obj format

Parsing module Parsing module flt format Vector Data For For 3D models vector data flt format

Parsing module Parsing module dae format Raster Data For For 3D models raster data dae format

Wrapper

Main System

terrains and underground geological models. Therefore, we entered into the virtual globe-based system be recognized should address the abovementioned challenges from the fol- and parsed by the wrapper. lowing three aspects: integrating the data types of various geospatial objects, unifying the different coordinate systems, Unifying the different coordinate systems and fusing the spatial contact relationships. All the location information of geospatial objects are from the data collection. Data may be converted in accordance with a Integrating the various data types certain standard, and different coordinate systems are selected in this process. Thus, it is improper to directly integrate these Data organizations of various geospatial objects are often dif- data without a unified coordinate system in the environment ferent. It is crucial to design a unified management approach on virtual globes. For the most geospatial objects, data is col- in a virtual globe which involves a large number of data types. lected and produced in a geographic coordinate system. But Modularization is a very useful strategy for such a complex aboveground and underground 3D models are usually built in situation. For each data type, it must be recognized by the a relative coordinate system. Therefore, we must integrate the virtual globe-based system in order to visualize them in a unified environment. The approach used to identify a data type is called a module, and the management of all the mod- Beijing54 Mercator ules is a wrapper. EPSG Projection For the aboveground and underground geospatial objects, Guass there are a variety of data formats since the industries involved Projection are rather special. But most of them can be converted to the Virtual Globe Relative WGS84 unified coordinate system common formats such as 3ds, obj, dxf and so on. So a data Coordinates parser is urgently needed to uniformly manage so many data types. As shown in Fig. 1, a transparent module and a wrapper are adopted to integrate and manage the various geospatial Integrated Visualization objects in a virtual globe-based system. In this way, only if Fig. 2 Unifying the different coordinate systems according to coordinate the data type has the corresponding module, can the data conversion Earth Sci Inform (2018) 11:591–603 595

Source Source Source data 1 data 2 data 3

Y N SRS ?

Relative coordinate Extract SRS system

Fig. 5 Fusion process between a wide range of geological model and the Reassign SRS Assign coordinates terrain surface. a original status of a underground object after unifying coordinate systems; b diagram of integrating the underground model and the terrain surface. Note that, length l and thickness d of the underground object should remain same in this process

in a coordinate system can be transformed into another coor- Compiling dinate system.

Fusing the spatial contact relationships Loaded into Virtual Globe The core of fusing the aboveground and underground Fig. 3 Loading process of multi-source data in a virtual globe geospatial objects is to find the common parts between these models by extracting the common boundaries, surfaces and so on, so as to seamlessly integrate and fuse the spatial objects. various data with different coordinate systems into the spher- The construction of any spatial objects is inseparable from the ical coordinate system of virtual globes according to coordi- three basic geometric elements: points, lines, and facet (trian- nate conversion (Fig. 2). gular facet and polygon). A polygon can also be divided into Virtual globes usually use a spherical coordinate system several triangular facets. Therefore, we can choose triangular where a spatial reference system (SRS) must be assigned for facets as the minimal logic unit to solve the contact relation- each type of data. The various geospatial data are compiled, ships between different geospatial objects. and then are loaded into a virtual globe-based system. The By using the terrain surface as an interface, the fusion strat- basic procedure is illustrated in Fig. 3. For a spatial object egy includes two aspects. One is the integration between without a SRS, we will directly assign its spatial location aboveground geospatial objects and the terrain surface, and when it is loaded into a virtual globe. For the data with a the other is the integration of underground geological models SRS, we can use the SRS or assign another one, and then and the terrain surface. compile and assign the corresponding location in the virtual globe-based system. The integration between aboveground objects and the ter- Compiling is used to place spatial objects in correct posi- rain surfaces In this process, it is necessary to reconstruct the tions due to the spatial transformation between different coor- terrain surface constrained by the boundaries of aboveground dinate systems. Through the spatial transformation, an object objects. The main process is illustrated in Fig. 4.

Fig. 4 Integrating approach of Terrain grids aboveground objects and the terrain surface Outline of entities

Reconsitution 596 Earth Sci Inform (2018) 11:591–603

Fig. 6 Schematic diagram of the (a) (b) scene clipping. a relationship surface surface between the viewing frustum, 1 surfaces, and underground A 1 objects; b view frustum clipping 2 2 with the terrain surface; c clipping B process of the terrain surface

C 3 3

(c) A Underground objects

Control points 1 Cutting points

Cutting line B 3 2 Control line

TIN grids C

The integration between underground models and the ter- al. 2009). We can reconstruct the terrain surface which rain surface The fusion approach between terrain surfaces and has merged the outline features of aboveground objects geological models is similar with aboveground objects and the according to the TIN surface of underground models to terrain surface. The surface of the geological models is achieve the fusion of the contact surfaces between under- usually constructed by TIN (Wu et al. 2005b; Jones et ground models and the terrain surface.

Fig. 7 Interaction framework of MFC and OSG OSG Rendering Thread MFC Main Thread

Organizing Data changes nodes CGlobeDoc CMainFrame

Control messages Scene updating COSG

No changes Scene CGlobeView rendering

Exit message Rendering loop

Exit Exit Earth Sci Inform (2018) 11:591–603 597

Table 3 Data formats which can be accepted by osgEarth Data formats Description

3DS (.3ds) The public data format of 3ds max AC (.ac) A simple and fast data format of AC3D models, which is saved as text format DAE (.dae) An interchange file format for interactive 3D applications, which uses XML schema DW (.dw) File format of Designer Workbench DXF (.dxf) Drawing exchange format of Autodesk, which is mainly used for the transmission and exchange of AutoCAD data OBJ (.obj) Standard 3D model format of Wavefront OGR (.ogr) A OGR-based vector geospatial data format OPENFLITGHT (.flt) 3D models data storage format of MultiGen-Paradigm OSG (.osg) A custom multi-data storage format on OSG

However, a set of independent geological bodies may cover often used to eliminate the unnecessary parts for the final hundreds or thousands of kilometres or even larger in a wide rendering. However, in this work, we use a scene clipping range of underground geological models. Under the circum- approach to show the underground geological models together stances, the fusion should be different from the approach be- with the other geospatial objects in a unified virtual globe- tween the terrain surface and relatively independent and small based environment. aboveground objects. If the same strategy is used to imple- In order to achieve this goal, we merge the cut processing ment the fusion of the large scale geological models and the of the terrain surface into the view frustum clipping according terrain surface, the results would be inconsistent with the fact, to the relationship between the viewing frustum and the terrain even error descriptions. Therefore, after the fusion between surface. External objects and the internal terrain surface are the terrain surface and the top surfaces of geological models, both removed from the viewing frustum. Of course, the cut- the coordinates of all nodes on the models should be adjusted ting for the terrain surface should be controlled within a cer- according to the distance between each node and the terrain tain distance. Displaying the underground geological models surface. Thus, all the geospatial objects will be integrated in at a large observing distance is meaningless, thus the cutting the spherical coordinate system of a virtual globe. The specific of the terrain surface will be performed only when the distance process is shown in Fig. 5. is less than a certain threshold. It should be noted that our workflow does not create new A designated area can also be cut to achieve the visualiza- data. In fact, our work will set up a bridge so as to integrate tion of underground models from a particular perspective. As those multi-source data into a unified environment. Different illustrated in Fig. 6, it is assumed that there are underground data sets are stored independently, but they are visualized in a objects in the area ABC which need to be shown and observed. unified virtual globe. Peripheral control points and TIN surfaces are used to control texture mapping and topographic fluctuation. In order to show View frustum-based scene clipping the underground objects in the area ABC, the terrain surface should be cut and the part within ABC will be hidden. Underground geological models are difficult to be shown in a Using the mask label of osgEarth is able to quickly hide the virtual globe-based system due to the blocking of the terrain polygon constructed by a series of points. Therefore, we need surface. In a large-scale 3D scene, view frustum clipping is to give the clipping area according to the above method as

Fig. 8 Integrated result of the aboveground and underground geospatial objects 598 Earth Sci Inform (2018) 11:591–603 well as the upper and lower corner points (−180, 90) and (180, cut in terms of the display hierarchy of the current model. The −90) of the scene firstly. And then the terrain surface will be example code is shown as the following.

POLYGON(( -180 -90 0, 119.3495 -90 0, 119.3495 26.0516 0, 119.3705 26.0516 0, 119.3705 -90 0, 180 -90 0, 180 90 0, -180 90 0))

global-geodetic

POLYGON(( 119.3495 26.0240 0, 119.3495 -90 0, 119.3705 -90 0, 119.3705 26.0240 0))

global-geodetic

Prototype system and application analysis MFC, OSG and osgEarth are jointly used to build a virtual globe-based framework, and we can easily do Prototype system design some incremental developments on the framework. To achieve a virtual globe scene, the view class of MFC In this work, we develop a virtual globe-based prototype sys- should be linked with the rendering engine of OSG. tem on the basis of OSG and osgEarth, which achieves the OSG is an independent class, which encapsulates the integration and visualization of aboveground and under- APIs provided by OSG and interacts with the view ground 3D geospatial objects, as well as inherits the multi- class by linking the window handle of application pro- scale and LOD rendering and scheduling mechanisms of gram. When the view class receives user messages, the osgEarth. operations of the 3D scene will be handed over to OSG

Fig. 9 Visual effect after using the scene clipping Earth Sci Inform (2018) 11:591–603 599

Fig. 10 Integrated visualization for aboveground and underground 3D models of Fuzhou on different levels of detail

Fig. 11 Integrated visualization for underground constructions in this prototype system

Zoom in 600 Earth Sci Inform (2018) 11:591–603

Table 4 Recorded information during the browsing No. Scenes Parameters Values Distance 6378137 Frame Rate 59.92 1 GPU 0.52 Vertices 7220 Triangles 12800 Distance 10000 Frame Rate 60.00 2 GPU 4.38 Vertices 371439 Triangles 33190 Distance: 8000 Frame Rate 60.01 3 GPU 6.71 Vertices 1226019 Triangles 84846 Distance: 5000 Frame Rate 60.00 4 GPU 13.20 Vertices 1775612 Triangles 474475 Distance 3000 Frame Rate 59.79 5 GPU 12.88 Vertices 2161512 Triangles 799799 Distance 2000 Frame Rate 60.06 6 GPU 15.75 Vertices 1862498 Triangles 643295 Distance 1000 Frame Rate 59.91 7 GPU 11.37 Vertices 1676668 Triangles 526179 Distance, viewing distance; Frame Rate, frame rate of the computer monitor; GPU, GPU acceleration efficiency; Vertices, number of vertices; Triangles, number of triangles

by the corresponding functions of the view class. The plug-in mechanism of osgEarth is very useful to load Figure 7 shows the interaction between MFC and OSG. remote sensing images, DEMs, vector data, external models Earth Sci Inform (2018) 11:591–603 601

Fig. 12 Curves of the recorded values in Table 4. a frame rate and viewing distance; b vertices and viewing distance; c triangles and viewing distance; d GPU acceleration efficiency and vertices

and so on. Remote sensing images, DEMs, and topographic objects including about 69,000 buildings, more than 4,500,000 vector data (roads, rivers, boundaries, etc.) can be obtained other facilities, and 7,2000 3D models of geobodies consisting easily. Aboveground geospatial models mainly rely on some of 85,000,000 triangular facets. Figure 11 shows underground professional 3D modeling software, such as 3D MAX, constructions of Fuzhou including nine subway tunnels (total AutoCAD, MAYA and so on. In this work, the underground length: 371.30 km), more than 3000 boreholes and so on. geological models came from a professional 3D geological The information on different levels of detail during the modeling software, QuantyView. These models can be output scene browsing is recorded in Table 4. as a recognizable format by osgEarth. The data formats which On the basis of Table 4, the curves between viewing dis- can be accepted by osgEarth are shown in Table 3. tance and frame rate, vertices, and triangles, as well as be- Figure 8 shows an integrated result of the aboveground and tween vertices and GPU acceleration efficiency are drawn underground geospatial objects in the virtual globe-based sys- respectively (Fig. 12). tem. Actually, the 3D underground models are added in this As shown in Fig. 12a, with changes of the viewing dis- scenario, but they cannot be observed due to the blocking of tance, the frame rate remains stable, which means that the the terrain surface. Therefore, the surface must be divided by scanning fluency can be ensured. As the viewing distance the approach described in section 2.3. decreases, the triangles within the scene increase firstly, and By using the scene clipping, we can see that the under- then the triangles begin to decrease (Fig. 12b, c). The reason ground geological models are visualized together with the for the increase is that the LOD nodes in the scene begin to aboveground geospatial objects (Fig. 9). increase gradually with decreasing of the viewing distance. Thus, the number of nodes and the triangles increase fast. However, as the further decrease, scene clipping starts to work Application analysis and the nodes without the visible scene are removed, which brings the reduction of the triangles. Fig. 12d shows that GPU This prototype system can support the integration and visual- acceleration efficiency increases with the increasing of verti- ization of massive and large-scale geospatial objects. Fuzhou, ces firstly; but when the vertices in the scene are more than a a coastal city of eastern China, is used as an application pilot to certain number, GPU acceleration efficiency begins to test the strategies proposed in this paper. The visualizing re- decline. sults and the efficiency of this system are also evaluated. Figure 10 shows the aboveground and underground 3D models of Fuzhou on different levels of detail. In this scene, the loaded objects include buildings, main roads, trees, other Discussion landscapes, and the underground 3D geological models of the whole Fuzhou basin (around 12,000 km2). The integrated In this work, we proposed an integrated visualization strategy landscape of Fuzhou is constructed on the whole geospatial for large-scale aboveground and underground geospatial 602 Earth Sci Inform (2018) 11:591–603 objects on the basis of the technical characteristics of virtual Conclusions globes. A prototype system was developed and implemented to test the effectiveness of the proposed approaches. Through This paper presented a virtual globe-based integration and analyzing the results, the advancement of the strategies and visualization framework for aboveground and underground system presented in this paper can be summarized as the 3D spatial objects, which aims to address an ongoing chal- followings. lenge in the field of geoinformatics. In this work, several use- A transparent module and a wrapper mode were ful strategies were adopted, which include the integrating adopted to parse various types of geospatial data to strategy of a variety of data types and formats in a virtual integrate and manage multiple data formats and data globe, the approach of unifying the different coordinate sys- models in a virtual globe-based system. This strategy tems, and the fusion method of contact relationships between eliminates the gaps between different data formats and aboveground geospatial objects, the terrain surface and under- various data models. Various geospatial models from ground geological models. An improved scene clipping meth- different applications are able to be perfectly integrated od was involved to achieve the display of underground models into a unified visual environment by using an interme- in a virtual globe-based environment. A prototype system was diate parser. developed and its performance was tested by using the actual Various coordinate systems were unified in a virtual globe- data of Fuzhou, a coastal city of eastern China. It is shown that based environment. For the geospatial objects from different the approaches and strategies proposed in this paper are effec- coordinate systems, the spatial position of each object is tive in integrating and visualizing aboveground and under- assigned in the virtual globe by using SRS and spatial coordi- ground geospatial objects. Furthermore, the challenge that nate transformation, which ensures all the geospatial objects large-scale underground geological models cannot be integrat- with a uniform coordinate system. ed and visualized in a virtual globe-based environment is An integration and fusion strategy was proposed to address overcome. The efficiency testing proves that the prototype the contact relationships between the geospatial objects by system can support the dynamic scheduling and management using the TIN-based terrain surface as an intermediate layer. of massive geospatial models containing millions of vertices For a wide range of underground models, the coordinates of and hundreds of thousands of triangles. the nodes on these models were adjusted according to the Although the prototype system inherits the advantage of distance from each node to the surface so as to ensure the LOD scheduling from osgEarth, the large-scale underground consistency of the models in the virtual globe-based spherical geological models cannot be organized by LOD model in this coordinate system. work. In the future, we will mainly focus on designing a rea- In order to display the underground geological models in a sonable LOD model to further speed up the scheduling for a virtual globe, an improved scene clipping method was de- wide range of underground models. signed. The designated area of the terrain surface is hidden automatically by using this method, which shows that the Acknowledgments We are grateful to Professor Babaie and anonymous blocking from the terrain surface in a virtual globe is reviewers for the insightful comments and suggestions which led to the improvements in the manuscript. This work was supported in part by the overcome. Natural Science Foundation of China (U1711267, 41172300) and the Finally, a virtual globe-based prototype system was National High-tech R&D Program of China (863 Program) built on the basis of OSG and osgEarth, which achieves (2012AA121401). the integration and visualization of 3D aboveground and underground geospatial models. 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