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Research on the Construction of standardized WebGL 3D scene Service for Power Grid Service

With the increasing maturity of WebGL technology as well as the development and construction of the 3D geographic information system, 3D geographic information sharing service has become the trend of future development. There are numerous three-dimensional data sources of the power grid and many applications across departments and systems, but the three-dimensional geographic information service based on unified standards is still rarely seen. The paper studies and discusses the selection of 3D service standards and the implementation technology of 3D geographic information service for the power grid.


Introduction
The construction of unified standard service for power grid 3D scene based on WebGL is the focus of power grid 3D data sharing and application, and it is necessary to ensure the integrity and reusability of power grid 3D data. Currently, the current power grid 3D data unified standard service is still rarely seen, and 3D platforms using their proprietary 3D data services to achieve 3D scene visualization, cannot achieve the cross-platform application of 3D data. Given the situation, the study focuses on the issue of 3D geographic information sharing based on the unified standard service of WebGL power grid 3D scene, so as to provide support for power grid 3D geographic information sharing and achieve cross-platform online access, lightweight browsing, professional applications, and other requirements.
Currently, such widely used geographic information platforms as Esri and hypermap could provide support for 3D geographic information. These platforms boast their 3D data standards respectively. In addition to the 3D data standards of some software manufacturers [1], there are also 3D Tiles 3D  There are three main 3D  service standards: (1) 3D service based on the Esri-I3S standard; (2) 3D service based on the hypergraph S3M standard; (3) 3D service based on the 3D Tiles standard.
Based on the above comprehensive analysis, it can be summarized that the main shortcomings of the current 3D geographic information sharing services based on these three standards include: (1) each service standard is supported by different platforms; (2) sharing can only be carried out among platforms of the same manufacturer. The services of different platforms cannot be integrated.

The implementation principle of Three-Dimensional standard service for power grid service
The realization of 3D scene service for power grid business needs to start from the following three aspects: (1) unifying the standard of service interface definition; (2) developing data processing tools; and (3) performance optimization. The technical route of implementation is shown in figure 1:

Unify the standards defined by the service interface
As a three-dimensional scene service for power grid business, it is necessary to unify the service standard, thus to realize the sharing of three-dimensional geographic information and the crossplatform application of three-dimensional data. The following principles should be followed when choosing service standards:(1) the number of platforms supporting this standard is relatively large；(2) adopted by the Open Geospatial Alliance (OGC).
After analysis, the platforms that support the I3s standard, the S3M standard, and the 3D Tiles standard are ArcGIS, hypergraph and hypergraph, Skyline, Cesium respectively. It shows that only the 3D Tiles standard is supported by a relatively larger number of platforms, while OGC also uses 3D Tiles as the community standard, so the 3D Tiles standard is adopted as the standard of 3D scene service for power grid business.

Tiles service standard structure definition
In 3D Tiles, a tileset is a tileset organized by tree spatial data structure. Each tile has a bounding body that completely surrounds its content (content). The tree is spatially coherent, and the content of the child tile is completely contained in the body surrounded by the parent tile. In order to reach the goal of flexibility, the tree can be any spatial data structure with spatial coherence, including kmurd tree, quad trees (quadtrees), octrees (octrees), and grids (grids). The development of data processing tools requires the use of multi-core parallel computing technology to improve the efficiency of data processing. The main functions of the data processing tool are to transform the point cloud data, artificial model data, and tilt model data, and to generate the results according to the standard of 3D Tiles.

Performance optimization and service releasing
The main work of the data processing tool includes two aspects: (1) according to the 3D Tiles standard, the manual modeling model, point cloud, and tilt photography model are processed into standard data files; (2) the manual modeling 3D model data is optimized, and the main work is to simplify the model by LOD [2], which includes the combination of LOD and texture of the model geometric features. Besides, for the repeated model combined with Instance technology to reduce the amount of data. After performance optimization, it is convenient for 3D customers based on 3DTiles standard to improve the efficiency of rendering.
The idea of geometric LOD algorithm: based on the analysis of the data characteristics, multicomponent aggregation characteristics, and some unique geometric features of the 3D building model, the paper proposes a simplified method that is suitable for non-manifold 3D building model. The whole simplification process is divided into two levels, namely: geometric component-level simplification and component-by-component-preserving geometric feature-preserving QEM weighted simplification [3] .  such as the vertex table, surface table, and point-surface topology table. (2) Quadratic error Q is calculated for each triangular patch, and the quadratic error Qi, of each vertex v, is the sum of the quadratic errors of all associated triangular patches of the vertex.
For example, it is known that the three vertices that make up a triangular patch are v1 and v3, respectively, and let the triangular patch.The plane equation is ax+by+cz+d=0, where a~2 + b~2 + c 2 = 1~n = [a, b, c] T.The quadratic error Q of the triangular patch is defined as Q = (A, b, c) = (nnT, dn, D2). Can be described as a matrix of the following 4 × 4.

Conclusion
Based on the analysis of common 3D geographic information service standards, the paper discusses the selection basis of 3D geographic information service standards for power grid business and puts forward the idea of 3D service implementation based on 3D Tiles [4][5] [6], which provides a reference scheme for the implementation of service-based 3D geographic information sharing. With the continuous development of WebGL technology and 3D geographic information service, the demand for 3D geographic information sharing based on the 3D Tiles standard will be further highlighted. The paper, a preliminary study of the above issues, can provide support for the construction of a 3D geographic information service platform for power grid business, and has important practical ICCBDAI 2020 Journal of Physics: Conference Series 1757 (2021) 012165 IOP Publishing doi:10.1088/1742-6596/1757/1/012165 5 significance for promoting the wide application of 3D power grid data and promoting the coconstruction and sharing of 3D geographic information resources.