Research on GIS+BIM technology integration

. The integration of GIS+BIM technology has gradually become a hot topic in the development of the construction industry. The paper analyzed the difficulties faced by technical integration in terms of data and models, studied the conversion methods of various data formats, optimized the processing of models at the business and technical levels, and provided data and model foundation reference for the collaborative application of GIS+BIM integration.


Introduction
The technical integration of GIS and BIM can give full play to their respective advantages.GIS offer a variety of spatial query and spatial analysis capabilities, and can provide based on BIM-based visualization, management, decision support and other technical methods for each stage of the project.BIM technology can provide high-precision model data and provide three-dimensional data support for the further application of GIS.The combination of the two can effectively support the collaborative analysis and shared application of large-scale projects.At present, the integration of GIS and BIM technology needs to solve the problem of data interoperability and sharing [1] .This paper analyzes the technical difficulties faced by the integration of GIS and BIM technology, and puts forward solutions to realize the deep integration of GIS and BIM.

Technical difficulties in integration
GIS and BIM belong to different fields, and they are different in the object and development direction.GIS belongs to spatial geography industry, focusing on analysis and management of spatial data.With a wide range of applications, and its advantages lie in the spatial analysis in real geographical scenes and strong data integration capabilities.BIM serves the construction field, focuses on the design of architectural space, which can be visualized and informationized to achieve the fine management of the project by using model.
There are usually technical differences between GIS and BIM.Firstly, the coordinate systems based on spatial management are different.The GIS system is positioned by the longitude and latitude of the earth, while the BIM model is positioned by the Cartesian coordinate system.In addition, so as to meet the needs of large-scale model display GIS usually uses multi-fine level "tiles" as the control mode of map loading and LOD display in graphic display.When applied to 3D GIS, 3D tiles are generated, that is, the triangular network of the model is multi-level cached according to the needs of LOD.For comparison, BIM usually splits, loads and displays the model in the unit of components.
The integration of GIS and BIM technology is not easy, as there are differences in data characteristics, model properties and application features, as shown in Table 1 [2] .The difficulty of GIS + BIM technology integration is the conversion and integration of the two at the data level, as well as the loading and fluency of large-scale models.Fig. 1 is the workflow chart of GIS + BIM integration platform solution at operation and maintenance stage.

Data transformation
There are many three-dimensional data types involved in GIS + BIM integration projects at operation and maintenance stage.and the data formats often involved in its integration application are listed in Table 2.At present, Revit is one of the commonly used software for BIM modeling in the field of engineering construction, which can create and carry the full professional BIM model.Using the conversion tool, the seamless cross-domain connection between BIM and GIS data interfaces can be realized, which provides data basis for the integration application of BIM and GIS.At the same time, the geometric vertex information, property information and family classification information of BIM components can be exported to ensure the integrity of BIM information.The derived model meets the requirements of efficient rendering of large-scale data and levels of detail ( LOD ) model, supports instantiated rendering, and can perform GIS analysis functions such as spatial relationship judgment, spatial operation and spatial analysis.The effect of BIM model data conversion is shown in Fig. 2.

Model data volume optimization
Model data is the core of 3D GIS.The 3D GIS model is composed of a large number of vertices and 3D tiles.Taking the statistics of the vertices derived from the single-storey BIM full-specialty model of a residence (with an area of about 700 square meters) as an example, it can be seen from Table 3 that the number of vertices derived from the BIM model exceeds 2 million, of which the number of vertices of MEP models accounts for about 97.5%.For the GIS platform, loading massive geometric data is undoubtedly the main factor affecting the fluency.
In addition, pipe fittings and pipes are the main components of MEP models, and the sum of the vertices of them accounts for 79.76%.Because of the detailed geometric representation, although MEP models have a small number of components, its single component vertex average is large.As a result, there is also some content that can be optimized.The optimization of GIS+BIM models loading can be considered at both business and technical levels.

Optimization at the business level
Define the model loading logic according to the scenario of the business application, give full play to the advantages of GIS in managing location information, and the characteristics of BIM in focusing on component geometry and attribute information, to achieve the purpose of loading different model data in an orderly manner in different business application scenarios.
Fig. 3 shows the model loading scheme in different scenarios.When displaying the overall data of the project or querying the information of the surrounding facilities of the project, in addition to loading the GIS model, only civil Engineering BIM models and the municipal engineering BIM models are loaded; When retrieving equipment information in the building or viewing the layout plan, the full-specialty BIM models should be loaded, and the GIS models only play a role in assisting spatial positioning.

Optimization at the technical level ① Instantiation technology
The instantiation technology is suitable for situations where there are many duplicate models, with it, the same geometry model is drawn only once, thereby reducing the pressure on hardware devices such as graphics cards and memory.For example, there are a large number of identical "door" objects in a building, only one of them is drawn using instancing technique rendering, so as to improve 3D scene performance.
② LOD technology LOD technology is the abbreviation of Levels of Detail, which means multi-level of detail technology.LOD technology refers to determining the resource allocation of object rendering according to the position and importance of the nodes of the object model in the display environment, reducing the number of faces and details of unimportant objects, so as to obtain efficient rendering operations.
When the 3D scene is zoomed in, models look meticulous, and when browsing the entire scene, the model will be displayed in a rougher way.This method can optimize the resource occupation to the greatest extent and improve the performance of the entire 3D scene.b.Simplified 3D tiles: BIM models are a type of high-density model with accurate modeling and detailed content.Its huge amount of data causes BIM models to be stuck when browsing in 3D scenes.Some BIM models have a large number of redundant vertices and 3D tiles, such as piers, door handles, lock cylinders, round pipes, etc.By simplifying 3D tiles of similar BIM models in batches, the memory usage can be reduced, thereby meeting the needs of large-scale data loading performance.
c. Delete sub-objects: Delete useless sub-objects in the BIM model to simplify the model, thereby improving the fluency of the 3D scene of models.As shown in Table 4, for a single object of door, the door handle and lock cylinder occupy 80%-90% of the data volume, and these components have no practical application value and belong to the category of optimization.Delete or simplify these redundant components can achieve the purpose of model lightweight.Under the premise of satisfying business application scenarios, by optimizing the model loading modes and processing methods in different scenarios, the best model loading optimization effect can be achieved, and can make models browsed smoother.

Conclusions
This paper puts forward the difficulties in the technology of the integration between GIS and BIM, and also the solutions of data conversion between them, so as to better integrate GIS and BIM models and data.At the business level, it is proposed that different scenarios load different detail models.At the technical level, the model can be optimized using techniques such as instantiation techniques, LOD techniques, and model lightweight processing techniques.In the future, combined with IoT technology, a smart operation and maintenance platform based on GIS + BIM will play an important role in the field of smart cities.

Fig. 1 .
Fig. 1.The workflow chart of GIS + BIM integration platform solution at operation and maintenance stage.

Fig. 2 .
Fig. 2. The effect of export model in GIS scape.

Fig. 3 .
Fig. 3. Orderly loading scheme of models in different scenarios.

Fig. 4 .
Fig. 4. Orderly loading scheme of models in different scenarios.③ Model lightweight processing technology Model lightweighting technology is to remove or simplify some of the frame of the model.BIM data in different fields has different levels of sophistication.When importing BIM data into the 3D GIS platform, it is necessary to carry out different degrees of lightweight operations according to the actual needs of the project.Common operations for model lightweighting usually include shells extraction, tiles simplification, and sub-object simplification.a. Shells extraction：Based on visual field analysis technology, The visible part (shell) and the invisible part (internal model) of BIM models are stored in different data sets respectively, so as to separate the internal and external models and achieve the purpose of loading models in the view scene.b.Simplified 3D tiles: BIM models are a type of high-density model with accurate modeling and detailed content.Its huge amount of data causes BIM models to be stuck when browsing in 3D scenes.Some BIM models have a large number of redundant vertices and 3D tiles, such as piers, door handles, lock cylinders, round pipes, etc.By simplifying 3D tiles of similar BIM models in batches, the memory usage can be reduced, thereby meeting the needs of large-scale data loading performance.c.Delete sub-objects: Delete useless sub-objects in the BIM model to simplify the model, thereby improving the fluency of the 3D scene of models.As shown in Table4, for a single object of door, the door handle and lock cylinder occupy 80%-90% of the data volume, and these components have no practical application value and belong to the category of optimization.Delete or simplify these redundant components can achieve the purpose of model lightweight.

Table 1 .
Comparison table of GIS and BIM characteristics.

Table 2 .
Common data formats of GIS + BIM integration.

Table 3 .
A typical 700 square meters single-storey residential full-specialty BIM model vertices statistics table.

Table 4 .
Statistical analysis table of the number of vertices of door components.