CityGML Building Models Research Articles

Automatic, Multiview, Coplanar Extraction for CityGML Building Model Texture Mapping

Most 3D CityGML building models in street-view maps (e.g., Google, Baidu) lack texture information, which is generally used to reconstruct real-scene 3D models by photogrammetric techniques, such as unmanned aerial vehicle (UAV) mapping. However, due to its simplified building model and inaccurate location information, the commonly used photogrammetric method using a single data source cannot satisfy the requirement of texture mapping for the CityGML building model. Furthermore, a single data source usually suffers from several problems, such as object occlusion. We proposed a novel approach to achieve CityGML building model texture mapping by multiview coplanar extraction from UAV remotely sensed or terrestrial images to alleviate these problems. We utilized a deep convolutional neural network to filter out object occlusion (e.g., pedestrians, vehicles, and trees) and obtain building-texture distribution. Point-line-based features are extracted to characterize multiview coplanar textures in 2D space under the constraint of a homography matrix, and geometric topology is subsequently conducted to optimize the boundary of textures by using a strategy combining Hough-transform and iterative least-squares methods. Experimental results show that the proposed approach enables texture mapping for building façades to use 2D terrestrial images without the requirement of exterior orientation information; that is, different from the photogrammetric method, a collinear equation is not an essential part to capture texture information. In addition, the proposed approach can significantly eliminate blurred and distorted textures of building models, so it is suitable for automatic and rapid texture updates.

Open-Source Tool for Transforming CityGML Levels of Detail

Urban Building Energy Modelling (UBEM) requires adequate geometrical information to represent buildings in a 3D digital form. However, open data models usually lack essential information, such as building geometries, due to a lower granularity in available data. For heating demand simulations, this scarcity impacts the energy predictions and, thereby, questioning existing simulation workflows. In this paper, the authors present an open-source CityGML LoD Transformation (CityLDT) tool for upscaling or downscaling geometries of 3D spatial CityGML building models. With the current support of LoD0–2, this paper presents the adapted methodology and developed algorithms for transformations. Using the presented tool, the authors transform open CityGML datasets and conduct heating demand simulations in Modelica to validate the geometric processing of transformed building models.

Automatic Unfolding of CityGML Buildings to Paper Models

3D city models are mainly viewed on computer screens, but many municipalities also use 3D printing to make urban planning tangible. Since 3D color printing is still comparatively expensive and the colors often fade over time, many of these models are monochrome. Here, color textured paper models offer an inexpensive and under-appreciated alternative. In this paper, a greedy algorithm adapted to CityGML building models is presented, which creates print templates for such paper models. These 2D layouts consist of cut edges and fold edges that bound polygons of a building. The polygons can be textured or left blank depending on the existence of CityGML textures. Glue tabs are attached to cut edges. In addition to the haptic 3D visualization, the quality of the 3D models can sometimes be better assessed on the basis of the print templates than from a perspective projection. The unfolding procedure was applied to parts of the freely available CityGML model of Berlin as well as to parts of models of the cities of Dortmund and Krefeld.

Urban energy simulations using open CityGML models: A comparative analysis

In this paper, simulations of 500 open-source CityGML building models for heating demand calculation are presented. Using an archetype-based approach and statistical enrichment, Modelica simulations are carried out using the TEASER tool and Dymola. The authors compare the simulation results against measurement data and investigate the impact of years of construction, building usages and refurbishment states in the context of urban building energy simulations. The individual buildings are assigned with information over the construction years and building functions using the measurement data. The years of construction and building usages are primarily obtained in the context of two other research projects and are used for the archetype assignments of the individual building models. Hence, these are well-informed assumptions by the authors, resp. the industry partner experts. This paper includes an in-depth investigation of individual building archetypes with respect to the years of construction. Firstly, the buildings are simulated using externally acquired construction years and pre-assigned CityGML building functions. The simulation results are compared against the measured primary heating demands. Due to high discrepancies between simulations and measured data, the authors secondly, simulate the building models with different refurbishment states. In this paper, the authors conclude that open datasets are indeed important, however, for energy performance simulations, attributes such as construction years, building usages and refurbishment states are necessary and should be included in the open datasets to present an added value.

Relation-Constrained 3D Reconstruction of Buildings in Metropolitan Areas from Photogrammetric Point Clouds

The complexity and variety of buildings and the defects of point cloud data are the main challenges faced by 3D urban reconstruction from point clouds, especially in metropolitan areas. In this paper, we developed a method that embeds multiple relations into a procedural modelling process for the automatic 3D reconstruction of buildings from photogrammetric point clouds. First, a hybrid tree of constructive solid geometry and boundary representation (CSG-BRep) was built to decompose the building bounding space into multiple polyhedral cells based on geometric-relation constraints. The cells that approximate the shapes of buildings were then selected based on topological-relation constraints and geometric building models were generated using a reconstructing CSG-BRep tree. Finally, different parts of buildings were retrieved from the CSG-BRep trees, and specific surface types were recognized to convert the building models into the City Geography Markup Language (CityGML) format. The point clouds of 105 buildings in a metropolitan area in Hong Kong were used to evaluate the performance of the proposed method. Compared with two existing methods, the proposed method performed the best in terms of robustness, regularity, and topological correctness. The CityGML building models enriched with semantic information were also compared with the manually digitized ground truth, and the high level of consistency between the results suggested that the produced models will be useful in smart city applications.

Three-dimensional CityGML building models in mobile augmented reality: a smartphone-based pose tracking system

ABSTRACT Many augmented reality (AR) systems are developed for entertainment, but AR and particularly mobile AR potentially have more application possibilities in other fields. For example, in civil engineering or city planning, AR could be used in combination with CityGML building models to enhance some typical workflows in planning, execution and operation processes. A concrete example is the geo-referenced on-site visualization of planned buildings or building parts, to simplify planning processes and optimize the communication between the participating decision-makers. One of the main challenges for the visualization lies in the pose tracking, i.e. the real-time estimation of the translation and rotation of the mobile device to align the virtual objects with reality. In this paper, we introduce a proof-of-concept fine-grained mobile AR CityGML-based pose tracking system aimed at the mentioned applications. The system estimates poses by combining 3D CityGML data with information derived from 2D camera images and an inertial measurement unit and is fully self-sufficient and operates without external infrastructure. The results of our evaluation show that CityGML and low-cost off-the-shelf mobile devices, such as smartphones, already provide performant and accurate mobile pose tracking for AR in civil engineering and city planning.

SEMANTICALLY ENRICHED HIGH RESOLUTION LOD 3 BUILDING MODEL GENERATION

Abstract. This paper reports about an effort to generate LoD3 models of buildings semi-automatically, with the highest possible level of automation. It is work in progress. We use multi-sensor data like aerial images from a 5-head camera with a GSD of 10 cm, UAV images, and aerial and mobile LiDAR point clouds. We distinguish two cases: LoD2 models are available and they are not. We apply Multi-Photo Geometrically Constrained Least Squares Matching for different kind of point measurements. The regularity of many building façades in Singapore leads us to the idea to generalize the measurement procedure towards using measurement macros (geometrical primitives, i.e. windows, doors, etc.) and combine reality-based with procedural modelling. In parallel we try to model these façade elements from LiDAR point cloud data. In another research line we do building detection by a novel approach to land-cover classification, incorporating features of the façades to improve the classification accuracy. To generate the semantic labels of the façades, we developed a spatially unrelated mean-shift clustering method to yield structurally confined segments. It is the characteristic of automated and even semi-automated procedures that the results need some amount of editing. We therefore work on interactive post-editing approaches on CityGML building models containing semantic information of each surface. Maintaining the semantic information throughout the editing process is essential but often lack the support from current tools. Accordingly, we implement a method to synchronize CityGML models. Overall this project consists of a great number of different algorithmic components, which can only be coarsely explained in this paper.

Evaluating the geometric aspects of integrating BIM data into city models

ABSTRACTAirborne Laser Scanning (ALS) is used to acquire three-dimensional (3D) city model data over large areas. However, because of the long ALS update cycle, building information models (BIM) could be utilized to maintain city models. In this study, we designed, implemented, and evaluated a methodology to formalize the integration of BIM data into city models. CityGML models were created from BIM data and ALS/footprint data based on common modelling guidelines. Both CityGML building models are modelled in a similar way and the relative differences between the models are on the order of decimetres.

Achieving Complete and Near-Lossless Conversion from IFC to CityGML

The Singapore Government has embarked on a project to establish a three-dimensional city model and collaborative data platform for Singapore. The research herein contributes to this endeavour by developing a methodology and algorithms to automate the conversion of Building Information Models (BIM), in the Industry Foundation Classes (IFC) data format, into CityGML building models, capturing both geometric and semantic information as available in the BIM models, and including exterior as well as interior structures. We adopt a Triple Graph Grammar (TGG) to formally relate IFC and CityGML, both semantically and geometrically, and to transform a building information model, expressed as an IFC object graph, into a city model expressed as a CityGML object graph. The work pipeline includes extending the CityGML data model with an Application Domain Extension (ADE), which allows capturing information from IFC that is relevant in the geospatial context but at the same time not supported by CityGML in its standard form. In this paper, we elaborate on the triple graph grammar approach and the motivation and roadmap for the development of the ADE. While a fully complete and lossless conversion may never be achieved, this paper suggests that both a TGG and an ADE are natural choices for supporting the conversion between IFC and CityGML.

Modeling Turkey National 2D Geo-Data Model as a CityGML Application Domain Extension in UML

This paper presents the
 generation of the 3D national geo-data model of Turkey, which is compatible
 with the international OGC CityGML Encoding Standard. We prepare an ADE named CityGML-TRKBIS
 that is produced by extending existing thematic modules of CityGML according to
 TRKBIS needs. All thematic data groups in TRKBIS geo-data model have been
 remodeled in order to generate the national large scale 3D geo-data model for
 Turkey. Within the concept of reference model-driven framework for modelling
 CityGML ADEs in UML, the first step is conceptual mapping between CityGML and
 national information model. To test all stages of the framework for Urban GIS
 Turkey, Building data theme is mapped with CityGML-Building thematic model. All
 classes, attributes, code lists and code list values of the TRKBIS is tried to
 be mapped with related CityGML concept. New classes, attributes and code list
 values that will be added to CityGML Building model are determined based on
 this conceptual mapping. Finally the new model for CityGML-TRKBIS.BI is
 established in UML with subclassing of the related CityGML classes. This study
 provides new insights into 3D applications in Turkey. The generated 3D geo-data
 model for building thematic class will be used as a common exchange format that
 meets 2D, 2.5D and 3D implementation needs at national level. 
 

A ROADMAP FOR GENERATING SEMANTICALLY ENRICHED BUILDING MODELS ACCORDING TO CITYGML MODEL VIA TWO DIFFERENT METHODOLOGIES

Abstract. The methodologies of 3D modeling techniques have increasingly increased due to the rapid advances of new technologies. Nowadays, the focus of 3D modeling software is focused, not only to the finest visualization of the models, but also in their semantic features during the modeling procedure. As a result, the models thus generated are both realistic and semantically enriched. Additionally, various extensions of modeling software allow for the immediate conversion of the model’s format, via semi-automatic procedures with respect to the user’s scope. The aim of this paper is to investigate the generation of a semantically enriched Citygml building model via two different methodologies. The first methodology includes the modeling in Trimble SketchUp and the transformation in FME Desktop Manager, while the second methodology includes the model’s generation in CityEngine and its transformation in the CityGML format via the 3DCitiesProject extension for ArcGIS. Finally, the two aforesaid methodologies are being compared and specific characteristics are evaluated, in order to infer the methodology that is best applied depending on the different projects’ purposes.

RESOLUTION IN PHOTOVOLTAIC POTENTIAL COMPUTATION

Abstract. In this paper, an analysis of the effect of the various types of resolution involved in photovoltaic potential computation is presented. To calculate solar energy incident on a surface, shadow from surrounding buildings has been considered. The incident energy on a surface has been calculated taking the orientation, tilt and position into consideration. Different sky visibility map has been created for direct and diffuse radiation and only the effect of resolution of the factors has been explored here. The following four resolutions are considered: 1. temporal resolution (1, 10, 60 minutes time interval for calculating visibility of sun), 2. object surface resolution (0.01, 0.1, 0.375, 0.75, 1.25, 2.5 and 5 m2 as maximum triangle size of a surface to be considered), 3. blocking obstacle resolution (number of triangles from LoD1, LoD2, or LoD3 CityGML building models), and 4. sky resolution (ranging from 150 to 600 sky-patches used to divide the sky-dome). Higher resolutions result in general in more precise estimation of the photovoltaic potential, but also the computation time is increasing, especially as realizes that this computation has to be done for every building with its object surface (both roofs and façades). This paper is the first in depth analysis ever of the effect of resolution and will help to configure the proper settings for effective photovoltaic potential computations.

RESOLUTION IN PHOTOVOLTAIC POTENTIAL COMPUTATION

In this paper, an analysis of the effect of the various types of resolution involved in photovoltaic potential computation is presented. To calculate solar energy incident on a surface, shadow from surrounding buildings has been considered. The incident energy on a surface has been calculated taking the orientation, tilt and position into consideration. Different sky visibility map has been created for direct and diffuse radiation and only the effect of resolution of the factors has been explored here. The following four resolutions are considered: 1. temporal resolution (1, 10, 60 minutes time interval for calculating visibility of sun), 2. object surface resolution (0.01, 0.1, 0.375, 0.75, 1.25, 2.5 and 5 m<sup>2</sup> as maximum triangle size of a surface to be considered), 3. blocking obstacle resolution (number of triangles from LoD1, LoD2, or LoD3 CityGML building models), and 4. sky resolution (ranging from 150 to 600 sky-patches used to divide the sky-dome). Higher resolutions result in general in more precise estimation of the photovoltaic potential, but also the computation time is increasing, especially as realizes that this computation has to be done for every building with its object surface (both roofs and façades). This paper is the first in depth analysis ever of the effect of resolution and will help to configure the proper settings for effective photovoltaic potential computations

ESTABLISHING A NATIONAL 3D GEO-DATA MODEL FOR BUILDING DATA COMPLIANT TO CITYGML: CASE OF TURKEY

This paper presents the generation of the 3D national building geo-data model of Turkey, which is compatible with the international OGC CityGML Encoding Standard. We prepare an ADE named CityGML-TRKBIS.BI that is produced by extending existing thematic modules of CityGML according to TRKBIS needs. All thematic data groups in TRKBIS geo-data model have been remodelled in order to generate the national large scale 3D geo-data model for Turkey. Specific attention has been paid to data groups that have different class structure according to related CityGML data themes such as building data model. Current 2D geo-information model for building data theme of Turkey (TRKBIS.BI) was established based on INSPIRE specifications for building (Core 2D and Extended 2D profiles), ISO/TC 211 standards and OGC web services. New version of TRKBIS.BI which is established according to semantic and geometric rules of CityGML will represent 2D-2.5D and 3D objects. After a short overview on generic approach, this paper describes extending CityGML building data theme according to TRKBIS.BI through several steps. First, building models of both standards were compared according to their data structure, classes and attributes. Second, CityGML building model was extended with respect to TRKBIS needs and CityGML-TRKBIS Building ADE was established in UML. This study provides new insights into 3D applications in Turkey. The generated 3D geo-data model for building thematic class will be used as a common exchange format that meets 2D, 2.5D and 3D implementation needs at national level.

ESTABLISHING A NATIONAL 3D GEO-DATA MODEL FOR BUILDING DATA COMPLIANT TO CITYGML: CASE OF TURKEY

Abstract. This paper presents the generation of the 3D national building geo-data model of Turkey, which is compatible with the international OGC CityGML Encoding Standard. We prepare an ADE named CityGML-TRKBIS.BI that is produced by extending existing thematic modules of CityGML according to TRKBIS needs. All thematic data groups in TRKBIS geo-data model have been remodelled in order to generate the national large scale 3D geo-data model for Turkey. Specific attention has been paid to data groups that have different class structure according to related CityGML data themes such as building data model. Current 2D geo-information model for building data theme of Turkey (TRKBIS.BI) was established based on INSPIRE specifications for building (Core 2D and Extended 2D profiles), ISO/TC 211 standards and OGC web services. New version of TRKBIS.BI which is established according to semantic and geometric rules of CityGML will represent 2D-2.5D and 3D objects. After a short overview on generic approach, this paper describes extending CityGML building data theme according to TRKBIS.BI through several steps. First, building models of both standards were compared according to their data structure, classes and attributes. Second, CityGML building model was extended with respect to TRKBIS needs and CityGML-TRKBIS Building ADE was established in UML. This study provides new insights into 3D applications in Turkey. The generated 3D geo-data model for building thematic class will be used as a common exchange format that meets 2D, 2.5D and 3D implementation needs at national level.

ROOF RECONSTRUCTION FROM AIRBORNE LASER SCANNING DATA BASED ON IMAGE PROCESSING METHODS

The paper presents a new data-driven approach to generate CityGML building models from airborne laser scanning data. The approach is based on image processing methods applied to an interpolated height map and avoids shortcomings of established methods for plane detection like Hough transform or RANSAC algorithms on point clouds. The improvement originates in an interpolation algorithm that generates a height map from sparse point cloud data by preserving ridge lines and step edges of roofs. Roof planes then are detected by clustering the height map’s gradient angles, parameterizations of planes are estimated and used to filter out noise around ridge lines. On that basis, a raster representation of roof facets is generated. Then roof polygons are determined from region outlines, connected to a roof boundary graph, and simplified. Whereas the method is not limited to churches, the method’s performance is primarily tested for church roofs of the German city of Krefeld because of their complexity. To eliminate inaccuracies of spires, contours of towers are detected additionally, and spires are rendered as solids of revolution. In our experiments, the new data-driven method lead to significantly better building models than the previously applied model-driven approach.

ROOF RECONSTRUCTION FROM AIRBORNE LASER SCANNING DATA BASED ON IMAGE PROCESSING METHODS

Abstract. The paper presents a new data-driven approach to generate CityGML building models from airborne laser scanning data. The approach is based on image processing methods applied to an interpolated height map and avoids shortcomings of established methods for plane detection like Hough transform or RANSAC algorithms on point clouds. The improvement originates in an interpolation algorithm that generates a height map from sparse point cloud data by preserving ridge lines and step edges of roofs. Roof planes then are detected by clustering the height map’s gradient angles, parameterizations of planes are estimated and used to filter out noise around ridge lines. On that basis, a raster representation of roof facets is generated. Then roof polygons are determined from region outlines, connected to a roof boundary graph, and simplified. Whereas the method is not limited to churches, the method’s performance is primarily tested for church roofs of the German city of Krefeld because of their complexity. To eliminate inaccuracies of spires, contours of towers are detected additionally, and spires are rendered as solids of revolution. In our experiments, the new data-driven method lead to significantly better building models than the previously applied model-driven approach.

3D Building Modeling and Indoor Fire Event Representation in CityGML

3-dimensional (3D) city model is increasingly popular in urban planning and disaster management. City Geographic Markup Language (CityGML) is an open data model and XML-based format for the representation and exchange of virtual 3D city model. This paper analyzed the modeling mechanism of 3D CityGML building model in LOD4 (Level of Detail) and proposed a method representing a fire event in CityGML. Our method included three operations：1) editing of building’s hierarchy and attributes information; 2) representing of semantic information of buildings surfaces, roof height and interior furniture; 3) representing of fire event by a vivid icon. With our method, 3D building model and indoor fire event can be both represented in CityGML, which is important to analyses urban emergency events.