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Three dimensional (3D) semantic city models are increasingly used for various fields such as urban planning and design, simulation, disaster management, etc. (Andreia, Luisa, Ana, & Sónia, 2016). The 3D City models are available in several formats such as OGC CityGML, ESRI 3D maps, 3DS, 3D BIM etc. 3D city models encoded in OGC CityGML standard are interoperable and contain rich information in terms of geometry, semantics, topology and appearance (Kravari & Bassiliades, 2015). The features in CityGML can be represented in five Levels-of-Detail (LoD) (Gröger & Plümer, 2012). All features defined in the modules have attributes and geometrical representations in different LODs. The geometry representation becomes more detailed with increasing LOD. More cities worldwide are representing their 3D city models according to the CityGML standard. CityGML allows users to employ virtual 3D city models for advanced analysis and visualization in many applications.
OpenStreetMap (OSM) is a project that creates and distributes geographic data for the world (“OpenStreetMap,” n.d.). The geographic data contains physical features on the ground such as road, building, water body etc. The physical features include both natural and manmade. Each physical feature is associated with tags that describes the type of the feature. OSM data is used in many applications such as routing services, education, research, etc. One of the challenging tasks in transforming 2D geospatial data into 3D city models is to find z coordinate (height) value. Digital Elevation Model (DEM) is a major source for obtaining heights of the objects on the terrain surface. DEM is a representation of terrain surface, created from elevation data (Hirt, 2014). NRSC Bhuvan (“NRSC Bhuvan,” n.d.), OpenDEM (“OpenDEM,” n.d.), ASTGTM (“ASTGTM,” n.d.), USGS EarthExplorer (“USGS EarthExplorer,” n.d.) and ALOS World 3D (“ALOS World 3D,” n.d.), etc. are the popular sources of DEM data.
Geospatial features such as Critical Infrastructure (CI) that includes power lines, drainage lines, water pipes etc. cannot be modeled into CityGML due to lack of schema definition for these features. These features can incorporate into CityGML model through extending CityGML application schema using the concept Application Domain Extension (ADE).
Due to significant features of CityGML 3D city models, number of applications are progressively increasing. Interoperability in spatial data plays a major role to exchange GIS data among different application domains of geoinformatics (Eder & Zipf, 2009). Interoperable sharing and exchanging of CityGML data among various applications is a major concern. Adopting interoperable information services can achieve interoperability in 3D city objects exchange. OGC web services facilitate GIS data access, management, manipulation, representation and sharing over computer networks. Many organizations and researchers have developed tools that facilitates GIS data dissemination through OGC web services. However, many of the tools support 2D spatial data. Tools that allows 3D city objects access through OGC WFS are under development.
3D City Database (“3D City Database,” n.d.) is a free geo database to store, represent, and manage CityGML 3D city models on top of a standard spatial relational database. 3DCityDB provides WFS interface that allows disseminating and accessing 3D city objects through OGC WFS service. 3DCityDB WFS implemented OGC WFS version 2.0 standard. OGC WFS 2.0 defines eleven operations that are grouped into four categories: Simple WFS, Basic WFS, Transactional WFS and Locking WFS. The current version of 3DCityDB WFS (v3.3) facilitates Simple WFS operations such as GetCapabilities, DescribeFeatureType, ListStoredQueries, DescribeStoredQuery and GetFeature. It supports only HTTP POST request and the WFS requests must be encoded in XML. The issues with the current version of 3DCityDB WFS are:
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User should be familiar with request building in XML
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User should know gmlid(s) of CityGML feature(s) to be accessed
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Accessing CityGML challenging features within a region
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WFS output cannot be viewed in CityGML which does not support WFS