CityGML is a widely adopted standard by the Open Geospatial Consortium (OGC)
for representing and exchanging 3D city models. The representation of semantic
and topological properties in CityGML makes it possible to query such 3D city
data to perform analysis in various applications, e.g., security management and
emergency response, energy consumption and estimation, and occupancy
measurement. However, the potential of querying CityGML data has not been fully
exploited. The official GML/XML encoding of CityGML is only intended as an
exchange format but is not suitable for query answering. The most common way of
dealing with CityGML data is to store them in the 3DCityDB system as relational
tables and then query them with the standard SQL query language. Nevertheless,
for end users, it remains a challenging task to formulate queries over 3DCityDB
directly for their ad-hoc analytical tasks, because there is a gap between the
conceptual semantics of CityGML and the relational schema adopted in 3DCityDB.
In fact, the semantics of CityGML itself can be modeled as a suitable ontology.
The technology of Knowledge Graphs (KGs), where an ontology is at the core, is
a good solution to bridge such a gap. Moreover, embracing KGs makes it easier
to integrate with other spatial data sources, e.g., OpenStreetMap and existing
(Geo)KGs (e.g., Wikidata, DBPedia, and GeoNames), and to perform queries
combining information from multiple data sources. In this work, we describe a
CityGML KG framework to populate the concepts in the CityGML ontology using
declarative mappings to 3DCityDB, thus exposing the CityGML data therein as a
KG. To demonstrate the feasibility of our approach, we use CityGML data from
the city of Munich as test data and integrate OpenStreeMap data in the same
area.

使用基于知识图谱的 CityGML KG 框架，将 CityGML 本体中的概念通过声明式映射到 3DCityDB 中进行填充，从而将其中的 CityGML 数据作为一个知识图谱公开。示范了该方法的可行性，并在慕尼黑市的 CityGML 数据中集成了 OpenStreetMap 数据。

通过知识图谱整合 3D 城市数据

Integrating 3D City Data through Knowledge Graphs

The international standard CityGML is both a data model and an exchange
format to store digital 3D models of cities. While the data model is used by
several cities, companies, and governments, in this paper we argue that its
XML-based exchange format has several drawbacks. These drawbacks mean that it
is difficult for developers to implement parsers for CityGML, and that
practitioners have, as a consequence, to convert their data to other formats if
they want to exchange them with others. We present CityJSON, a new JSON-based
exchange format for the CityGML data model (version 2.0.0). CityJSON was
designed with programmers in mind, so that software and APIs supporting it can
be quickly built. It was also designed to be compact (a compression factor of
around six with real-world datasets), and to be friendly for web and mobile
development. We argue that it is considerably easier to use than the CityGML
format, both for reading and for creating datasets. We discuss in this paper
the main features of CityJSON, briefly present the different software packages
to parse/view/edit/create files (including one to automatically convert between
the JSON and GML encodings), analyse how real-world datasets compare to those
of CityGML, and we also introduce \emph{Extensions}, which allow us to extend
the core data model in a documented manner.

本研究提出 CityJSON，一种基于 JSON 的城市数字模型交换格式，用于取代 CityGML 的 XML-based 交换格式，并探讨了 CityJSON 的主要特点，软件包和现实世界数据与 CityGML 的比较，以及如何通过文档方式扩展核心数据模型。