Open geospatial tools and standards for 3D GIS
Three-dimensional GIS datasets are being increasingly used as input in different applications . Nowadays, the 3D GIS community is strongly focused on semantic 3D city modelling (e.g. based on international standards such as CityGML  and IndoorGML ), but, as highlighted by Biljecki et al. , in practice the quality of available 3D datasets is often sub-par since they may contain geometric and topological errors. However, the validity of 3D primitives in 3D GIS datasets is often a prerequisite for using them in simulation and decision-making software. In order to address this concern, Ledoux has developed val3dity, an open-source software to validate 3D primitives according to the international definitions of ISO 19107 . Practitioners can use it directly, without limitations: its code is freely available under the GPLv3 license, both binaries and a web-application are publicly available. It takes as input several formats (including the international standard CityGML), and outputs a report that helps users identify and understand the errors. Ledoux’ paper describes some of the engineering decisions supporting val3dity, and show that it can be used to validate real-world datasets .
The next open-source implementation of this thematic collection is VI-Suite: a set of environmental analysis tools with geospatial data applications. VI-Suite combines the functionality of 3D design software with performance simulation. In their paper, Southall and Biljecki present the history of VI-Suite development along with a review of its capabilities of relevance to geospatial analysis . Furthermore, some of the benefits of this tool are discussed including aspects that make it suitable for the processing and analysis of potentially large geospatial datasets. As an example use case, a 3D city model of The Hague is used to demonstrate some of the geospatial workflows possible and some of its visualisation functionalities.
In the same context, an open-source, platform-independent 3D geo-database solution for CityGML-based 3D city models called 3D City Database (3DCityDB) is developed and presented by Yao et al. . The 3DCityDB software package consists of a database schema for spatially enhanced relational database management systems (both for ORACLE Spatial and PostgreSQL/PostGIS) with a set of database procedures and software tools allowing to import, manage, analyse, visualize, and export virtual 3D city models according to the CityGML standard. Within this paper, the authors illustrate the software suite and explain the related technical implementations and the underlying conceptual software design in detail. Moreover, the utilization of 3DCityDB in different projects and practical application fields are also presented.
Last but not the least, Agugiaro et al. present the Energy Application Domain Extension (ADE) for CityGML with the aim of enhancing interoperability for urban energy simulations . According to the authors the Energy ADE is meant to offer a unique and standard-based data model to allow for both detailed single-building energy simulation (based on sophisticated models for building physics and occupant behaviour) and city-wide, bottom-up energy assessments, with particular focus on the buildings sector. Their article presents the rationale behind the Energy ADE and describes its main characteristics, the relation to other standards, as well as provides some examples of current applications and case studies .
Open source geospatial solutions for urban/environmental spatial analyses
Open-source geospatial solutions have been effectively employed in various geospatial use cases which involve studies of the urban environment. In this regard, this thematic collection includes two further studies focused on spatial analysis and one study on employing volunteered geographic information (VGI), both for urban/environmental applications. A brief overview of the studies is presented below.
GeoJModelBuilder is an open-source geo-processing workflow tool which allows distributed geo-processing algorithms, models, data, and sensors to be chained together to support geospatial data analysis for environmental monitoring, as a use case example . In their article, Zhang et al. present the system architecture as well as the technical implementation details of GeoJModelBuilder. Their developed framework leverages open standards, Sensor Web, geoprocessing commands and services, OpenMI-compliant models together . The paper justifies the ability of the proposed workflow tool with discussing three environmental use cases.
In another urban study, Molloy and Moeckel present an iterative algorithm to design optimally sized spatial zones suitable for spatial modelling, while respecting municipal boundaries . The authors argue that the appropriate resolution of a zone system is key to the development of any transport model as well as other spatial analyses, since the number and shape of zones directly impacts the effectiveness of any further modelling steps, with the trade-off between computation time and model accuracy being a particularly important consideration. Their article as well as their source code are openly accessible for detailed investigation and usage in projects .
Crowdsourcing geographic information and participatory GIS have been among the most intensively studied topics in geospatial research and industry over the past two decades. Various projects have implemented participatory-sensing concepts within their workflow in order to benefit from the power of volunteers, and improve their product quality and efficiency [14, 15]. The datasets generated or collected by volunteer citizens are referred to as Volunteered Geographic Information (VGI), which has been one of the most interesting and challenging type of open geospatial data in the past years. The next study in this thematic collection employs OpenStreetMap data; the most popular form of VGI data source for producing up-to-date litter maps . OpenLitterMap rewards users with Littercoin for doing the work of collecting, mapping, processing and producing open geospatial data on an increasing variety of pre-defined types of litter. This process is currently accomplished through a web-app, which is built with focus on the mobile and generally available also on-line. As described by the developer of the project, in order to ensure data quality, all data goes through a manual verification process, which is being done by a small team of trained volunteers. This verified data is being used to develop machine-learning algorithms that will make manual processing and verification easier . OpenLitterMap is an interesting example of implementing useful open data platforms and infrastructures on top of OpenStreetMap among many other existing platforms such as OpenTopoMap, OpenCycleMap and WheelMap , to name a few.