Improving the usability and discoverability of CityJSON Extensions

The data model CityGML supports domain-specific extensions via Application Domain Extensions (ADEs); users can add new attributes, classes, and even new geometries for specific domains. A 2018 survey found only 10 of 44 ADEs had publicly available schemas, and I show in this paper that a follow-up survey of 16 post-2018 ADEs shows similar problems, with only 7 providing schemas and 5 providing example datasets (both very useful for reuse by others). During its development, the encoding CityJSON addressed some of the limitations of ADEs through its Extensions mechanism: Extensions are simpler and more restricted, for instance no new geometries. However, creating and hosting Extensions remains cumbersome, limiting adoption in practice. This paper presents two contributions to lower that barrier. First, a web application (hosted on the CityJSON website) lets practitioners build Extension files interactively, without needing to write JSON Schema by hand. Second, a public registry on GitHub collects, hosts, and versions all known Extensions at stable, predictable URLs; 9 Extensions are currently included. The registry improves discoverability, provides worked examples, and resolves hosting difficulties such as CORS restrictions.