## 4 Implications for the Design of 3D Data Infrastructures
A consequent implication is to serve a high-demanded profile, such as long-term preservation, and offer many models, which can be achieved by incorporating reusable and publicly available 3D content. * **Enhancing findability and reusability:** Findability and accessibility increasingly become challenging as the number of 3D models increase, as evidenced by our previous analysis on image repositories [109]. Stable identification (ID) systems are a major prerequisite to citing and accessing model data. Indexing and findability of 3D data primarily rely on metadata. Despite extensive research [48] and numerous methods/tools [110; 111; 112; 113; 114; 115; 116], for example, reverse engineering tools to compare models with sources [117; 118], documentation tools and methods to record steps and decisions taken during the 3D modeling process [114; 119; 120] metadata still need to be assigned by the creators in manual processes. With regards to the relevant schema, CIDOC CRM became fixed as an ISO standard for heritage documentation [47]. Nevertheless, the widespread adoption of CIDOC CRM into systems remains of limited outreach, and its implementation into application ontologies is of heterogeneous quality. Regarding metadata creation, currently most metadata descriptions are set manually by users. Numerous initiatives are targeting the development of domain-specific thesauruses to formalize the tagging by metadata; for example, art and architectural history content. Despite the unification by ontologies, manual tagging is limited; for instance, with regard to necessary workload, as well as a limited suitability for massive amounts of data or retro-tagging. Therefore, ensuring and/or automating the process of sufficient metadata generation and verification becomes an important issue. * **Serving heterogeneous user communities:** Three-dimensional heritage objects are used in various disciplinary contexts such as art and architectural history studies, museology, archaeology, and heritage conservation for a wide range of purposes in research, education, and heritage management [44]. Several sectoral standards such as IFC for building information modeling [121], GML for geo- and city-scale models [122], and the Digital Twin [123] as a domain overarching paradigm are relevant. In addition to content generated by professionals, there is a substantial amount of 3D heritage content created by enthusiasts. The creation of user-generated 3D content is facilitated by the development and availability of ready-to-use photogrammetric software tools [2] supporting the crowdsourced collecting and processing of images as a prerequisite for 3D photogrammetry [124] and the crowd-based creation of 3D models [125; 126]. One of the key success factors of repositories is their ease of use [109]; for instance, due to their slick and user-friendly user interfaces and workflows. Consequently, repositories are required to address a large variety of users and incentivize them to provide content by keeping requirements low, nurturing the content provision, and providing rewards. * **Feature and quality requirements:** The visual qualities of 3D web-based viewers have been discussed in various articles [30], with a notable trend to increase (photo) realism. However, these required visual qualities seem highly dependent on use cases. Regarding tools and functionalities, many application frameworks provided by public institutions are VREs that provide a complex working environment for particular communities, such as archaeology [127] or architectural history [128], but require much more experience. For general-purpose 3D viewers, low-level requirements, such as model viewing and viewport navigation, are considered extremely important [28; 45]. Higher-level requirements include measurement and editing tools and the ability to show/hide parts of the model [24]. Consequently, viewer tools should be kept simple and focused on the most relevant features.