From Geometric Coverage to Task-Aware Observability: A Systematic Mapping Review of Geospatial and 3D Placement of Fixed Urban Sensing Infrastructure

Fixed urban sensing infrastructure underpins smart-city monitoring, transportation, environmental sensing, and digital twins, yet placement decisions often reduce deployment to geometric coverage or connectivity. This systematic mapping review is informed by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. It examines geospatial and three-dimensional (3D) placement methods for fixed urban sensors and synthesizes them through task-aware observability. The retained Scopus export dated 19 May 2026 contained 83 candidates; 20 were excluded before full-text assessment. After eligibility screening, 51 Scopus studies, four citation-identified studies, and eight studies from a focused assessment of known references formed a 63-publication analytical corpus comprising 56 direct-placement and seven placement-enabling studies. Studies were coded by spatial representation, objective, method, evaluation context, and reproducibility assets. Twenty-eight use 3D scenes or digital-twin representations, including 23 direct-placement studies. Only three report operational field evaluation, and four provide documented public-code links. Stream proportions are sensitive to the twelve non-database additions, all concentrated in the roadside, low-power wide-area network (LPWAN), and acoustic stream. Coding was collaborative rather than independently duplicated, so frequencies are descriptive of the agreed corpus. The review provides a cross-domain evidence map, reporting checklist, and research agenda for reproducible, transferable, task-aware urban sensing placement.