Ontologies
How the data of a place is cut into things and relations a model can reason about.
Between the data and the engine sits a decision about what the things are. A river model's world is reaches and flows; a city model's world is buildings with heights and uses; an agent-based model's world is agents and rules. These choices are the model's ontology, and most engines carry one without saying so. Joining two models is mostly the work of reconciling two of them.
The choices matter because the world does not arrive in pieces. A "thing" is a cluster of tightly entangled relationships that is useful to name because it is roughly separate from other clusters: a dog, a watershed, a government. People are very good at recognising such clusters, and the categories they form are graded rather than sharp, with some members more typical than others (prototype theory; George Lakoff, Women, Fire, and Dangerous Things). John Muir, in My First Summer in the Sierra: "When we try to pick out anything by itself, we find it hitched to everything else in the universe." A model has to pick things out anyway. See the term entry Ontology.
This shelf holds the data models and standards that twins are built on, the papers that propose ways to order the subsystems of a place or a planet, methods for building a vocabulary up into a knowledge graph in order (reviewed here: the book Ontology Pipeline), and tools that build or discover vocabularies.
Tools
Open-source Python library by Yuta Sato and colleagues that builds graphs from buildings, streets, transit feeds, origin–destination matrices, zones and points of interest, for network analysis in NetworkX and graph neural networks in PyTorch Geometric.
Cities & placesMovement & logistics open sourceAPIopen participation
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A JSON encoding of the OGC CityGML data model for 3D city models, with open tools to convert, validate, view and edit files; version 2.0 is an official OGC standard.
Cities & places open sourceopen participation
The open standard in which shared bike, scooter, moped and car systems publish their stations, vehicles, availability and prices as JSON files; maintained by MobilityData.
Movement & logistics open sourceopen participation
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Open standard for public transport data: routes, stops, timetables and fares (GTFS Schedule), and live delays, vehicle positions and alerts (GTFS Realtime). Maintained by MobilityData.
Movement & logistics open sourceopen participation
An experiment by Anselm Hook in which a machine reads the news, finds the things each story is about and grows a vocabulary of kinds and parts from the text, with the things mapped over time.
People & society open sourceopen participation
An open specification from Google Cloud's Data Cloud team for keeping curated knowledge as a directory of markdown files with YAML frontmatter and ordinary links between them, so that people, AI agents and other tools can read the same files without a translation layer.
Cities & places open sourceopen datano APIopen participation
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Universal Scene Description, an open file format and library for exchanging 3D scenes, created by Pixar and promoted by the Alliance for OpenUSD; the format under NVIDIA Omniverse.
Cities & places open source
Open knowledge graph from the University of Maine and partners that links US test results for PFAS with facilities, streams, wells, aquifers, drinking water systems, farmland and county boundaries, so they can be queried together.
WaterFood & landCities & places open sourceAPI
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An open file format for compressed 3D Gaussian splats, from Niantic Spatial, about ten times smaller than the usual PLY files; MIT licence, with C++ and TypeScript libraries.
Cities & places open sourceopen participation
Books
Jessica Talisman's 2026 book from Technics Publications, setting out a six-stage method for building a knowledge system from controlled vocabulary to knowledge graph, with a browser Studio and a SKOS editor.
Essential
A just world on a safe planet: Earth-system boundaries, translations, and transformations article draftA 2024 report in The Lancet Planetary Health by Gupta and colleagues for the Earth Commission, on Earth-system boundaries, their translations and transformations. The Commission's safe and just boundaries cover Earth systems including biodiversity, land, freshwater, nutrients, climate and air pollution.
Solid
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A 2021 Earth System Dynamics paper by Donges and colleagues proposing three taxa of modelled subsystems (biophysical, socio-cultural, socio-metabolic) and a scheme for classifying their interactions.
Weather & climateLiving systems open source
Of note
- Conceptualizing Small and Medium-Sized Smart Cities in the Mediterranean Region: An Ontological Approach article draft
A 2019 paper in the International Journal of E-Planning Research by Panagiotopoulou, Kokla and Stratigea of the National Technical University of Athens that extends an existing smart city ontology with concepts and relationships drawn from small and medium-sized Mediterranean cities.