BOT: the Building Topology Ontology of the W3C Linked Building Data Group

In this article, the authors introduce the Building Topology Ontology (BOT), a lightweight OWL ontology that allows reasoning with and publishing Building Information Models (BIM) on the Web, including the nested concepts building, storey, construction elements and sites as well their topological interfaces. This allows them to share building models in terms of 3D data of diverse formats, and to query over a building's nested form, e.g., in order to get all appliances in a building or the thickness of the building's walls. The authors compare BOT with the state of the art in building information models, propose a lightweight formalization and align it with DUL and other state-of-the art ontologies, show how 3D geometric models can be linked, discuss application scenarios and evaluate the ontology on three BIM models using six queries including DL and SWRL inference, listing time performance measures.

Without doubt this is a description of a solid and rather mature ontology development, based on the work of the W3C LBD community group and taking into account years of technical development in the BIM community. The design choices made sense to me and the text is well written and well argued. Also the evaluation and application scenarios are convincing.

Still I think the text would require a major revision to focus on some essential points:

- Since BOT was published in earlier papers of the authors (ref no. 50, 51), it is necessary to say more clearly what this article adds in terms of novelty.
- The introduction requires a much stronger motivation and a more concrete research goal for this particular paper. The current goals (REQ1-4) like "support of web-based information exchange" are much too vague to serve as research questions for this specific article. A good way to evaluate ontologies is in terms of concrete competency questions, by showing that questions motivated in the introduction of the paper can be translated into answerable queries later on, similar to the queries Q1-6 or the application scenarios in sect 4. I would recommend the authors therefore to formulate and motivate such concrete questions in the introduction based on their examples. It is necessary to show which specific research problems can be solved with BOT, beyond just claiming to improve information exchange in more general terms, as formulated in BIM maturity level 3. The latter goal is also problematic because difficult to show in the paper. As a matter of fact, it was not shown in the paper how BOT improves all these aspects mentioned in REQ1-4.
- What does "definitely not fully web-ready" (page 5) mean? Similarly, it remains unclear what "as the file-based exchange mechanism prevails" refers to. What is wrong with the exchange of files? RDF is also exchanged in terms of files, so what is the point the authors want to make here...? It remains unclear how this relates to the goals RE1-4. Please reformulate.
- The authors should rethink whether their goal is really to unify building ontologies, as claimed in several parts in the text, given there might be legitimate reasons to have different ontologies for different purposes. Please reformulate.
- Regarding the design choices, I was wondering what the ontological distinction between "elements" and "zones" really entails, given that these two classes have very similar/equivalent mereo-topological structures and that rooms and buildings are indeed constructed and have functions, too. What is the core distinction, and more importantly, why is it needed? This design decision now enforces a lot of repetitive formal structure which could be avoided otherwise. Similarly, the meaning of bot:Space remains a bit vague for me, and also: why is it considered an artefact?
- Why do the authors not reuse previous work in mereo-topologies to encode building topology? What about RCC8? Mereological parthood relation ontologies?
- Shouldn't interfaces overlap with at least 2 elements or zones, instead of one (Otherwise, why should it be called "inter"-face)?
- It would be helpful to explain better what a zeroPoint is (as far as I understood a reference point for turning a local coordinate into a geocoordinate?)
- The related work section lacks a discussion of how BOT relates to 3D interoperable models developed in GIS, such as CityGML

This paper presents Building Topology Ontology (BOT) developed for facilitating Web-based exchange of building models. The authors present the necessity of developing BOT, classes and properties of BOT, how BOT can be combined and aligned with existing ontologies, and how BOT can be used to annotate real building models to support queries and reasoning. Overall, this paper is very well written. I only have a few minor suggestions.

- Lines 103-111: The authors listed four requirements for reaching BIM Maturity Level 3. It would be great if the authors can explicitly state how the developed BOT help fulfill these four requirements in the conclusion of this article.

- Line 437: "bot:containsZone": Is it possible that two Zones occupy exactly the same 3D spatial extents? If so, can the property "bot:containsZone" be applied to such a situation? Some topological relations, such as DE-9IM, differentiate "equals" and "contains". If "bot:containsZone" is applicable, maybe the authors could add a one-sentence clarification here.

- Lines 506-507: The authors provided an example of assigning a single WGS 84 point to a bot:Site. What if one would like to provide a more accurate spatial footprint of the site by e.g., including the coordinates of four corner points of the site (assuming the shape of the site is a rectangle)? Could the authors explain how such a situation can be handled?

My review is via the dimensions:

(1) Quality and relevance of the described ontology (convincing evidence must be provided).

This is a mature work reporting about BOT, which is released as a public draft by the authors' Linked Building Data Community Group. Authors do great job in placing the work among related work, and provide evidence via examples (of the ontology elements), via practise (when using with other ontologies), and via evaluation (both "BOT and BOT exporters").

(2) Illustration, clarity and readability of the describing paper, which shall convey to the reader the key aspects of the described ontology.

This is a very good piece of work, very clear, lots of illustrations, excellent read. I am totally convinced that this can be published as it is.

One philosophical question & comment though. What the authors think of different kinds of "spaces" within buildings, and what they might be? In my previous projects we have listed quite a few of them, but still I am unaware of any attempt to formally represent them. Perhaps interesting future work, or is there already some substantial work done in the area? Happy to discuss this further.