Proposal: Industry Foundation Classes (IFC) ifcXML or STEP Physical Format (SPF or IFC-SPF) as the Main Open Format

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Industry Foundation Classes (IFC)

From Industry Foundation Classes IFC 4.3

Introduction

The Industry Foundation Classes (IFC) are an open international standard for sharing Building Information Model (BIM) data. The standard comprises:

1. A schema (provided in various forms, see scope)
2. Documentation (provided in HTML, authored in Markdown)
3. Property and Quantity Set definitions (standardized definitions for an extensibility mechanism realised in the schema - provided in XML)
4. Exchange or serialization mechanisms of data files, see scope

The prevalent exchange format for IFC is the Step Physical File Format (ISO 10303-21:2002) based on the schema publication using the EXPRESS language (ISO 10303-11:2004). It is a clear-text encoding of the entity instances that make up the exchange, in which attribute values are provided as an ordered sequence of unnamed values.

Conventions

The IFC specification includes terms, concepts and data specification items that originate from use within disciplines, trades, and professions of the construction and facility management industry sector. Terms and concepts use the plain English words, the data items within the data specification follow a naming convention.

• the data item names for types, entities, rules and functions start with the prefix "Ifc" and continue with the English words in CamelCase naming convention (no underscore, first letter in word in upper case);
• the attribute names within an entity follow the CamelCase naming convention with no prefix;
• the property set definitions that are part of this standard start with the prefix "Pset_" and continue with the English words in CamelCase naming convention;
• the quantity set definitions that are part of this standard start with the prefix "Qto_" and continue with the English words in CamelCase naming convention.

buildingSMART International publishes translations of those terms and concepts into other human languages on translations.buildingsmart.org.

Model View Definitions

Official model view definitions (MVDs) exist as related specifications. The official MVD policy for IFC 4.3 currently holds 3 levels of implementation for IFC:

• Reference View
• Alignment Based Reference View
• Design Transfer view

These three MVDs can be seen as three levels of implementation for IFC 4.3. They are gradual levels adding more advanced features to the implementations. The documentation is deposited at standards.buildingsmart.org.

Architecture

The data schema architecture of IFC defines four conceptual layers, each individual schema is assigned to exactly one conceptual layer. The figure below shows the schema architecture of the IFC layered architecture.

Figure A

1. Resource layer --- the lowest layer includes all individual schemas containing resource definitions, those definitions do not include a globally unique identifier and shall not be used independently of a definition declared at a higher layer;
2. Core layer --- the next layer includes the kernel schema and the core extension schemas, containing the most general entity definitions, all entities defined at the core layer, or above carry a globally unique id and optionally owner and history information;
3. Interoperability layer --- the next layer includes schemas containing entity definitions that are specific to a general product, process or resource specialization used across several disciplines, those definitions are typically utilized for inter-domain exchange and sharing of construction information;
4. Domain layer --- the highest layer includes schemas containing entity definitions that are specializations of products, processes or resources specific to a certain discipline, those definitions are typically utilized for intra-domain exchange and sharing of information.

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ifcXML

.ifcXML is a Extensible Markup Language (XML) provides enhanced readability and benefits from a broad range of software tools. ifcXML is based on the ISO standard for representation of STEP data in XML format ISO 10303-28

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STEP Physical File (SPF) .ifc

STEP Physical Format (SPF or IFC-SPF) is the most widely used format for IFC in practice, which is the most compact of the formats listed that can be read as text. IFC-SPF is based on the ISO standard for clear text representation of EXPRESS data models ISO 10303-21

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Reference Links

• IFC Specifications Database
• 8.9 Resource definition data schemas > IfcGeometryResource
• IFC Formats