What Is A GUID And Why Is It A Core Component Of A Model?

GUIDs & BIM

GUIDs are an essential component of collaborative BIM.

You may not know what a GUID is, they are embedded deep within model data, and hopefully, you will never need to know the technical details of a GUID.

But, I'm often asked to provide an explanation of GUIDs. Which, I'm always happy to provide.

When producing an EIR (Employer's Information Requirement) or a BEP (BIM Execution Plan), I'm always super clear about the need for unique GUIDs within BIMs. They ensure that data remains structured, unique and traceable.

What's A GUID?

A GUID is a unique identifier.

GUID - Globally Unique IDentifier

It's sometimes referred to as a UUID (Universal Unique ID). They both refer to the same thing.

All sorts of IDs are widely used in everyday life, such as National Insurance numbers, serial numbers and registration numbers.

It's pronounced gu as the starting sound of queue and id as the ending sound of bid (or at least that's how I pronounce it!)

It's definitely not pronounced as one dictionary definition of guid (meaning a Scottish form of good) as in gid.

What Does a GUID Look Like?

A GUID is a series of hexadecimal numbers. Hexadecimal numbers include the digits 0-6 plus letters A-F. It's 32 digits long and arranged in an 8-4-4-4-12 series.

Technically speaking it's a 128-bit integer number.

So a typical GUID would look like this:

393e7c63-4848-4a44-8d8e-3bf8595a15f7

But equally could look like this:

44b5910a-d3e1-49e7-94cf-8508d915e6fd

Or any other possible combination or hexadecimal units.

Although, (confusingly) IFC has a slightly different implementation of a GUID (it's designed to slightly decrease the data size of the number) using base 64 character encoding. So if you inspect the underlying data of an IFC file you will see different numbers such as:

3harydHYP498i3qpkg66os

Why Use GUIDs?

GUIDs provide a unique ID without requiring a central database or system for issuing and tracking IDs.

But, how can this be?

GUIDs do not require a method of controlling the issued numbers due to the very small likelihood of ever finding two identical numbers.

The 128-bit number is big enough that should 1,000,000,000 (1 billion) GUIDs be produced every second for 1 year (that's 31,536,000 seconds). Then the probability of finding two identical GUIDs out of all those GUIDs produced would only be 50%.

This means that for the vast majority of applications, using software (or algorithm) generation of GUIDs, you can be confident that any GUID will provide a unique reference (or ID).

What is the Benefit in Having a Unique ID?

GUIDs allow the precise tracking of information.

  • Everything within a BIM will have a GUID, the facility, floors, spaces, assets etc will all have a GUID
  • Changes to objects in different models versions can be tracked as an object maintains its GUID
  • Data can be associated with objects by referencing the object GUID (the data would also have its own GUID)
  • GUIDs form the structure and referencing mechanism of a BIM
  • Related data such as BCFs (BIM Collaboration Format) refer to objects in models using GUIDs and then have their own GUIDs for tracking the status of an issue

Revit Unique IDs and GUIDs

Revit, as most BIM authoring tools, use unique IDs to structure its database. However, the unique ID, known as a Revit UniqueID, is not the same as a GUID.

It has a similar format to GUID, it contains a hexadecimal string in the GUID format with the 8-4-4-4-12 digits. But it also contains 8 additional hexadecimal digits. Making the full string 8-4-4-4-12-8. The last 8 digits are referred to as the Revit ElementID.

When exporting Revit as an IFC, GUIDs will be produced during the export process. The GUIDs produced will be based upon the Revit UniqueID (but may not be the same due to the logic used when creating GUIDs).

If you are required to produce a COBie export from Revit, I recommend using the COBie extension for Revit, this has a global option to export GUIDs rather than ElementIDs.

A Unique Reference

The unique nature of a GUID provides a one-off label for every BIM object, space, floor and building.

A GUID can remain with an object throughout its life.

The benefits of uniquely labelling an object are only just being used. In the future, you will be able to track the history of an object throughout its life cycle from cradle to grave.