BIM Technical Standards: Referencing & Linking
An external reference is any data that is viable yet does not reside within a particular file. The practice of referencing external data exists within all major BIM platforms and goes by several different names including: Xrefs, References, Link. This section applies to all of these. GSA encourages the minimization of external referencing, especially to any 2D drawings, including AutoCad files. An excessive amount of referencing such 2D files can clog up the linking process and result in improper linking or can even crash the computer. 2D drawings should be generated in the authoring BIM software to minimize any negative consequence of opening the files.
- Master files should always be referenced origin to origin (base point to base point, 0,0,0 to 0,0,0)
- Reference nesting should be avoided, especially on project-based BIM platforms.
- When ever possible, all references must have relative paths, as opposed to absolute paths.
Referencing vs. Inserting
There are a few basic rules to consider when deciding whether files should be referenced together or inserted into one another.
- Each real world object may exist in only one BIM. At no time should this data be duplicated.
- By inserting, you are taking ownership of the inserted objects. If you do not own the objects they should be referenced.
- Whenever possible, attempt to deal in native formats. For example the direct insertion of AutoCAD data into a Revit model should be avoided, instead, convert the DWG data into native Revit geometry and then merge the two Revit models.
The naming of linked files should adhere to the standards already defined in this document. The one exception to this is when referencing another party's data into your model. If at the time of model submittal you will have another party's data referenced to your model, you must append the beginning of that file with an "x" to denote that it is used as reference only. For example, a file named 00000000_AI_B4-50_B.rvt that is being used as reference must be renamed and linked under the name x00000000_AI_B4-50_B.rvt.
Using Uniform Resource Locators (URLs)
Most BIM applications allow for the attaching of URLs (Uniform Resource Locators) to individual BIM objects. This allows for many forms of extended data to be connected directly to the BIM object. When using URLs in as-built or record data for GSA, there are certain rules that need to be followed to ensure that the URLs will continue to work once the data is submitted.
Note: The proper use of URLs on GSA projects requires that the project be established using the GSA standard folder structure. This folder structure is available as a template and is documented within the Data Submittal Standard.
- All URLs must be verified as working prior to the submittal of the data to GSA.
- All URLs must point to either a folder or file which resides within the folder structure outlined in the Data Submittal Standards.
- Several URL fields, parameters, or attribute names are defined in the GSA BIM Guidelines for Revit. These URLs must exactly match the naming outlined in the Minimum Attributes Standard URL section.
- Additional URL attributes may be added to the BIM if required for the project.
URL to Local Files
- All URLs used in submitted data must be of the relative and never pathed to a absolute network location.
- Valid use:
- Concrete\Warranty Info\Warranty.pdf or
- .\Concrete\Warranty Info\Warranty.pdf
- Invalid use:
- E:\Models\Concrete\Warranty Info\Warranty.pdf
- Valid use:
URL to Local Files
- URLs should always go deeper into the folder structure. Backlinking should be avoided whenever possible.
- Valid use: .\Concrete\Warranty Info\Warranty.pdf
- Invalid use: ..\..\Concrete\Warrant Info\Warranty.pdf
URL to Internet Address
- Care must be used when linking to internet based resources. It is unlikely that they will be available, with address unchanged, for the life-cycle of the building.
- Internet URLs must only be used when there is no means of making the content local to the data submittal.