📐 DGN File Reference

What Is a DGN File?

A .DGN file (Design file) is the native drawing format for MicroStation, made by Bentley Systems. Think of it as MicroStation's equivalent of AutoCAD's .DWG file — it stores all the geometry, annotations, levels (layers), and settings for a drawing.

The name comes from "Design" — Intergraph Corporation (later Bentley) created the format in the 1980s, and it has been the standard for MicroStation ever since.

You don't need MicroStation to work with DGN files. AutoCAD has had built-in DGN support since AutoCAD 2007, so you can attach, import, and even export DGN files without ever opening MicroStation.

Who Uses DGN Files — and Why Will You Encounter Them?

DGN files are most common in industries where government agencies set the CAD standard. If you work with any of the following, you will encounter DGN:

DGN vs DWG — The Plain English Comparison

Both formats do the same job — store CAD drawings. The key difference is which software created them. Here's what that means in practice:

DGN Terminology Translator — MicroStation → AutoCAD

The biggest confusion when first encountering DGN files is the different names for the same concepts. Here's the complete translator:

Opening a DGN File in AutoCAD — Your First Steps

When someone sends you a DGN file, you have two main options in AutoCAD: attach it as an underlay (best for using it as a background reference) or import it (converts geometry to native AutoCAD objects). Here's how to decide:

DGNATTACH — Attaching a DGN File as an Underlay

Just like XATTACH attaches a DWG file, DGNATTACH attaches a DGN file as a non-editable underlay. The geometry displays in your drawing but stays in the original DGN file.

Step-by-Step: Attaching a DGN Underlay

DGNIMPORT — Converting DGN Geometry to AutoCAD Objects

DGNIMPORT reads a DGN file and creates native AutoCAD objects from the DGN elements. Lines become Lines, text becomes MText, levels become Layers. This is a one-time translation — the DGN file is not linked afterward.

Step-by-Step: Importing a DGN File

What Translates — and What Doesn't

DGNEXPORT — Sending AutoCAD Drawings to DGN Format

When a client or agency requires you to deliver drawings in DGN format, use DGNEXPORT. AutoCAD translates the DWG content into a DGN V8 file.

Level Mapping — Controlling How DGN Levels Become AutoCAD Layers

By default, AutoCAD translates DGN level names directly to AutoCAD layer names. But in practice — especially with V7 DGN files where levels are numbers (1, 2, 3...) — you often need to map DGN levels to specific AutoCAD layer names, colors, and linetypes. This is done with a .dgnmapping file.

The .dgnmapping File

A DGN mapping file is an XML file that tells AutoCAD how to translate between DGN and DWG properties during DGNIMPORT and DGNEXPORT. You create and manage mapping files with the DGNMAPPING command.

Example Level Mapping Table (V7 DOT Standard)

Note: Actual DOT level standards vary by state and project. Always check the project's CAD standards manual for the official level-to-layer mapping.

DGN Units — Master Units and Sub-Units

DGN files use a two-tier unit system that is different from AutoCAD's single drawing-unit system. Getting this right is critical for correct scaling when importing or attaching DGN files.

How DGN Units Work

Master Units (MU) are the primary measurement unit — like feet or meters. Sub-Units (SU) are the subdivision of the master unit — like inches within feet, or millimeters within meters. Positional Units (PU) are the internal precision units that MicroStation uses for calculations.

DGN File Versions — V7 vs V8 vs V8i

There are two distinct DGN file formats. Knowing which version you have determines what features are available and how well AutoCAD can translate the content.

Key V7 Limitations You'll Hit

• Only 63 levels — complex drawings may have everything squeezed onto a few levels, making layer mapping difficult
• Level names are numbers, not descriptive — you need the project's CAD standards document to know what Level 5 means
• No embedded level descriptions — you can't look at the DGN to understand the level purpose
• Cell libraries are separate files — cells in a V7 DGN may not resolve unless the original .cel files are present
• Custom line styles often don't translate — they typically come in as Continuous

Multi-Model DGN Files

Unlike DWG files which have one Model Space and multiple Layout tabs, a single V8 DGN file can contain multiple completely separate models. A typical DOT project DGN might contain:

When you DGNATTACH or DGNIMPORT, you select which model to bring in. You can only process one model at a time. For a DOT bridge project, you might need to separately import the Plan model, the Profile model, and the Cross-Section model — each stored as a separate model in the same DGN file.

Government / DOT Workflow — Working with DGN on Real Projects

State DOTs typically require that all roadway design drawings be prepared or delivered in MicroStation DGN format to their specific standards. Here's how that workflow typically intersects with AutoCAD:

Scenario 1: You Receive DOT Background DGN Files

The DOT provides you with base map DGN files (survey, existing road geometry, right-of-way) and you need to overlay your utility/structural design on them.

Scenario 2: You Receive a DGN That Needs to Become a DWG

The project was done in MicroStation and you're taking it over in AutoCAD, or a subcontractor delivered DGN files and you need them as DWG for your drawing set.

DGN System Variables

These system variables control how DGN underlays display and behave. They work like the equivalent DWF/PDF underlay variables.

Common DGN Problems and Fixes