Sustainability of Digital Formats: Planning for Library of Congress Collections

Design files, abbreviated as DGN, is a drawing file format for CAD applications used by architects and engineers to draft technical drawings of buildings.

There are two versions of DGN, after Intergraph was acquired by Bentley.

1) The original Intergraph Standard File Formats (ISFF) specification published in the late 1980s by Intergraph. This version is referred to as Intergraph DGN or V7 DGN. Versions 7 and below are this version.

2) Bentley V8 DGN, a version of DGN updated by Bentley Systems in 2000 and includes a set of DGN's capabilities but a different internal data structure. It is used by MicroStation 8 software. This version is also referred to as the Bentley Systems version or MicroStation DGN. Versions 8 and above are this version.

There may be compatibility issues between versions of the MicroStation software and versions of the DGN format. Different versions of MicroStation save DGN files in slightly different formats.

ISFF Structure

The ISFF structure consists of several types of binary files. As summarized by Wikipedia:

• Design files (DGN): binary files with variable-length records for graphic elements, and non-graphic data.
• Cell library files (CEL): binary files that store cell definitions for placement in design files.

DGN and Cell library files are defined in the same ISFF specification.

DGN files have a specific byte order:

• Short integers are stored with little-endian ordering
• Long integers are stored with middle-endian byte ordering

DGN files store all floating-point variables in the VAX D-Float format. The specification also notes, "MDL and MicroCSL automatically convert floating-point variables to the native format of the CPU in use. Bits are labeled from the right, 0-63."

The first three elements of the DGN are the design file header. The header holds symbology (color, line style, and line weight) and primitive elements, and complex elements. Primitive elements are considered to be lines, line strings, shapes, ellipses, arcs, text, and cones. Complex elements are a set of elements that form a single entity. Cells, surfaces, solids, and B-splines are all under this category of complex elements. The maximum combined length of the header and all component elements cannot be greater than 65535 words.

DGN elements have a fixed header, with the first 18 values, considered "words" in the specifications parlance, containing the element type, level, words to follow, and range information.

The first word in the header defines the element's type and level. The second word in the element header indicates the total word count (excluding words 1 and 2). For complex elements, this defines the length of only the header element and not component elements.

Words 3-14 of the element header contain the six long integers that define the element's range -- its low and high x, y, and z coordinates in absolute units of resolution (UOR). These words are stored as middle endian, offset binary.

Word 15 contains the element's graphic group number. If zero, the element is not in a graphic group. If non-zero, the element is in a graphic group with all other elements that have the same graphic group number.

Word 16 defines the number of words existing between (and excluding) word 16 and the first word of the attribute data. Attribute data is optional and may or may not be present.

Word 17 describes the element's properties.

Word 18 defines the element's symbology.

DGN files can store a MicroStation concept of "tag sets" and "tags" which allows text with a "tag" identifier that is part of a named "tag set" to be stored in DGN files.

Files can also store non-graphic data such as reference file attachments and named views.

MicroStation V8 DGN Structure

From Bentley (link via Internet Archive): "A MicroStation V8 DGN file is composed of one or more models. A model is a container for elements. The V8 DGN removes all file format-based constraints that were present in previous versions of MicroStation." According to the ArchiveTeam wiki, DGN versions 7 and below are binary formats, and version 8 was changed to be an "OLE2" document container format.

The MicroStation V8 DGN format is different from the previous format in a few ways. Levels, references, components, graphic groups, text nodes, no longer have a limit. Each level is named and stored as an element. Bentley DGN files do not have a maximum size limit. Cells have no size limits but have a name limit of approximately 500 characters. Single elements are 128 KB maximum. A line string, shape, or point curve can have up to 5000 vertices. Other details are in the MicroStation V8 DGN File Format documentation (link via Internet Archive).

Intergraph Standard File Formats (Element Structure). An HTML copy of this PDF is also available online.

MicroStation V8 DGN File Format documentation lists the differences between the V8 and ISFF formats.

The format was listed as open by Bentley (link via Internet Archive), but it is available only by request from the company. There is a conversation on the Bentley MicroStation Programming Forum that contests the status of the specification as "open."

Requires advanced tooling. Limited documentation and technical changes between versions will require effort to understand the format.

According to FileFormat.com's entry on DGN, DGN can be converted to several other formats such as DWG, BMP, JPEG, PDF, GIF and others. It can be opened with Autodesk AutoCAD, Bentley View and Bentley Systems MicroStation in addition to other software applications such as Corel PaintShop Photo Pro and IMSI TurboCAD Deluxe versions.

Comments welcome.

DGN supports complex elements, which the specification defines as lines, line strings, arcs, curves, and open B-Spline curves.

Text node header elements are available to store text strings, spacing, origins, node numbers, and the maximum number of characters in a string.

DGN stores symbology, defined in the specification as style, weight, and color values.

DGN defines a surface or solid as a complex 3D element that is projected or rotated from a planar boundary element (line, line string, curve, arc, or ellipse).

DGN supports three raster formats: straight binary, run-length encoded binary, and a byte for each pixel (used to store images). Transparent bits, background colors, foreground colors, and clip boxes are supported.

Supported. In a DGN file, a point string element consists of a number of vertices with orientations defined at each vertex, which can support viewpoint orientation. The specification states: "Sometimes a point that is not within the design plane needs to be specified. For example, the center point for an arc may be far from the design plane, although the design plane must completely contain the arc. In these cases, points are specified as two or three double-precision (64-bit), floating point values."

The specification also notes "an element's orientation is represented by the transformation matrix to design file coordinates. These transformations are stored in a compressed format called quaternions."

The V8 DGN format is derived from MicroStation. The first MicroStation that could write DGN files was MicroStation 2.0, released in February 1987.

Non-profit consortium OpenDWG Alliance announced in a press release (link via Internet Archive) on May 16, 2003 that they will support V8 DGN. The Alliance was creating software (link via Internet Archive) that helped people use DGN files, called Teigha. This product consolidated support of both DGN and DWG files in its 2012 release. Similarly, Bentley offered OpenDGN (link via Internet Archive), an initiative for open DGN tools. Comments welcome.