Explainer
What Is a CGM File? Aerospace's Illustration Format
The Format
Vector Graphics, Before the Web
The Computer Graphics Metafile is an ISO-standardized vector format from the 1980s: an illustration stored as drawing instructions, not pixels. Technical illustration adopted it for the reasons vector always wins for line art — exploded views and schematics that stay razor-sharp at any zoom, print cleanly at any size, and remain editable — and because aerospace couldn't wait for the web to invent SVG. By the time browsers had vector graphics at all, fleets were flying with illustration libraries in the hundreds of thousands of CGMs.
The version that matters for publications is WebCGM, the profile standardized for interactive use — and its defining feature is the hotspot: regions of the drawing carrying identifiers that link them to content. The illustration stops being a picture and becomes an interface.
Why It Persists
The Illustration Is Navigation
Inside an IETM, hotspots do real work: click the pump in the figure and the viewer opens its removal procedure or its parts entry; touch a callout and the matching row highlights. Those links are data — identifiers connecting figure regions to content — which makes a hotspot-linked illustration library one of the most valuable holdings a program owns, and one of the easiest to destroy through careless handling. The panic move we warn against is flattening everything to raster “for compatibility”: it discards the scalability and severs every link, the charts-as-pictures mistake at industrial scale.
New programs can and often do go SVG-first — but CGM isn't mere inertia. Contracts specify it, toolchains speak it, and decades of content in it still flies, the same survival logic that keeps SGML in service. The right posture is delivery-focused: serve the library through a platform that renders it well — natively or via converted renditions with the source preserved — and carry the hotspot identifiers forward as data, whatever the pixels do. Handled that way, a thirty-year-old illustration library isn't technical debt. It's the most structured graphics estate in your building.
FAQ
Questions We Hear
What is CGM?
The Computer Graphics Metafile — an ISO-standardized vector graphics format (ISO/IEC 8632) dating to the 1980s, storing an illustration as drawing instructions rather than pixels. Aerospace and defense technical illustration standardized on it long before the web existed, because the needs were the same ones vector formats always serve: line art that stays crisp at any zoom, prints cleanly, and remains editable — for parts diagrams, schematics, and exploded views by the hundreds of thousands.
What is WebCGM, and what are hotspots?
WebCGM is the profile of CGM standardized for web and interactive use, and its defining feature for technical publications is the hotspot: a region of the illustration carrying an identifier that links it to content. Click the pump in the exploded view and the viewer opens the pump's removal procedure or its parts entry; highlight a callout number and the matching row lights up. That figure-to-content linking is a load-bearing feature of IETMs — the illustration is navigation, not decoration.
Why not just use SVG?
New programs often can — SVG is the native vector language of the web, and modern viewers and standards accommodate it. But CGM's position isn't inertia alone: decades of illustration libraries exist in it, illustration toolchains and viewer ecosystems in aerospace speak it fluently, and contracts and program baselines specify it. Format migrations in regulated documentation are projects, not preferences — so CGM persists for the same reason legacy formats always persist: the content in it still flies.
We hold a large CGM library. What should we do with it?
Treat it as an asset, not a liability — vector sources with structured hotspot linking are the highest-value illustration holdings there are. The practical question is delivery: modern platforms either render CGM directly or convert renditions to web-native formats while preserving the source, and hotspot identifiers should be preserved as data either way, because they encode the figure-to-content links a viewer needs. What to avoid is the panic move: flattening everything to raster images 'for compatibility,' which throws away exactly the scalability and linking that made the library valuable.
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An Illustration Library Worth Keeping?
If your CGM estate is headed for a flatten-to-PNG 'modernization,' stop the truck. We deliver legacy illustration libraries with their links and their sharpness intact.