FSX BGL compression format explained: learn why BGL is not a ZIP archive and how imagery, terrain mesh and object scenery differ.
FSX scenery BGL files do not use one universal compression format. A BGL is a compiled Microsoft scenery container, not a ZIP-style archive. Object and airport BGLs store binary scenery records, while Resample-generated imagery and elevation BGLs use content-specific internal compression that FSX reads directly.
This applies to both boxed Microsoft Flight Simulator X and FSX: Steam Edition. If a file type must be specified, call it a proprietary FSX BGL binary, not ZIP, RAR, CAB or another general-purpose compression format.
Is an FSX BGL a compressed archive?
No: an FSX BGL is not an archive that an ordinary compression utility can unpack. Microsoft used the same .bgl extension for several scenery record types, including airports, object placement, terrain mesh, vector data and aerial imagery.
Renaming a BGL to .zip will not expose its contents. A ZIP file may contain BGLs for distribution, but those files must be extracted before FSX can load them. The BGL normally belongs in an active scenery area's Scenery subfolder, as covered in our FSX scenery folder and activation instructions.
Which FSX BGL files use internal compression?
Internal encoding depends on the scenery content and the Microsoft SDK compiler used to create it.
| Scenery content | Typical compiler | How compression works |
|---|---|---|
| Airports and object placement | BGLComp | Compiled scenery records with no general archive-compression setting. |
| Aerial or photo scenery | Resample | Tiled raster data encoded inside the BGL; the compiler can apply its supported quality and compression controls. |
| Elevation mesh | Resample | Terrain samples are packed into specialised blocks. Source resolution, LOD coverage and compression settings all affect size. |
| External textures | Texture conversion tools | DDS or BMP files may use their own texture encoding, but that is separate from BGL compression. |
Claims that every BGL uses JPEG, DXT or a particular LZ algorithm are therefore misleading. Such terms may describe an individual payload or texture, but they do not define the BGL format as a whole.
Large terrain add-ons illustrate the distinction. This global mesh package built from compiled terrain BGLs contains elevation data rather than an unpackable image archive, while this 10-metre USA mesh example specifically uses MultiLOD compilation and slight compression.
Can you recompress an existing BGL file?
You normally cannot recompress a finished BGL safely; rebuild it from the original source with the appropriate FSX SDK compiler.
- Identify the scenery type. Airport and object BGLs require a different compiler workflow from imagery or terrain mesh.
- Return to the source files. For Resample projects, adjust supported settings such as
CompressionQuality, source resolution and LOD selection. BGLComp projects do not have one equivalent archive-compression switch. - Compile a new BGL. Do not modify compressed blocks with a generic archive or hex-editing tool.
- Check the result in FSX. Inspect imagery for block artefacts and terrain for lost detail, spikes or seams at coverage boundaries.
A decompiler may understand certain BGL record types, but compilation is not guaranteed to be reversible. It may not recover the original source, names or project structure.
Does stronger BGL compression improve FSX performance?
Stronger compression does not automatically increase frame rates. It can reduce the storage and data-reading requirements of imagery or terrain, but FSX must still decode and render the scenery.
Aggressive lossy settings may introduce visible imagery artefacts or reduce elevation fidelity. They also do nothing to reduce airport object counts, draw calls or complex scenery logic. Choose stronger compression when download or storage size is the priority; use conservative settings when terrain and image quality matter more.