From FSDeveloper Wiki
BGL is the file format used by Microsoft Flight Simulator to store scenery. It uses the .bgl file extension. BGL stands for BAO Graphical Language, where BAO stands for Bruce Artwick Organisation who was the original creator of the Flight Simulator series.
There are a lot of different 'types' of BGL file, each with their own structure, each containing different things, but all with the *.bgl extension. It is best to think of the BGL format as a 'container' into which Flight Simulator objects are placed.
(Created using Resample)
- Mesh: A 3D surface representing terrain onto which textures and models are placed.
- Aerial Imagery: An aerial photo converted for use in Flightsim.
- Landclass: A low resolution raster where each pixel represents and type of landcover (i.e. farmland, bush) and which, in turn, loads the relevant texture to 'paint' the mesh.
- Waterclass: A low resolution raster where each pixel represents and type of water (I.e. Tropical, muddy) and which, in turn, loads the relevant texture to alter the colour of the water.
- Population Density: A raster where each pixel represents the number of people living in the area.
 Vector data
(Created using Shp2Vec)
- Roads and Railway information: A vector line calling the relevant texture and placed on the mesh.
- Freeway information: An 'invisible' vector line calling the road traffic effect (i.e. moving cars)
- Water polygons: 3D vector polygons using a water texture/effect and placed using their 3D vertice values (I.e. they override the terrain mesh)
- LandClass polygons: Vector Polygons where the attribute of the polygon represents the type of landcover and which, in turn, loads the relevant texture to 'paint' the mesh.
- Shoreline information: A vector line calling the wave effect (I.e. moving waves)
In the default FSX data, bgl files containing these items are prefixed with cvx (i.e. cvx8845.bgl)
 Airport & Objects
(Created using BGLComp for FSX or FS2004)
- Compiled Runway, taxiway, apron information, etc (XML)
- Custom buildings placed in a certain position (MDL + XML)
- Custom buildings and put into an object library (MDL + XML)
- Library buildings placed in a certain position (XML)
- Effects placed in a certain position (XML)
Legacy BGL files were made with BGLC or SCASM.