Format of the Cvx BGL Files

Introduction

CVX Files can be read by the TmfViewer.exe application provided with the FS SDK. They contain TERRAIN_VECTOR_DB sections. These sections and subsections describe the different layers (roads, railways, water polygons, etc.) along with the geographical coordinates.
In this document, I will often take examples from the cvx2815.bgl file since that is the one I used to figure out the structure of the cxv file and it describes the area I live in. This file is located in: (....)\Microsoft Flight Simulator X\Scenery\0301\scenery.

BGL Common Format

All BGL files share the same generic format. A BGL file is made of a header, sections, subsections and subsection data. Subsections are children of sections. The sections are here to help us locate the subsections in the file. Data specific information is contained in the subsections data and their format is dependent on the section type.
A BGL file always start with a header (Size = 0x38 bytes), followed by a list of section pointers. The number of section pointers is defined in the header.

File Header

The header consists of 0x38 (56) bytes. It contains the number of sections defined in the file as well as the bounding geographical coordinates of the covered squared area.

Offset	Number of bytes	Description
0x00	4 - DWORD	Magic Number #1 – Must be 0x01, 0x02, 0x92, 0x19
0x04	4 - DWORD	Header size : 0x38
0x08	4 - DWORD	Unknown – Maybe a timestamp
0x0C	4 - DWORD	Unknown – Maybe a timestamp
0x10	4 - DWORD	Magic Number #2 – Must be 0x03, 0x18, 0x05, 0x08 Maybe to identify the FS version
0x14	4 - DWORD	The number of sections following this header.
0x18	32	Array[8] of unsigned integers (DWORD). Each value describes the bounding coordinates of a squared subarea. Even if 8 slots are provided, it is not necessary to have all 8 values filled. The list stops at the first null (0x00000000) value. I don’t know what these subareas are used for. To compute the bounding coordinates, please see Annexe A. To get the bounding coordinates of the square area covered by the file, just keep the minimal and maximal values of each bounding coordinates.

Example

For example, in cvx2815.bgl :

Offset	Values	Description
0x00	01 02 92 19 | Magic Number #1 |- | 0x04 | <pre>38 00 00 00	Header size
0x08	EF 82 DF E2
0x0C	E8 C7 C6 01
0x10	03 18 15 08	Magic Number #2
0x14	01 00 00 00	1 section following this header
0x18	E8 07 02 00	MinLatitude(Deg) = 46.40625 MaxLatitude(Deg) = 47.8125 MinLongitude(Deg) = -75.0 MaxLongitude(Deg) = -73.125
0x1C	E9 07 02 00	MinLatitude(Deg) = 46.40625 MaxLatitude(Deg) = 47.8125 MinLongitude(Deg) = - 73.125 MaxLongitude(Deg) = -71.25
0x20	EA 07 02 00	MinLatitude(Deg) = 45.0 MaxLatitude(Deg) = 46.40625 MinLongitude(Deg) = -75.0 MaxLongitude(Deg) = -73.125
0x24	EB 07 02 00	MinLatitude(Deg) = 45.0 MaxLatitude(Deg) = 46.40625 MinLongitude(Deg) = -73.124 MaxLongitude(Deg) = -71.25
0x28	00 00 00 00
0x2C	00 00 00 00
0x30	00 00 00 00
0x34	00 00 00 00

You’ll notice that only 4 subareas are defined (on a possibility of 8) and the last 4 available slots are empty (Value = 0)
So the bounding coordinates of the area covered by cvx2815.bgl are:

MinLatitude(Deg) = 45.0

MaxLatitude(Deg) = 47.8125

MinLongitude(Deg) = -75.0

MaxLongitude(Deg) = -71.25

Sections

Following the header, at offset 0x38, there are as many sections as defined at offset 0x14 of the header. Each section has a size of 20 bytes and has the following structure:

Relative Offset	Number of bytes	Description
0x00	4 - DWORD	Section type: one of the following values: None = 0x0 Copyright = 0x1 Guid = 0x2 Airport = 0x3 Nav = 0x13 Ndb = 0x17 Marker = 0x18 Boundary = 0x20 Waypoint = 0x22 Geopol = 0x23 SceneryObject = 0x25 AirportNameIndex = 0x27 VorIcaoIndex = 0x28 NdbIcaoIndex = 0x29 WaypointIcaoIndex = 0x2A ModelData = 02B AirportSummary = 0x2C Exclusion = 0x2E TimeZone = 0x2F TerrainVectorDb = 0x65 TerrainElevation = 0x67 TerrainLandClass = 0x68 TerrainWaterClass = 0x69 TerrainRegion = 0x6A PopulationDensity = 0x6C AutogenAnnotation = 0x6D TerrainIndex = 0x6E TerrainTextureLookup = 0x6F TerrainSeasonJan = 0x78 TerrainSeasonFeb = 0x79 TerrainSeasonMar = 0x7A TerrainSeasonApr = 0x7B TerrainSeasonMay = 0x7C TerrainSeasonJun = 0x7D TerrainSeasonJul = 0x7E TerrainSeasonAug = 0x7F TerrainSeasonSep = 0x80 TerrainSeasonOct = 0x81 TerrainSeasonNov = 0x82 TerrainSeasonDec = 0x83 TerrainPhotoJan = 0x8C TerrainPhotoFeb = 0x8D TerrainPhotoMar = 0x8E TerrainPhotoApr = 0x8F TerrainPhotoMay = 0x90 TerrainPhotoJun = 0x91 TerrainPhotoJul = 0x92 TerrainPhotoAug = 0x93 TerrainPhotoSep = 0x94 TerrainPhotoOct = 0x95 TerrainPhotoNov = 0x96 TerrainPhotoDec = 0x97 TerrainPhotoNight = 0x98 FakeTypes = 0x2710 IcaoRunway = 0x2711