FlightGear, a premier open-source flight simulator, offers a level of customization that rivals commercial alternatives. Among its most compelling features is the automatic scenery generation pipeline, which empowers users to create highly detailed, custom maps from real-world geographic data. This guide provides an in-depth walkthrough of FlightGear's automatic scenery generation tools, focusing on the TerraGear suite, to help you build immersive and realistic environments for your virtual flights.

Why Use Automatic Scenery Generation?

Manual scenery creation is labor-intensive and often impractical for large areas. Automatic generation leverages global datasets like OpenStreetMap (OSM) and digital elevation models (DEMs) to produce terrain, water bodies, roads, railways, and even 3D buildings with minimal manual intervention. This approach ensures geographic accuracy, consistency, and scale, allowing you to recreate any region on Earth within FlightGear. By mastering these tools, you can transform your simulator into a platform for real-world navigation practice, exploration, or simply a more authentic flying experience.

Understanding FlightGear's Scenery Generation Tools

FlightGear’s scenery generation ecosystem is built around TerraGear, a suite of command-line tools that process raw geographic data into the simulator's proprietary format. TerraGear automates the entire pipeline: data ingestion, terrain mesh creation, land cover classification, water body mapping, road and rail routing, and building placement. It also handles projection and coordinate transformations to ensure compatibility with FlightGear's Earth-centered coordinate system.

Key Components of TerraGear

  • Data Conversion: Converts raw vector (e.g., OSM) and raster (e.g., DEM/SRTM) data into intermediate formats readable by TerraGear.
  • Terrain Generation: Produces a triangulated irregular network (TIN) from elevation data, creating a realistic 3D surface. Resolution and simplification parameters control detail vs. performance.
  • Land Cover Classification: Assigns texture types (forest, urban, farmland, etc.) to polygons based on OSM tags, using a priority system to avoid overlaps.
  • Object Placement: Automatically inserts 3D models for roads, rivers, railways, pylons, and buildings. TerraGear includes a building synthesizer that generates generic 3D structures from OSM building footprints.
  • Lighting and Material: Generates material files that define how surfaces interact with light, including seasonal variations (snow cover, leaf colors).

Preparing Your Environment and Data

Before diving into generation, you need a properly set up development environment. TerraGear runs on Linux, macOS, and Windows (via WSL or Cygwin). We recommend using a dedicated directory for scenery projects to keep things organized.

Installing TerraGear

The easiest way is to compile from source or use pre-built binaries. For detailed instructions, refer to the official FlightGear Wiki: TerraGear. On Ubuntu/Debian, you can install dependencies and build from the GitLab repository. Ensure you have tools like cmake, g++, libboost-dev, and libgdal-dev installed.

Obtaining Geographic Data

You will need two primary data types:

  • Elevation Data: SRTM (Shuttle Radar Topography Mission) is widely used. Download .hgt files from sources like the USGS Earth Explorer or use pre-processed versions available in FlightGear community repositories.
  • Vector Data: OpenStreetMap is the best choice for roads, water, railways, and building footprints. You can download region-specific OSM extracts from providers like Geofabrik or the Planet OSM file.

Organizing Your Working Directory

Create a folder structure like this:

project/
├── data/
│   ├── osm/           # OSM .pbf or .osm files
│   ├── elev/          # SRTM .hgt files
│   └── output/        # Generated scenery
├── terragear/          # Compiled tools
└── config/             # Configuration files and work scripts

Configuring and Running the Generation Pipeline

TerraGear uses a modular pipeline controlled by shell scripts. The most common workflow involves four main stages: data preprocessing, terrain generation, land cover and object placement, and final packaging.

Stage 1: Data Preprocessing

Convert raw OSM and elevation data into TerraGear's intermediate format. Use ogr-decode for OSM files. For example:

ogr-decode --area 48.0:-124.0:49.0:-122.0 \
  --max-segment-length 500 \
  data/output-ogr/ \
  data/osm/region.osm.pbf

For elevation data, use gdal-decode to convert SRTM files into TerraGear's .btg format. Set the appropriate projection (usually EPSG:4326) and desired resolution.

Stage 2: Terrain Generation

Run tg-construct to build the terrain mesh. Create a configuration file (config/work.conf) specifying your data paths and parameters:

[Terrain]
Elevation data = /path/to/elev
Output dir = /path/to/output
Triangulation mode = 5
Simplification tolerance = 10.0
Texture mapping = ortho

Execute with:

tg-construct --config config/work.conf

The Simplification tolerance controls mesh quality: lower values (e.g., 5) give high detail but large file sizes; higher values (e.g., 20) prioritize performance.

Stage 3: Land Cover and 3D Objects

Generate material classifications and 3D objects using tg-construct with additional flags. For buildings, you need a building rules file. A basic example:

[BuildingLayer]
Height from tag = building:levels
Default height = 6.0
Randomize height = true

Run: tg-construct --buildings=config/buildings.conf

Stage 4: Final Assembly and Export

After all layers are processed, use tg-merge to combine them into a single scenery tile. Then copy the resulting .btg.gz and .mat files into your FlightGear scenery directory (usually ~/fg-scenery/Scenery/).

Advanced Tips for Realistic Results

Optimizing OSM Data Filters

To avoid clutter, filter out unnecessary OSM tags. Use a --filter argument in ogr-decode to include only relevant features (e.g., highways, rivers, buildings). For example, to keep only primary and secondary roads:

--filter "highway IN ('primary', 'secondary')"

Using Orthoimagery

For stunning visual fidelity, tile orthophoto imagery and apply it as a ground texture. Use tools like vf-tools or terraphoto (third-party) to generate an orthophoto layer. This replaces the generic land cover textures with real satellite imagery. Be mindful of disk space—ortho tiles can be large.

Adjusting Building Density

OSM building footprints may be incomplete or overly dense. Use tg-construct's --building-density parameter to cull smaller buildings or adjust the minimum area threshold. This can improve performance in urban areas without sacrificing the overall look.

Integrating 3D Models

Place custom 3D models (e.g., landmarks, airports) using the ObjectPlacement stage. Provide an OSM file of points of interest and a mapping file that links OSM tags to specific .ac models. The FlightGear model database (see wiki) offers a collection of community-contributed models.

Troubleshooting Common Issues

  • Missing Textures or Black Terrain: Ensure your material files (.mat) reference correct texture paths. Check that the texture directories exist in $FG_ROOT/Textures.
  • Generation Errors: Examine log files for warnings. Common problems include OSM data clipping errors (ensure your area bounds are correct) or missing elevation data for coastal tiles.
  • Performance Stutters: Reduce terrain simplification tolerance or restrict the number of buildings. Disable orthoimagery if using it—switch back to procedural textures.
  • Coordinate Offsets: Verify that all data uses WGS84 (EPSG:4326). Mismatched projections cause scenery to appear in the wrong location.

Conclusion

FlightGear's automatic scenery generation tools unlock the power to create any landscape on Earth with remarkable fidelity. By mastering TerraGear’s pipeline—from data acquisition through configuration and final packaging—you can produce custom maps that enhance navigation training, visual exploration, or simply personal enjoyment. Start with small areas to iterate quickly, then scale up as you refine your workflow. The FlightGear community is rich with resources; refer to the official guide on building custom scenery for further reading. With persistence, you'll be flying over your own perfectly rendered custom territory in no time.