# Earth Gravitational Model (EGM2008)

The coordinate system used by GPS is [WGS 84](https://en.wikipedia.org/wiki/World_Geodetic_System#WGS_84), which expresses altitudes as a "height above ellipsoid".
Because this ellipsoid is a mathematical simplification, the height given by GPS does not necessarily reflect the height above the actual ground beneath.

The [Earth Gravitational Models](https://en.wikipedia.org/wiki/Earth_Gravitational_Model) published by the National Geospatial Intelligence Agency are a way to approximate how high an altitude given in WGS 84 coordinates truly is above the ground.

This library includes data derived from [EGM 2008](https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84) along with an interpolation function. Together, these allow for an approximate offset to be obtained for arbitrary points on the globe.

The `geoid.rs` file containing terrain data is [generated using the Fortran program](./tools/README.md) provided by NGA.

## Updating `geoid.rs`

The `src/geoid.rs` file is generated by a Python script (`generate.py`) that
runs NGA's Fortran interpolation program. In order to run the script, you'll
need to download the EGM model data from the NGA's website.

You will also need to install `gfortran`. There are a few ways to do this:
- On macOS, through [Homebrew](https://brew.sh) with `brew install gfortran`.
- Use the included [Nix](https://nix.dev) dev shell with `nix develop`.

For convenience, you can run `generate_geoid.sh` which will download and verify
the model data from NGA, compile the interpolation program, and update
`src/geoid.rs`.