Editing Natural Area Code (section)

== Extension to three dimensions ==
The full NAC system provides a third coordinate: altitude.  This coordinate is the [[arctangent]] of the altitude, relative to the Earth's radius, and scaled so that the zero point (000...) is at the centre of the Earth, the midpoint (H00...) is the local radius of the geoid, i.e. the Earth's surface, and the endpoint (ZZZ...) is at infinity.

For example, the three-dimensional NAC for the centre of Brussels, at ground level, is HBV6R&nbsp;RG77T&nbsp;H0000.