Natural Area Code
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The Natural Area Code is a proprietary geocode system for identifying an area anywhere on the Earth, or a volume anywhere in the universe. The use of thirty alphanumeric characters instead of only ten digits makes a NAC shorter than its numerical latitude/longitude[/altitude] equivalent.
The system is copyrighted by NAC Geographic Products, Inc.
Two-dimensional system
Instead of numerical longitudes and latitudes, a grid with 30 rows and 30 columns - each denoted by the numbers 0-9 and the twenty consonants of the Latin alphabet - is laid over the globe. The datum used is the World Geodetic System (WGS) of 1984. A NAC cell can be subdivided repeatedly into smaller NAC grids to produce an arbitrarily small area.
The NAC represents an area on the earth. When the represented area is small enough, a NAC represents a location. Therefore, it is a unified representation of both an area and a location. An eight or ten character NAC is also called a Universal Address, because it can uniquely specify every building, house, parking meter, fire hydrant, street light, tree or any other fixed object in the world. An eight-character NAC specifies an area no larger than 25 metres by 50 metres, while a ten-character NAC cell is no larger than 0.8 metres by 1.6 metres.
For example, the ten-character NAC for the centre of the city of Brussels is HBV6R RG77T.
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 RG77T H0000.