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Positions on and near the Earth's surface can be described by systems of =
spatial referencing. These are of two basic types:

a) those using coordinates;

b) those based on geographic identifiers (for example postal addresses, =
administrative areas).

Coordinates are unambiguous only when the coordinate reference system to =
which those coordinates are related has been fully defined. A coordinate =
reference system is a coordinate system that has a reference to the =
Earth. This document describes the elements that are necessary to fully =
define various types.