Part 4: C
The Spatial Reference Model (SRM) precisely and unambiguously specifies the context in which positions, directions, and distances are defined. It also provides the concepts needed to convert locations, directions and distances accurately among multiple spatial reference frames.
Access to the data types and concepts defined by this specification is through an application program interface (API). The abstract specification of this API is defined in ISO/IEC 18026. Each part of ISO/IEC 18042 defines the binding of that abstract specification to a particular programming language. This document specifies a standard binding for the C computer programming language.
http://standards.iso.org/ittf/PubliclyAvailableStandards/ISO_IEC_18042-4_Ed1.html