A vivid relic under rapid transformation

Thomas Knudsen


TRLIB, the transformation system of the Danish National Survey and Cadastre (KMS), has served as the official tool for transformation between Danish map projections and horizontal and vertical datums for half a century. TRLIB system originated as a fairly simple system using one of the first compilers for the Algol-60 language, but it has evolved dramatically since then. Recently the extended transverse mercator implementation of TRLIB was included in PROJ, the leading open source projection package. This is well in line with other efforts towards making TRLIB more interoperable with other open source projection and transformation packages.

The main objective of this paper is to briefly outline the historical origins of TRLIB, and to describe the architectural design considerations behind an ongoing effort to make TRLIB ready for today’s requirements, most notably by making it 64-bit clean and thread safe. But since the TRLIB code is used in a large number of commercial GIS products, the design considerations are also influenced by the importance of maintaining a stable, backwards compatible version, while also utilizing this golden opportunity to simplify and improve the application program interface.


Geodesy; cartography; history; transformation; datum shifts

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