GeoDLL supports the development of geodetic software on various platforms by providing geodetic functions. GeoDLL contains precise functions of the themes 2D and 3D coordinate transformation, geodetic datum shift and reference system convertion, meridian strip changing, user defined coordinate and reference systems, distance calculation, Digital Elevation Model, NTv2 handling and map function. GeoDLL enables to perform professional grade coordinate transformations in own programs: 1. The current and many historical coordinate and reference systems of all countries of the European Union (EU) including the eastern extensions from 2004 and 2007 and the systems of the European non-EU countries. 2. The US and Canadian State Plane Coordinate Systems (SPCS) on NAD27 and NAD83 and other coordinate and reference systems of the North American continent. 3. The coordinate and reference systems of the Australasian continent. 4. Many coordinate and reference systems of the countries of other continents. 5. World-wide NTv2 grid file supported coordinate transformations. 6. All world-wide coordinate and reference systems in current use. 7. The German coordinate and reference systems of the old and new federal states, the 40 Prussian Soldner Land Registers, the German ‘Lagestatus’, and exact reference systems of the German federal states. User coordinate systems, local reference systems and earth ellipsoids can be defined, based on the coordinate system parameters, the semi axes of the earth and seven Helmert or three Molodenski datum parameters. GeoDLL comes with extensive documentation and is supplied as a DLL file or as C++ or CA-Visual Objects source code. The functions of GeoDLL are suitable for multithreading. The DLL is written in the C++ language. That gives very fast performance, compact code, and high stability in use.