FOSS4G 2023 Prizren in various surveying projects, the use of local coordinate reference systems (CRS) tailored to project sites is common, especially when stringent accuracy requirements cannot be met by standard GNSS surveying techniques. Creating such local systems involves complex processes, often necessitating specialized tools and skills like point triangulation with total stations. However, not all project tasks require such precision; many can be performed using GNSS receivers with real-time kinematics (RTK) or post-processing kinematics (PPK), which reduces costs and time. Nonetheless, coordinates still need to be aligned with the project's local system, leading to the concept of site calibration – establishing a correspondence between a well-known CRS and the local system. This paper introduces an interoperable and open-source solution for site calibration. Unlike proprietary algorithms tied to specific software, this solution is based on open standards and can be fully implemented using open-source software. The authors outline the workflow to mathematically solve the site calibration problem and represent it as a self-contained coordinate reference system. They detail how the Eigen open-source library and the OGC 18-010r7 standard (WKT version 2) are employed. This approach ensures compatibility and interoperability among various applications. A significant contribution of the paper lies in comparing and analyzing two mathematical methods to derive WKT2 representations for site calibration. The first method generates a derived projected system using a PROJ-specific 3D transformation, while the second method splits the problem into horizontal and vertical components, resulting in a compound coordinate reference system. The paper discusses the advantages and drawbacks of each approach in terms of solution clarity and sensitivity to measurement errors, particularly in the vertical axis. Overall, the paper outlines a comprehensive, open-source solution for site calibration with potential implications for various surveying applications.