In order to identify differences between physical and digital mock-ups, which do not match exactly, and to transfer those differences back into CAD format, we developed an AR based measuring system. The system allows matching the CAD data with real mock-ups and documenting differences between them. Essential functions like measurement and online construction are provided, allowing the end-users to create information in AR space and feeding them back into the CAD model. The application was developed within an authoring tool based on the instant*reality*-framework. Furthermore, it was presented on ISMAR ’07. The tool was developed for the needs of the submarine engineering of (Howaldtswerke – Deutsche Werft GmbH), who also funded the project.