The development of new measurement techniques is changing the process of documentation and assessment of built heritage. An accurate planimetric basis is essential for the constructive evaluation of a building. This paper presents the methodology used in the geometric assessment of Gothic vaults in the Cathedral of Tortosa (Spain). The use of a Terrestrial Laser Scanner allows the assessment of the vaulting to a level of detail that has never been possible to achieve previously. The accurate identification of geometric deviations in the masonry can be combined with the information from primary sources, revealing new information about the construction process of the apse.