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Recent advances in structure-from-motion techniques are enabling manyscientific fields to benefit from the routine creation of detailed 3D models.However, for a large number of applications, only a single camera is available,due to cost or space constraints in the survey platforms. Monocularstructure-from-motion raises the issue of properly estimating the scale of the3D models, in order to later use those models for metrology. The scale can bedetermined from the presence of visible objects of known dimensions, or frominformation on the magnitude of the camera motion provided by other sensors,such as GPS.  This paper addresses the problem of accurately scaling 3D models created frommonocular cameras in GPS-denied environments, such as in underwaterapplications. Motivated by the common availability of underwater laser scalers,we present two novel approaches. A fully-calibrated method enables the use ofarbitrary laser setups, while a partially-calibrated method reduces the needfor calibration by only assuming parallelism on the laser beams, with noconstraints on the camera. The proposed methods have several advantages withrespect to the existing methods. The need for laser alignment with the opticalaxis of the camera is removed, together with the extremely error-prone manualidentification of image points on the 3D model.  The performance of the methods and their applicability was evaluated on bothdata generated from a realistic 3D model and data collected during anoceanographic cruise in 2017. Three separate laser configurations have beentested, encompassing nearly all possible laser setups, to evaluate the effectsof terrain roughness, noise, camera perspective angle and camera-scenedistance. In the real scenario, the computation of 6 independent model scaleestimates using our fully-calibrated approach, produced values with standarddeviation of 0.3%.

Automatic Scale Estimation of Structure from Motion based 3D Models using Laser Scalers