A comparison of calibration methods and system configurations of underwater stereo-video systems for applications in marine ecology

Abstract

Stereo-video systems are used by marine ecologists to count and measure the size of aquatic organisms.Most marine ecologists are not experts in underwater photogrammetry and are not necessarily aware of someof the key design and implementation issues that influence the accuracy and precision of the measurements.Two factors that can influence the measurement accuracy and precision of a stereo-video system are the cali-bration strategy and the physical orientation of the system. We compare the accuracy and precision of lengthmeasurements made by systems calibrated using two regularly used calibration approaches. We also comparelength measurements recorded from cameras with base separations of 150 mm, 400 mm, and 800 mm. Athree-dimensional (3D), network calibration method using a purpose built calibration cube was used to deter-mine key calibration and camera orientation parameters (e.g., base separation, focal length, and lens distor-tions) with much improved precision in comparison to a two-dimensional method using either an A3 or A4planar calibration pattern. As a result, measurements made with a 3D cube displayed improved accuracy andprecision compared to either the A3 or A4 planar calibration pattern across a range of typical operational dis-tances. The distance between the cameras on the base bar (base separation) affects the accuracy and precisionof measurements with the 800 mm system producing length estimates of improved accuracy and precisionthan the systems with narrower baselines. The 150 mm system resulted in measurements with poor precisionand accuracy, especially for measurements made at distances greater than six meters.