A practical guide to the use of consumer-level digital still cameras for precise stereogrammetric <I>in situ</I> assessments in aquatic environments

Abstract

Scientists planning to use underwater stereoscopic image technologies are often faced with numerous problems during the methodological implementations: commercial equipment is too expensive; the setup or calibration is too complex; or the imaging processing (i.e. measuring objects in the stereo-images) is too complicated to be performed without a time-consuming phase of training and evaluation. The present paper addresses some of these problems and describes a workflow for stereoscopic measurements for marine biologists. It also provides instructions on how to assemble an underwater stereo-photographic system with two digital consumer cameras and gives step-by-step guidelines for setting up the hardware. The second part details a software procedure to correct stereo-image pairs for lens distortions, which is especially important when using cameras with non-calibrated optical units. The final part presents a guide to the process of measuring the lengths (or distances) of objects in stereoscopic image pairs. To reveal the applicability and the restrictions of the described systems and to test the effects of different types of camera (a compact camera and an SLR type), experiments were performed to determine the precision and accuracy of two generic stereo-imaging units: a diver-operated system based on two Olympus Mju 1030SW compact cameras and a cable-connected observatory system based on two Canon 1100D SLR cameras. In the simplest setup without any correction for lens distortion, the low-budget Olympus Mju 1030SW system achieved mean accuracy errors (percentage deviation of a measurement from the object's real size) between 10.2 and -7.6% (overall mean value: -0.6%), depending on the size, orientation and distance of the measured object from the camera. With the single lens reflex (SLR) system, very similar values between 10.1% and -3.4% (overall mean value: -1.2%) were observed. Correction of the lens distortion significantly improved the mean accuracy errors of either system. Even more, system precision (spread of the accuracy) improved significantly in both systems. Neither the use of a wide-angle converter nor multiple reassembly of the system had a significant negative effect on the results. The study shows that underwater stereophotography, independent of the system, has a high potential for robust and non-destructive in situ sampling and can be used without prior specialist training.

First year of practical experiences of the new Arctic AWIPEV-COSYNA cabled Underwater Observatory in Kongsfjorden, Spitsbergen

“…This correction was performed with the "stereo_gui" modified MATLAB routine (Wehkamp and Fischer, 2014). After this step, all the objects that were detected in the first image analysis step were measured (standard length in fish, carapax length in macrocrustacea, and max.…”

“…The profiling sensor carrier was fully remote controlled via the Internet and was operated year-round from October 2013 to November 2014 from Germany. It was equipped with a CTD for the assessment of the main hydrographical parameters and the RemOS1 stereo-optical camera system (Fischer, 2017;Fischer et al, 2007b;Wehkamp and Fischer, 2014) for macrobiota assessments. Using the stereooptic sensor, we assessed the macrobiota, jellyfish, and fish community along the vertical depth profile from 11 m water depth to the surface with the sensors looking from a distance of about 2.5 m towards a steep wall that reached from 11 m of water depth to 3 m below the mean sea level (Fig.…”

“…Based on these data, we analysed the temporal succession of the shallow water fish, jellyfish, and macrozoobenthos community in this kelp-dominated shallow water Arctic habitat in Kongsfjorden. Organisms on the stereoscopic images were analysed in a two-step procedure following the routines described in Wehkamp and Fischer (2014). The 48 stereoscopic image pairs of each day were first scanned manually for the presence of organisms.…”

“…In benthic crustaceans, the carapax length from the tip of the rostrum to the end of the telson (in a normal body position) was measured; for fish, the standard length; for jellyfish, the largest body dimension (either longitudinal or transversal); and for chaetognaths and pteropods, the longitudinal body axes were measured. The system allowed for an accuracy in length measurements of approximately 3 % (Wehkamp and Fischer, 2014). Figures 8 to 10 show the size-frequency distributions of the six measured groups per month over the seasonal cycle from October 2013 to November 2014.…”

“…To the best of our knowledge, there are only a small number of studies and observatories available worldwide that are trying to also assess higher trophic levels with remote controlled optical systems (Aguzzi et al, 2011;Buckland et al, 2005;Fischer et al, 2007b;Wehkamp and Fischer, 2014), and even fewer with regard to quantitative assessments with respect to a specimen's abundances and species-specific length-frequency analysis in an area. Because these technologies will certainly develop and improve over the next years, this study also discusses certain specific requirements and challenges for such systems, especially for shallow water Arctic areas.…”