PhotoQuad: A dedicated seabed image processing software, and a comparative error analysis of four photoquadrat methods

Abstract

Photographic quadrat sampling is commonly used for the study of sessile benthic communities. However, photoquadrat data handling is fragmented into several processing methods, and there is a scarcity of dedicated software tools that integrate all major analysis options. photoQuad is a new software system for advanced image processing of photographic samples, dedicated to ecological applications. The software integrates a series of methods for the extraction of species area, percentage coverage, or presence/absence information, including random point counts (RP), grid cell counts (CL), freehand regions (FH), and image segmentation-based regions (SG). These are simultaneously functional in a layer-based environment, further supported by a variety of tools for image enhancement, image calibration, automatic quadrat boundary detection, and management of user-specific species libraries. The paper documents the main features of photoQuad, and demonstrates its performance through the simultaneous application of the RP, CL, FH, and SG methods on identical datasets, and the comparison of errors in species area and coverage measurements. The simulated data used for reference are disk-shaped patches, whose area and density statistics are equivalent to three benthic species characteristic of Mediterranean coralligenous communities. The analysis indicated that measurement methods differed in area and coverage bias, as well as in their sensitivity to species size. Large patches were accurately measured by all methods in terms of mean scaled error, but CL and RP provided high error variance, and their performance deteriorated with decreasing patch size; the region-based SG and FH methods provided the lowest errors and were both robust to patch size. The image and quadrat calibration process showed no statistically significant effect on the outputs, although further analysis is needed to validate this result. Overall, photoQuad constitutes a powerful software for elaborate analysis of photoquadrat images, facilitating fast and comparable evaluation of the ecological information contained therein. The photoQuad software is freely available to download and use from: http://www.mar.aegean.gr/sonarlab/photoquad.