Precision and accuracy of common coral reef sampling protocols revisited with photogrammetry.

The rapid decline of coral reefs calls for cost-effective benthic cover data to improve reef health forecasts, policy building, management responses and evaluation. Reef monitoring has been largely based on divers' observations along transects, and secondarily on quadrat-based protocols, video and photographic records. However, the accuracy and precision of the most common sampling approaches are not yet fully understood. Here, we compared benthic cover estimates from three common sampling protocols: Reef Check (RC), Atlantic and Gulf Rapid Reef Assessment (AGRRA) and photoquadrats (PQ). The reef cover of two contrasting sites was reconstructed with ∼450 m2 orthomosaics built with high resolution Structure-from-Motion (SfM) photogrammetry, which were used as references for comparisons among protocols. In addition, we explored sample size requirements for each protocol and provided cost-effectiveness comparisons. Our results evidenced between-reef differences in the accuracy and precision of estimates with the different protocols. The three protocols performed similarly in the reef with low macroalgal cover (<0.5%), but PQ were more accurate and precise in the reef with relatively high (∼20%) macroalgal cover. The sample size for estimating coral cover with a 20% error margin and a 0.05 significance level was lower for PQ, followed by AGRRA and RC. Considering performance, cost surrogates and equipment needs, cost-effectiveness was higher for PQ. We also discuss costs, limitations and advantages/disadvantages of SfM photogrammetry as a sampling approach for coral reef monitoring.

Image dataset acquired from an unmanned aerial vehicle over an experimental site within El Soldado estuary in Guaymas, Sonora, México.

It is well known that remote sensing is a series of procedures which detects physical characteristics of the earth surface by remotely-measuring its reflected and emitted radiation using cameras or sensors. Lately, the increasing use of unmanned aerial vehicles (UAVs) as remote sensing platforms and the development of small-size sensors have resulted in the expansion of continuous monitoring of earth surface at smaller spatial scales. For this reason, the integration of UAV- and consumer-grade cameras can be useful to acquire surface characteristics at plot or footprint scale. This dataset contains 314 aerial images covering an area of aproximately 18,800 m2 within the footprint of an Eddy covariance and meterorological station. The monitoring site was deployed at "El Soldado" estuary (27°57'14.4″ N and 110°58'19.2″ W) located in the southern coast of the Mexican State of Sonora. UAV flight path was programmed to flight in autonomous mode with an altitude of 30 m, a velocity of 5 m/s and a frontal and side overlap of 85 and 75% respectively. This dataset was created to support mapping surveys for surface classification and site description. This dataset is aimed to support researchers, stakeholders and general public interested in coastal areas, natural resources management and ecosystem conservation. Finally, this dataset could be also used for those interested in digital photogrammetry and 3D reconstruction as benchmark example to develop high resolution orthomosaics.