Objective stratification and sampling-effort allocation of ground-truthing in benthic-mapping surveys

Clements, A. J., Strong, J. A., Flanagan, C., and Service, M. 2010. Objective stratification and sampling-effort allocation of ground-truthing in benthic-mapping surveys. – ICES Journal of Marine Science, 67: 628–637. The application of statistical procedures for objective stratification of sampling effort during map ground-truthing is presented. Marine benthic mapping is usually undertaken in two stages: a remotely sensed acoustic survey followed by ground-truthing to confirm ground-type and habitat classification. The objective of this study was to assess the application of optimum allocation analysis (OAA) through the use of remotely sensed data to direct expensive ground-truthing sampling effort. At an offshore site in the Irish Sea, classification of remotely sensed data, namely bathymetry and slope angle, generated six predicted ground-types. Calculated data variances within each ground-type were assumed to be a predictor of substratum heterogeneity, and these were used in an OAA to apportion ground-truthing effort objectively within each ground-type in order to achieve a set level of sampling precision. The sampling effort recommended by the OAA was realistic and practical with regard to video footage, but the collection of grabs was limited by resource constraints. The coefficient of variation (CV) of the video ground-truthing data matched that estimated by OAA, but the inability to collect all the recommended grabs resulted in CVs greater than expected for sediment grain-size parameters. The efficient identification of substratum classes using OAA represents a first stage whereby this method could direct ground-truthing that could ultimately be used for habitat mapping.