Stock, Andy (2015): Secchi depth maps for the Baltic Sea, 2003-2012, with links to GeoTIFs [dataset]. PANGAEA, https://doi.org/10.1594/PANGAEA.834265,

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Supplement to: Stock, A (2015): Satellite mapping of Baltic Sea Secchi depth with multiple regression models. International Journal of Applied Earth Observation and Geoinformation, 40, 55-64, https://doi.org/10.1016/j.jag.2015.04.002

Secchi depth is a measure of water transparency. In the Baltic Sea region, Secchi depth maps are used to assess eutrophication and as input for habitat models. Due to their spatial and temporal coverage, satellite data would be the most suitable data source for such maps. But the Baltic Sea's optical properties are so different from the open ocean that globally calibrated standard models suffer from large errors. Regional predictive models that take the Baltic Sea's special optical properties into account are thus needed. This paper tests how accurately generalized linear models (GLMs) and generalized additive models (GAMs) with MODIS/Aqua and auxiliary data as inputs can predict Secchi depth at a regional scale. It uses cross-validation to test the prediction accuracy of hundreds of GAMs and GLMs with up to 5 input variables. A GAM with 3 input variables (chlorophyll a, remote sensing reflectance at 678 nm, and long-term mean salinity) made the most accurate predictions. Tested against field observations not used for model selection and calibration, the best model's mean absolute error (MAE) for daily predictions was 1.07 m (22%), more than 50% lower than for other publicly available Baltic Sea Secchi depth maps. The MAE for predicting monthly averages was 0.86 m (15%). Thus, the proposed model selection process was able to find a regional model with good prediction accuracy. It could be useful to find predictive models for environmental variables other than Secchi depth, using data from other satellite sensors, and for other regions where non-standard remote sensing models are needed for prediction and mapping. Annual and monthly mean Secchi depth maps for 2003–2012 come with this paper as Supplementary materials.

Monthly and annual (April-September) mean Secchi depth maps for 2003-2012, as well as 10-year averages for each month and the April-September period.

The maps were made with a Generalized Additive Model (GAM) that used MODIS data (chlorophyll a and remote sensing reflectance at 678nm) as well as long-term mean salinity to predict Secchi depth. Tested against regional-scale field observations, the monthly mean Secchi depth maps had a mean absolute error of 0.86 m.

All data are in GEOTIF format and come with spatial resolution of 2 km, 4 km and 6 km. The coarser resolution data were derived from the lower resolution data using ArcGIS' cell statistics tool. The coordinate system is Lambert Azimuthal Equal Area (LAEA), ETRS 1989.