ALGORITHMS
GFM's flood products are based on an ensemble approach integrating three robust, cutting edge algorithms developed independently by three leading research teams.
The motivation for choosing such a methodology is to substantially improve accuracy of the derived Sentinel-1 flood and water extent maps and to build a high degree of redundancy into the production service.
As stated elsewhere, the data processing architecture underlying the different scientific algorithms is based on the data cube concept, whereby SAR images are geocoded, gridded and stored as analysis ready data (ARD) in an existing spatio-temporal SAR data cube.
By using a data cube, where the temporal and spatial dimensions are treated alike, each Sentinel-1 image can be compared with the entire backscatter history, allowing to implement different sorts of change detection algorithms in a rather straightforward manner. Importantly, the entire backscatter time series can be analysed for each pixel. Therefore, model training and calibration may be carried out systematically for each pixel.
The advantages of working with data cubes are:
- (a) algorithms are better able to handle land surface heterogeneity;
- (b) uncertainties can be better specified;
- (c) regions where open water cannot be detected for physical reasons (e.g. dense vegetation, urban areas, deserts), can be determined a priori,
- (d) historic water extent maps can be derived, essentially as a by-product of the model calibration, which may serve as a reference for distinguishing between floods and the normal seasonal water extent.
The (internal) availability of three separate flood and water extent maps tackles, by readily identifying them, the shortcomings a single algorithm, by itself, might be suffering of in specific circumstances and/or part of the world due to many well-known factors like topography or environmental conditions.
For these very reasons, Users have access to consensus flood maps where a pixel is marked as flooded when at least two algorithms classify it as water.
Accordingly, the implemented quality assurance procedures (see INSERT REFERENCE) allow for differentiating between classification errors that can be attributed to shortcomings of individual algorithms and errors that are inherent to the SAR sensing instruments and their difficulty to capture the appearance or disappearance of surface water in particular situations.
Algorithms description
A detailed description of the three algorithms and examples of applications in an operational context is provided in the PDD, this sections anf the following links provide their description in a nutshell.
All the three algorithms make use of historical time series of SAR intensity data and use topography-derived indices to refine the initial classification of water bodies. However, differences appear in the ways historical time series of intensity data are finally used to parameterize the retrieval algorithms and the way ancillary data such as topography data are used in the production system. Other differences relate to the inclusion of a region growing step or not, the scale at which the thresholds are determined and applied to each pixel’s backscatter value and other nuances in the way the retrieval algorithms are setup.
The most relevant features of the algorithms are summarized in the table below.
