HASARD-MM service specifications

Service Description

The HASARD Multi Mission (HASARD-MM)_ service performs a mapping of standing waters from a co-pol backscatter image. It supports backscatter images from multiple missions. This service relies on the “HASARD”¹ flood mapping algorithm that enables systematic, automatic and reliable SAR mapping of terrestrial water bodies. HASARD implements the Hierarchical Split-Based Approach (HSBA), described by Chini et al. 2017², and is specifically designed to support flooding-related disaster risk reduction at global scale. The underlying end-to-end image processing chain is based on a patented technology from the Luxembourg Institute of Science and Technology (LIST)³ that combines and fully integrates innovative hierarchical image splitting and open water backscatter modelling.

HASARD-MM exploits a single satellite image (Sigma Nought) to rapidly produce accurate standing water maps. The service employs Height Above the Nearest Drainage (HAND) auxiliary data to exclude false alarms in slope areas due to layover. HASARD-MM generates a water extent map in both raster and vector format.

Workflow

The HASARD-MM processing service applies the workflow below:

Inputs

Input of the HASARD-MM service is a single band Sigma0 asset in dB from a Calibrated Dataset [CD] and the single band asset from an HAND Auxiliary Dataset. Thus to employ the service you need the following two Datasets:

1. An event SAR Calibrated Dataset including a co-pol Sigma Nought asset in dB (e.g. s0_db_x_hh for TerraSAR-X) acquired during crisis,

2. HAND Auxiliary Dataset including the hand single band asset providing relative elevations in m.

Parameters

The HASARD-MM service requires a specified number of mandatory and optional parameters. All service parameters are listed in the below Table 1.

Table 1 - Service parameters for the HASARD-MM processor.

Input SAR calibrated dataset and asset name

First two mandatory parameters are dedicated to specify the input calibrated dataset and define which single band backscatter asset from this dataset shall be used for the water mapping computation. The system does not automatically select the co-pol asset from the given Calibrated Dataset to be used in the computation.

Input HAND auxiliary dataset and asset name

With the third HAND auxiliary dataset and fourth mandatory parameters, ‘HAND auxiliary datasetandHAND asset namerespectively, the user must specify the input auxiliary dataset and specify the name of the single band asset to be used for the water mapping computation. The name of the asset in the forth parameterHAND asset nameis pre-filled by default and the user must just drag and drop the auxiliary dataset in the third parameterHAND auxiliary dataset`.

AOI (optional)

The fifth parameter (optional) can be used to define the area of interest expressed as a Well-Known Text value.

The input area of interest must have an area higher than 1600 skm or 160000 ha. If a smaller area is inserted the job will not be executed and a warning will appear.

An AOI higher than 160000 ha can be easily drawn in the ESA Charter Mapper by using the Spatial Filter button available in the Navigation and Search Toolbar in the upper left side of the map. During the drawing of the rectangular area, a popup showing the size of the current area in ha will appear in the map next to the cursor.

In the definition of “Area of interest as Well Known Text” it is possible to apply as AOI the drawn polygon defined with the area filter. To do so, click on the button in the left side of the "Area of interest expressed as Well-known text" box and select the option AOI from the list. The platform will automatically fill the parameter value with the rectangular bounding box taken from the current search area in WKT format.

HSBA parameters

The following four mandatory parameters: Starting tiling level, Check bimodal distribution (Ashman D), Minimum number of pixels per tile, Number of pixels for small object removal represents the parameters of the retrieval algorithm. Default values given in Table 1 are the ones recommended for a general use.

HAND threshold

In the tenth mandatory parameter the user must specify a relative elevation threshold to be considered when mapping water extent. Pixels classified as water that have values of Height Above the Nearest Drainage (HAND) above this threshold in meters (e.g. 15m) are masked out. In particular, this threshold is used to derive a binary mask of potential floodplains from the HAND dataset which is then applied for masking the water extent.

where((hand<=15),1,0);

Backscatter threshold

In the last mandatory parameter the user must insert a threshold value in decibel (e.g. -20 db) to be used for the computation of water extent from the backscatter signal.

Outputs

The HASARD-MM processor provides as output the following products:

1. Water extent overview

2. Water extent single band

3. Water extent polygon vector

HASARD-MM Product Specifications can be found in the below tables.