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HASARD service specifications


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Service Description

The Flood Extent Mapping (HASARD) service performs a flood mapping computation between two SAR EO data products (reference and flooded images). The “HASARD”1 flood mapping algorithm 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 exploits pairs of satellite images (flood and reference from Ground Range Detected calibrated data) to systematically, rapidly and automatically produce, record and disseminate accurate floodwater maps. Output flood maps are obtained by applying an automatic unsupervised Change Detection algorithm to pairs of SAR images only over pixels which are classified as non-permanent water bodies.

The result is a binary raster bitmask with the potential flooded area in the disaster zones. It also provides backscatter coefficient in dB for the two input images.

Note

This service currently support only Sentinel-1 data.

Workflow

The processing service applies the workflow below:

graph TB c[(COS-2<br>&<br>External EO data catalogues)] style c fill:#ffde86,stroke:#ffb900,color:#282828,stroke-width:2px c --> d2(Reference SAR Dataset) c --> d1(Flood SAR Dataset) d1 --> calib[SAR<br>Products<br>Calibration] d2 --> calib[SAR<br>Products<br>Calibration] calib --> a2[input 2] calib --> a1[input 1] subgraph Inputs style Inputs fill:#e8e8e8,stroke:#818181,color:#282828 a1[/Reference SAR Calibrated Dataset/] style a1 fill:#acc8ff,stroke:#87afff,color:#282828,stroke-width:2px a2[/Flood SAR Calibrated Dataset/] style a2 fill:#acc8ff,stroke:#87afff,color:#282828,stroke-width:2px a3[/AOI as WKT/] style a3 fill:#acc8ff,stroke:#87afff,color:#282828,stroke-width:2px a4[HSBA parameters] style a4 fill:#acc8ff,stroke:#87afff,color:#282828,stroke-width:2px end subgraph HASARD a1--> aa1[Reference backscatter co-pol asset] a2--> aa2[Flood backscatter co-pol asset] aa1[Reference backscatter co-pol asset]--> clipping((Clip)) aa2[Flood backscatter co-pol asset]--> clipping((Clip)) a3[AOI]--> clipping[Spatial<br>subset] a4[HSBA parameters]--> hasard[HSBA algorithm] clipping --> stacking[Co-located<br>Stacking] stacking --> hasard[HSBA algorithm] hasard --> cog[Convert to COG] cog --> p1(Flood mask) style p1 fill:#cfdfff,stroke:#87afff,color:#282828 p1 --> over1[Overview creation] over1 --> p2(Flood-extent-overview) style p2 fill:#cfdfff,stroke:#87afff,color:#282828 clipping --> p3(s0_db_c_vv-post) style p3 fill:#cfdfff,stroke:#87afff,color:#282828 clipping --> p4(overivew-vv-post) style p4 fill:#cfdfff,stroke:#87afff,color:#282828 clipping --> p5(s0_db_c_vv-pre) style p5 fill:#cfdfff,stroke:#87afff,color:#282828 clipping --> p6(overview-vv-pre) style p6 fill:#cfdfff,stroke:#87afff,color:#282828 p1 --> stac[Create STAC item] p2 --> stac[Create STAC item] p3 --> stac[Create STAC item] p4 --> stac[Create STAC item] p5 --> stac[Create STAC item] p6 --> stac[Create STAC item] end subgraph Outputs style Outputs fill:#e8e8e8,stroke:#818181,color:#282828 stac --> o1[/Binary Flood Mask/] style o1 fill:#acc8ff,stroke:#87afff,color:#282828,stroke-width:2px stac --> o2[/Flood Extent<br>Overview/] style o2 fill:#acc8ff,stroke:#87afff,color:#282828,stroke-width:2px stac --> o3[/Flood Sigma0/] style o3 fill:#acc8ff,stroke:#87afff,color:#282828,stroke-width:2px stac --> o4[/Flood Sigma0<br>overview/] style o4 fill:#acc8ff,stroke:#87afff,color:#282828,stroke-width:2px stac --> o5[/Reference Sigma0/] style o5 fill:#acc8ff,stroke:#87afff,color:#282828,stroke-width:2px stac --> o6[/Reference Sigma0<br>overview/] style o6 fill:#acc8ff,stroke:#87afff,color:#282828,stroke-width:2px end

Inputs

Input of the Flood Extent Mapping service is a couple of co-pol Sigma0 images in dB from Calibrated Dataset [CD] obtained from the Radar Calibration processor. This pair shall be made of the following Datasets:

  1. Secondary SAR Calibrated Dataset including a co-pol Sigma Nought asset in dB (e.g. s0_db_c_vv for Sentinel-1) acquired during crisis,

  2. Reference SAR Calibrated Dataset including a co-pol Sigma Nought asset in dB (e.g. s0_db_c_vv for Sentinel-1) acquired before the event.

Warning

HASARD requires as input a couple of Sigma0 co-pol assets in dB from SAR calibrated dataset. Such image pairs must come from the same sensor and shall have the same radar geometry (incident angle, orbit path). Input Flood and Reference backscatter images must be both in VV polarization.

Warning

This service does not support SAR complex data (e.g. Sentinel-1 SLC).

Parameters

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

Parameter Description Required Default value
Flood event calibrated dataset Secondary SAR Calibrated Dataset including a post-event Sigma0 in db asset YES
Reference calibrated dataset Reference SAR Calibrated Dataset including a pre-event Sigma0 in db asset YES
Splitting level Area of interest expressed in WKT YES 3
Check bimodal distribution (Ashman D) Ashman D coefficient to be used to check bimodal distribution YES 2.4
Minimum number of pixels per tile Minimum number of pixels per tile for checking the bimodality hypothesis YES 1000
Number of pixels for small object removal Number of pixels for small objects (blobs) removal YES 20
Area of Interest Area of interest expressed in WKT NO

Table 1 - Service parameters for the HASARD processor.

Flood and Reference SAR calibrated datasets

First two mandatory parameters are dedicated to define input "Flood" and "Reference" co-pol backscatter images from SAR Calibrated Datasets. This pair must come from the same sensor and shall have the same radar geometry (incident angle, orbit path). Input for Flood event image and Reference event image parameters are the references to the Calibrated Datasets. The system will automatically select the co-pol asset from the given Calibrated Dataset to be used in the computation.

Warning

The drag and drop of the the single-band asset (e.g. "s0_db_c_vv") is not possible. Users must drag and drop only a Calibrated Dataset (e.g. "[CD] SENTINEL-1A C-SAR IW VV/VH 134 2021/11/06 21:43:07") into both Flood event image and Reference event image fields.

HSBA parameters

The other four mandatory parameters, Splitting 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 recommended to be used for these four HSBA parameters.

AOI (optional)

This last parameter (optional) may define the area of interest expressed as a Well-Known Text value. If set, it overrides the automatic determination of the maximium common area between the input-reference products geometry.

Warning

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.

Tip

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.

Image-note-001

Tip

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 :fontawesome-solid-magic: 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 the from current search area in WKT format.

Outputs

The HASARD processor provides as output the following products:

  1. Flood binary mask

  2. Flood extent mapping overview

  3. Sigma nought in dB from secondary calibrated dataset

  4. Grayscale overview from secondary sigma nought co-pol asset

  5. Sigma nought in dB from reference calibrated dataset

  6. Grayscale overview from reference sigma nought co-pol asset

HASARD Product Specifications can be found in the below tables.

Attribute Value / description
Long Name Flood Binary flood mask from HASARD
Short Name flood-mask
Description Flood bitmask: 1=Flooded, 0 =Not-flooded
Processing level L2
Data Type 1-bit
Band 1
Format COG
Projection Native or EPSG:4326 - WGS84
Valid Range [0 - 1]
Fill Value N/A
Attribute Value / description
Long Name SAR Amplitude Change RGB composite
Short Name overview-flood-mask
Description Flood bitmask in the blue channel including transparency
Processing level L2
Data Type UnSigned 8-bit Integer
Band 4
Format COG
Projection Native or EPSG:4326 - WGS84
Valid Range [1 - 255]
Fill Value 0
Attribute Value / description
Long Name Secondary Sigma Nought Asset in dB
Short Name s0_db_c_vv-post
Description Single-band Sigma Nought asset in dB from the SAR Calibrated Dataset acquired during the crisis
Processing level L1
Data Type Float 32 bit
Band Single for each polarization
Format COG
Projection Native or EPSG:4326 - WGS84
Units dB
Attribute Value / description
Long Name Full resolution grayscale overview of secondary sigma nought asset
Short Name overview-pp-post (where "pp" stands for polarization e.g. hh or vv)
Description Grayscale RGBA from the secondary Sigma Nought co-pol asset (including alpha band)
Processing level L1
Data Type UnSigned 8-bit Integer
Band 4
Format COG
Projection Native or EPSG:4326 - WGS84
Valid Range [1 - 255]
Fill Value 0
Attribute Value / description
Long Name Reference Sigma Nought Asset in dB
Short Name s0_db_c_vv-pre
Description Single-band Sigma Nought asset in dB from the SAR Calibrated Dataset acquired before the event
Processing level L1
Data Type Float 32 bit
Band Single for each polarization
Format COG
Projection Native or EPSG:4326 - WGS84
Units dB
Attribute Value / description
Long Name Full resolution grayscale overview of reference sigma nought asset
Short Name overview-vv-pre
Description Grayscale RGBA from the reference Sigma Nought co-pol asset (including alpha band)
Processing level L1
Data Type UnSigned 8-bit Integer
Band 4
Format COG
Projection Native or EPSG:4326 - WGS84
Valid Range [1 - 255]
Fill Value 0

  1. LIST, An automatic satellite image processing tool for flood hazard prevention on a global scale, available at www.list.lu

  2. M. Chini, R. Hostache, L. Giustarini and P. Matgen (2017), "A Hierarchical Split-Based Approach for Parametric Thresholding of SAR Images: Flood Inundation as a Test Case," in IEEE Transactions on Geoscience and Remote Sensing, vol. 55, no. 12, pp. 6975-6988, Dec. 2017, DOI: 10.1109/TGRS.2017.2737664

  3. LIST, HASARD developed by the Remote Sensing and Natural Resources Modelling group of the Environmental Research and Innovation Department, available at www.list.lu