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HASARD service specifications


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Service Description

The Flood Delineation (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(cos2) --> d1[Dataset pre-event] c --> d2[Dataset post-event] d1 --> b1((SAR<br>calibration)) d2 --> b2((SAR<br>calibration)) b1 --> a1[input flood dataset] b2 --> a2[input reference dataset] subgraph Inputs a1 a2 a3 a4 end subgraph HASARD a1[input flood image]--> clipping((Clip)) a2[input reference image]--> clipping((Clip)) a3[AOI]--> clipping((Spatial<br>subset)) a4[HSBA parameters]--> hasard((HSBA algorithm)) clipping --> stacking((Co-located<br>Stacking)) stacking --> hasard((HSBA algorithm)) end subgraph Outputs hasard --> o1[Flood<br>mask] hasard --> o2[Flood<br>mask<br>overview] clipping --> o3[Sigma0<br>flood<br>image] clipping --> o4[Overview<br>flood<br>image] clipping --> o5[Sigma0<br>reference<br>image] clipping --> o6[Overview<br>reference<br>image] end

Inputs

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

  1. flood image (Sigma0 VV during crisis)

  2. reference image (Sigma0 VV before the event)

Warning

HASARD requires as input a couple of Sigma0 images 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 image Sigma0 in db asset from post-event SAR Calibrated Dataset YES
Reference image Sigma0 in db asset from pre-event SAR Calibrated Dataset 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 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 delineation overview

  3. SAR Backscatter Intensity of flood image in dB

  4. Flood Image overview

  5. SAR Backscatter Intensity of reference image in dB

  6. Reference Image overview

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 Backscatter coefficient from Flood SAR calibrated data
Short Name post_s0_db_c_vv
Description Backscatter gray scale single band image from Flood SAR Calibrated Dataset
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 single band image from SAR flood image
Short Name overview-vv-post
Description Grayscale RGBA from calibrated SAR Flood image in VV polarization (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 Backscatter coefficient from Reference SAR calibrated data
Short Name pre_s0_db_c_vv
Description Backscatter gray scale single band image from Reference SAR Calibrated Dataset
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 single band image from SAR reference image
Short Name overview-vv-pre
Description Grayscale RGBA from calibrated SAR Reference image in VV polarization (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