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HASARD Dual Image service specifications


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This service provides a binary flood mask by applying an automatic change detection algorithm between a pair of pre- and post-event Sentinel-1 datasets selected by the user (one before and one after the event). The service analyzes the reference and flooded images and retrieves the increased water extent with respect to the reference (areas considered as standing water bodies are removed).

📕 The tutorial of the HASARD Dual Image service is available in this section.


Service Description

The HASARD Dual Image service performs a flood mapping computation between two SAR Calibrated Datasets selected by the user (one before and one after the event).

The “HASARD”1 flood mapping algorithm enables systematic, automatic and reliable SAR mapping of terrestrial water bodies. The HASARD Dual Image service 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. The service is developed by LIST and WASDI.

The HASARD Dual Image processor 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. This binary mask represents the increased water extent with respect to the reference (areas considered as standing water bodies are removed). It also provides co-located Sigma Nought assets in dB at the same polarisation derived from the two input datasets.

Note

This service currently supports only Sentinel-1 data.

Workflow

The processing service applies the workflow below:

graph TB a[(Radar <br> Calibrated Datasets)] style a fill:#ffde86,stroke:#333,color:#282828,stroke-width:2px a --> a2[input 2] a --> 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 Dual Image a1 --> aa1[Reference backscatter co-pol asset] style aa1 fill:#cfdfff,stroke:#333,color:#282828 a2 --> aa2[Flood backscatter co-pol asset] style aa2 fill:#cfdfff,stroke:#333,color:#282828 aa1 --> stack[Co-located <br> stacking <br> and subset] aa2 --> stack[Co-located <br> stacking <br> and subset] a3 --> stack[Co-located <br> stacking <br> and subset] a4 --> hasard[HSBA algorithm] stack --> 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 stack --> p3(s0_db_c_vv-post) style p3 fill:#cfdfff,stroke:#87afff,color:#282828 stack --> p4(overivew-vv-post) style p4 fill:#cfdfff,stroke:#87afff,color:#282828 stack --> p5(s0_db_c_vv-pre) style p5 fill:#cfdfff,stroke:#87afff,color:#282828 stack --> 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

Input

Input of the HASARD Dual Image 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 polarisation.

Warning

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

Parameters

The HASARD Dual Image 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 Hierarchical level of splitting. When the level is equal to 1 the image is split into 4 tiles, equal to 2 in 16 tiles, equal to 3 in 64 tiles, etc. 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 Dual Image 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 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, and 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 maximum 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 from the current search area in WKT format.

Output

The HASARD Dual Image 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 Dual Image Product Specifications can be found in the below tables.

Attribute Value / description
Long Name Flood Binary flood mask from HASARD Dual Image
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

Vectorize HASARD flood mask single band asset

HASARD-DI's binary flood mask can be spatially filtered and / or converted to polygon using the FilterVectorize service.

To polygonize the flood-mask single band asset employ the FilterVectorize service in Vectorize mode by selecting only true values DN=1 (flood).

If not satisfied with the spatial filtering offered by the HASARD Dual Image service with the Number of pixels for small object removal parameter, the FilterVectorize service can be used to further post-process the binary mask and remove small isolated clusters of flooded pixels. Thus to obtained further spatially filtered flood extent vector employ the FilterVectorize service in the Filter and Vectorize mode.

Warning

Only the flood-mask single band asset can be used in the FilterVectorize on-demand service, being the only discrete raster produced by the HASARD Dual Image service.


  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