HASARD Dual Image service specifications

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.

Workflow

The processing service applies the workflow below:

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.

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.

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.

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.

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.

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.