Help Content > Basic Module > Feature Extraction

Purpose

Single-date and multi-temporal features based on first order statistics Intensity image(s) or interferometric image pairs can be derived from SAR data. These features enable, depending upon the targeted product, to detect and extract structures or temporal changes, which can be additionally used for segmentation and/or classification purposes.

Technical Note

Given two coregistered complex SAR images (S1 and S2), one calculates the interferometric coherence (γ) as a ratio between coherent and incoherent summations. Note that the observed coherence - which ranges between 0 and 1 - is, in primis, a function of systemic spatial de-correlation, the additive noise, and the scene de-correlation that takes place between the two acquisitions.

The estimated coherence - which ranges between 0 and 1 - is function of:

-	Systemic spatial de-correlation (i.e. the additive noise).

-	Temporal de-correlation between Reference and Secondary acquisitions.

The coherence product has essentially a twofold purpose:

-	To determine the quality of the measurement (i.e. interferometric phase). As a rule of thumb, InSAR pairs with low coherence should not be used to derive reliable phase related measurements.

-	To extract thematic information relevant to the ground features properties and their temporal changes. This information is enhanced when coupled with the backscattering coefficient (σo) of the Reference and Secondary Intensity data.

As a general rule it can be assumed that the coherence decreases with increasing Reference - Secondary acquisition time distance. The relationship between the coherence (γ) and the Signal to Noise Ratio (SNR) is:

SNR = γ2/1-γ2

In case the two images are very much different in terms of areal coverage, the smallest one should be used as "Input Reference file" in order to avoid the coregistration windows to be located in areas with null pixel values, which can eventually cause the coregistration process to fail. For details about the coregistration process, refer to the relevant Technical Note.

In case of baseline conditions - or topographic conditions - which cause the coherence to get lost due to the very fast change of the interferometric phase, it is suggested to over sample the range pixel size by entering negative values in the Range Multilooking (Preferences>Interferometry>Multilooking). As an example using ERS-ASAR interferometric tandem pairs, which are characterized by a very small ambiguity height (hence very dense/frequent interferometric fringes), the Interferometric multilooking factors shall be set to -2 and 3 (respectively in Range and Azimuth).

The use of the Digital Elevation Model in input (optional) enables to generate the coherence image using the interferogram flattened on the basis of the known topography.

In case of inaccuracy in the satellite orbits or in the Digital Elevation Model geolocation, a Ground Control Point (i.e. "GCP" file) is required to correct the SAR data (i.e. "Input Reference SLC" file) with respect to the reference Digital Elevation Model. In this case the shift calculated in the coregistration process is combined with the Ground Control Point shift in order to correct the data of the Input file list according to the Input reference file.

It is important to note that:

❖In case the "Input Reference SLC file" has already been corrected with the manual or the automatic procedure the GCP is not needed.

❖In case the "Input Reference SLC file" is correct (i.e. the nominally geocoded image fits with the DEM), while the orbital parameters of the "Input Slave SLC file" are not accurate (i.e. the nominally geocoded image does not fit with the DEM), the GCP is not needed but the flag "Automatic Secondary Orbit Correction", in the Preferences>Flattening, must be checked.

The coherence image can be generated from the input unfiltered interferogram or from the filtered one, using one of the available filtering methods. The selected option can be specified in the relevant Preferences.

Input Files

Input Reference file

Input file name of the (not coregistered) Single Look Complex (_slc) Reference data. This file is mandatory.

Input Secondary file

Input file name of the (not coregistered) Single Look Complex (_slc) Slave data. This file is mandatory.

Optional Files

Geometry GCP file

Either a previously created Ground Control Point file (.xml) is loaded (Load GCP File) or the interface to create a new Ground Control Point file is automatically loaded (Create GCP File, refer to the "Tools>Generate Ground Control Point" for details). This file is optional.

Shift Parameter file

Name of the file with the shift parameters used for the coregistration (_par). This file is mandatory. If the Compute shift parameter flag is set, it is generated as output.

DEM/Cartographic System

Digital Elevation Model file

Digital Elevation Model file name. This should be referred to the ellipsoid. In case a list of input files is entered, the DEM must cover the whole imaged area. This file is optional.

Output Projection

In case that the Digital Elevation Model is not used, it is mandatory to define the Cartographic System.

To use the same coordinate system as another dataset, click the Import from Existing Dataset button and select the source dataset.

To apply the same Coordinate System of the current selected layer

The reset icon allows to reset the coordinate system field.

Parameters - Principal Parameters

Range looks

Number of looks in range.

Azimuth looks

Number of looks in azimuth.

Grid Size for Suggested Looks

The grid size, in meters, used to tune range and azimuth looks. If the other parameters are manually set, the grid size will not imply a change in their values.

Compute Shift Parameters

By setting this flag, the coregistration shifts between Reference and Secondary image are calculated and saved into the _par file.

Coregistration with DEM

By setting this flag, the input Digital Elevation Model is used in the coregistration process.

Filtering method

The filtering methods can be selected:

•Boxcar

•Adaptive Non-Local

Coherence AZ Box Range

Window Size in Azimuth direction for Boxcar method.

Coherence RG Box Range

Window Size in Azimuth direction for Boxcar method.

Parameters - Global

It brings to the general section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

Parameters - Interferogram

It brings to the Interferometry section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

Parameters - Flattening

It brings to the Flattening section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

Parameters - Filtering

It brings to the Adaptive Filtering section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

Parameters - Coregistration

It brings to the coregistration section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

Parameters - Other Parameters

It brings to the general section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

Output Files

Output file

Output file name of the Coherence data. This file is mandatory.

_cc

Coherence and associated header files (.sml, .hdr).

_master_fil_pwr: fileterd of the master file if the Non Local InSAR fileter is selected.

_slave_fil_pwr. fileterd of the master file if the Non Local InSAR fileter is selected.

Details specific to the Units of Measure and Nomenclature of the output products can be found in the Data Format section.

General Functions

Exec

The processing step is executed.

Store Batch

The processing step is stored in the batch list. The Batch Browser button allows to load the batch processing list.

The window will be closed.

Help

Specific help document section.

Specific Function(s)

None.

Basic Module - Feature Extraction - Coherence

Basic Module - Feature Extraction - Coherence