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<< Click to Display Table of Contents >> Interferometry - Stereo Radargrammetry - 1 -Stereo Matching Process |
  
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<< Click to Display Table of Contents >> Interferometry - Stereo Radargrammetry - 1 -Stereo Matching Process |
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Purpose
This functionality is intended to estimate the elevation by means of the amplitude (intensity) data in a stereo-matching workframe.
Technical Note
As for photogrammetry, also radargrammetry exploits a stereo acquisition configuration to estimate a precise topographic height. Data in slant range (_pwr or _slc) or ground range (_gr) are supported. In case of _pwr files, users have to mooltilook range 1: azimuth 1. The software retrieves a shift in pixel, along the range direction, that is proportional to the topographic height. The first step of this tool is to consider the known topography by coregistering the Secondary acquisition over the Reference acquisition. The residual mismatch (in range direction) will correspond to the residual topography with respect to the reference DEM. The matching
algorithm is performed in a pyramidal way, by estimating an initial coarse shift on the multilooked acquisitions and refining it iteration by iteration to obtain a finer estimate.
The separation angle between the two acquisition should be between 15 to 25-30 degree. The higher the separation angle, the higher the sensitivity to the topography, while presenting the matching algorithm with a harder task. The radargrammetry tool can provide good results with high resolution sensors, and mostly in natural areas.
This estimate of the shift is performed by means of a coregistration process using the amplitude cross correlation (refer to the relevant Technical Note for details). The values of the output cross correlation vary between 0 (worst conditions) and 1 (best conditions), same as the coherence product.
The relationship between the Signal to Noise Ratio (SNR) value and the coherence (γ) value is:
SNR = γ2/1-γ2
The shift is calculated with steps (in terms of number of pixels) defined by the "Azimuth Looks" and "Range Looks" factors. We suggest using factors which are five times bigger than the default ones used in the multilooking process.
The coregistration shift estimate is optimized by means of the input Digital Elevation Model.
A flattening process is executed to remove the systematic effects due to the topography. It is executed by transforming the input Digital Elevation Model into the Reference slant range image geometry. In case of precise orbits and accurately geocoded reference Digital Elevation Model, this process is run in a fully automatic way. However, in case of inaccuracy in the satellite orbits or in the Digital Elevation Model geolocation, a Ground Control Point (GCP file) is required to correct the Reference acquisition of the SAR pair 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 Secondary data according to the Reference data.
It is important to note that:
❖In case the "Input Reference file" has already been corrected with the manual or the automatic procedure the "GCP file" is not needed.
❖In case the "Input Reference file" is correct (i.e. the nominally geocoded image fits with the DEM), while the orbital parameters of the "Input Secondary file" are not accurate (i.e. the nominally geocoded image does not fit with the DEM), the "GCP file" is not needed but the flag "Automatic Secondary Orbit Correction", in the Preferences>Flattening, must be checked.
The shift between Reference and Secondary data is calculated in pixel units and it is measured in the satellite viewing geometry.
Input Files
Input Reference File
File name of the Reference data (_slc, _pwr, _gr). This file is mandatory.
Input Secondary File
File name of the Secondary data (_slc, _pwr, _gr). 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 an 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 used to average the shift estimate in range direction.
Azimuth Looks
Number of looks used to average the shift estimate in azimuth direction.
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.
CC Range Window Size
Range dimension, in pixels, of the windows where the cross correlation based shift is estimated.
CC Azimuth Window Size
Azimuth dimension, in pixels, of the windows where the cross correlation based shift is estimated.
CC Oversampling
The cross-correlation based estimate is applied on over sampled data. The higher this value the longer the processing time and the accuracy. Values higher then 16 are typically not required.
Cross Correlation Threshold
If the correlation value is below this threshold, then the window is not used for the shift estimate.
Max Residual Topography (m)
Maximum residual height difference (in meters) with respect to the input DEM considered in the matching process.
By setting this flag, the input Digital Elevation Model is used in the coregistration process.
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 - Flattening
It brings to the flattening 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 Root Name
Name of the output root. This file is mandatory.
_dr
Shift measured in range direction with the associated header files (.sml, .hdr).
_rg_sint
Synthetic shift in range with the associated header files (.sml, .hdr).
_az_sint
Synthetic shift in azimuth with the associated header files (.sml, .hdr).
_cc
Cross correlation image with the associated header files (.sml, .hdr).
_srdem
Digital Elevation Model in slant range geometry (.sml, .hdr).
_par
ASCII file containing the coregistration shift parameters in range and azimuth. This file is generated only if the shift parameters are calculated.
_pwr
Multi-looked Reference and Secondary image with the associated header files (.sml, .hdr).
_orb.sml
Xml file containing the scene orbital parameters.
_orbit_off.shp
Shape file with the points used to estimate the orbit based shift. This file contains the following information:
| - | Pixel position in range direction (Range), in Single Look pixel units. |
| - | Pixel position in azimuth direction (Azimuth), in Single Look pixel units. |
| - | Shift measured in range direction (Dr), in Single Look pixel units. |
| - | Shift measured in azimuth direction (Da), in Single Look pixel units. |
| - | Calculated polynomial shift, to apply in range direction (Drfit), in Single Look pixel units. |
| - | Calculated polynomial shift, to apply in azimuth direction (Dafit), in Single Look pixel units. |
The file, which is generated only when the shift parameters are calculated, is multilooked (i.e. Azimuth and Range looks) as specified in the Input Parameters.
_winCC_off.shp
Shape file with the points used to estimate the cross correlation based shift from the Intensity data. In addition to the information provided by the "_orbit_off.shp" file, this file contains also the cross-correlation value (CC), which is dimensionless and it can vary from 0 to 1.
The file, which is generated only when the shift parameters are calculated, is multilooked (i.e. Azimuth and Range looks) as specified in the Input Parameters.
Details specific to the Units of Measure and Nomenclature of the output products can be found in the Data Format section.
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.
Close
The window will be closed.
Help
Specific help document section.
Specific Function(s)
None.
Task, SARscapeBatch object, SARscapeBatch script example
References None.