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Q. - When it makes sense to use the Sample Selection tool?

 

A. - It is typically used when you have a temporal series (two images or more acquired with same viewing geometry) and you are interested only on a small portion of the full frame; it is then possible to define the area of interest in one scene/acquisition (this must be the first in the input list) and get the other images of the series automatically cut on the same area. The process of estimating the relative shift of each input image, with respect to the first of the list, is exactly the same used for the Coregistration; the only difference being that here only the shift parameters are estimated and the images are not resampled/coregistered.

 

This tool is usually adopted for preparing a sample data set to input into a DInSAR, Persistent Scatterers or SBAS processing; nevertheless it is also useful to prepare a temporal series which have to be processed in the Basic module for amplitude related application.

 

Q. - What is the method adopted for the Phase Editing and when should I perform this step?

 

A. - First of all it must be outlined that the phase editing can be performed only under the "ENVI Classic" interface. It is based on the following procedure:

1.The original unwrapped phase (_upha) is copied and the editing is executed on the duplicated file (in order to always keep the original product).

2.The "phase jumps" are identified visually as well as checking the pixel value variation (i.e. phase variation in radians) using "ENVI View Tools" such as the "Profiles>Arbitrary Profile(Transect)" or the "Cursor Location/Value".

3.Each "phase island" is contoured with an ENVI ROI.

4.A Manual or semi-automatic estimate of the number of 2π cycles, which have to be added (or subtracted) to each "phase island", is computed.

5.The phase correction is performed.

 

It is recommended not to perform the contouring and correction of more "phase islands" at the same time.

 

The term "phase jumps" indicates discontinuities in the unwrapped phase.These can be due to unwrapping error, which are typically induced by low coherence. However it must be noted that there can be "phase jumps", due for instance to steep topography, which have not to be corrected.

 

The term "phase island" indicates an area which is bordered by a "phase jump". Due to this the island is higher (or lower) of a certain number of 2π cycles with respect to the surrounding area.

 

It must be noted that an attempt of reducing (or completely avoid) the presence of "phase jumps" in the unwrapped phase can be done by:

- Change the unwrapping coherence threshold (usually reducing it).

- Use a different unwrapping method.