GAMMA Interferometric Point Target Analysis Software (IPTA): Reference Manual

expand_pt

ANSI-C program: expand_pt.c

NAME
expand_pt - expand existing point list by applying phase quality check to candidate point list

SYNOPSIS
expand_pt <plist1> <pmask1> <plist2> <pmask2> <pSLC_par> <itab> <pbase> <base_flag> <pdiff1> <pdiff2> <pdiff_type> [pres] [pdh] [pdef] [punw] [psigma] [sigma_max] [dh_max] [def_min] [def_max] [model] [bmax] [dtmax]

<plist1>	(input) point list of known good points (int)
<pmask1>	(input) point data stack for plist1 of mask values of known good points (uchar)
<plist2>	(input) point list of candidate points (int)
<pmask2>	(output) point data stack for plist2 of mask values resulting from phase quality check
<pSLC_par>	(input) stack of SLC/MLI parameters (binary)
<itab>	(input) table associating interferogram stack records with pairs of SLC stack records (ascii) (line entries are: pSLC_rec1 pSLC_rec2 itab_rec_num switch_flag)
<pbase>	(input) stack of interferogram baselines (binary)
<base_flag>	baseline flag (0:initial baseline 1:precision baseline)
<pdiff1>	(input) point data stack for plist1 of differential interferogram (float or fcomplex)
<pdiff2>	(input) point data stack for plist2 of differential interferogram (type as pdiff1)
[pdiff_type]	data type of pdiff1 and pdiff2 (0=float (unwrapped phase), 1=fcomplex, default=1)
[pres]	(output) point data stack of residual unmodeled phase (enter - for none, float, atmos. phase, non-linear def., baseline error)
[pdh]	(output) point data stack of height correction value (enter - for none, m, float)
[pdef]	(output) point data stack of linear deformation rate (enter - for none, m/day, float)
[psigma]	(output) point data stack of phase standard deviation from fit (enter - for none, float)
[sigma_max]	threshold for maximum phase standard deviation (enter - for default) (radians): 0.70
`[dh_max]`	maximum height difference for initial search (enter - for default) (m): 30.000
[def_min]	minimum deformation rate for initial search (enter - for default) (m/year): -0.005
`[def_max]`	maximum deformation rate for initial search (enter - for default) (m/year): 0.005
[model]	phase regression model: 1: a0 + a1bperp[i] 2: a0 + a1bperp[i] + a2delta_t[i]) (default) 3: a1bperp[i] 4: a1bperp[i] + a2delta_t[i] 5: a0 + a2delta_t[i] 6: a2delta_t[i]
[bmax]	maximum perp. baseline (m) considered, (-1 = all records, default=-1)
[dtmax]	maximum time interval (days) considered, (-1 = all records, default=-1)

EXAMPLE
expand_pt plist1 pmask1 plist2 pmask2 ppar itab pbase 1 pdiff1 pdiff2 1

For the large list of candidate points (plist2) the quality of each point is evaluated based on the phase standard deviation of the differential interferometric phase (pdiff2) from the selected 2-D regression model using nearby validated points (plist1, pmask1) as reference. Good/bad points, i.e. points with a phase standard variaion smaller/larger than the indicated threshold, are accepted/rejected by setting the corresponding mask values in pmask2 to 1/0.

DESCRIPTION
In the IPTA point lists contain the coordinates (range and azimuth pixel number relative to the reference SLC geometry) of the selected points. Data at these points (SLC values, interferogram values, ...) are stored in point data stacks with each record corresponding to a different SLC or interferogram. The point data stack of mask values contains for each point a flag (0 or 1) which indicates if the data of a point are considered (1) or not considered (0).

expand_pt supports an important step in the generation of a long list of valid points. Input used for this step includes an existing and validated short list as well as a candidate list for out of which the long list shall be generated. This candidate list shall include the valid points of the long list as well as other points. expand_pt determines the validity of these candidate points using the phase standard deviation of a regression model as the quality criteria. The available short list of validated points permits to improve the robustness and efficiency of the quality assessment for the points of the long list.

The validated "short" list is defined through the point list, plist1, the point data stack of mask values, pmask1, the stack of SLC parameters, pSLC_par, the interferogram table, itab, and the stack of interferogram baselines, pbase. The corresponding differential interferogram values are in the point data stack pdiff1.

The candidate point list is defined through the point list, plist2, the point data stack of mask values, pmask2, the stack of SLC parameters, pSLC_par, the interferogram table, itab, and the stack of interferogram baselines, pbase. The corresponding differential interferogram values are in the point data stack pdiff2. If the differential interferometric data are in fcomplex form the phase of the data in the pdiff must be unwrapped. An initial search of the possible parameter values is performed to unwrap the data. These parameters include the differential height, deformation rate, and phase constant.

The command line parameters def_max and dh_max determine the range of values for the initial search. The search range for the differential height between the reference point and the test point is +/- def_max meters. Similarly, the range of deformation rates is +/- def_max meters/year. The processing time of the program strongly depends on the range of values used for the test. If the terrain is known to be relatively flat, then dh_max can be reduced. Similarly, if the expected deformation rate between the reference point and the test point is expected to exceed 5mm/year, then the value of def_max can be increased. For the program expand_pt the reference point is chosen to be within the same area to the test points and the values of the parameters can be adjusted. to speed up processing

expand_pt goes through the candidate point list and determines for each point a quality measure based on a phase regression analysis (using a one or two dimensional regression model) using a local phase reference of good quality. The regression analysis can be restricted to shorter baselines and/or shorter time intervals only. The baseline considered is the absolute value of the perpendicular component. The time interval considered is the absolute value of the acquisition time difference between the reference and slave SLC.

Chances to identify "good" points increase if the differential interferogram is of high quality, which is assured by subtracting a high quality simulated phase. Topographic, deformation, and atmospheric phase terms should be modeled as good as possible, which means the most accurate terrain height estimates, baselines, deformation models, and atmospheric phase terms should be used. On the other hand the chances are also increased by using a very local and validated reference as correction terms (height corrections, relative deformation, relative atmospheric difference) remain small over short distances. This second criteria permits to use expand_pt even with relatively poor (initial) estimates for terrain height and baseline and without correction for deformation and atmosphere.

The point data stacks of differential interferograms can be provided in fcomplex or float (unwrapped phases) format.

Initial or precision baseline estimates can be used.

As regression models there are options:
1. 1-d model depending on height differentials alone 2. 1-d model depending on deformation rate alone 3. 2-d model depending on both height differentials and deformation rate Each of these three models can include an optional phase constant parameter. The phase constant is required if a single SLC used as the reference for all interferograms. If there are different SLCs are used as the reference for the for the interferograms in the stack the phase constant is 0.0 and does not need to be estimated. Use of the one-dimensional regression models makes constrains the regression and gives better results for small stacks or early on in the analysis when large height corrections are expected.

Programs to generate new point lists (e.g. the candidate point list) are image2pt, mkgrid, merge_pt, prox_pt, pwr_stat, and sp_stat.

Programs to validate existing point lists are qc_pt and expand_pt. The funcitionality of qc_pt is similar to the one of expand_pt in the sense that a mask is generated for a candidate list based on a quality measure. qc_pt differs from expand_pt in that it does not require a validated list of good points which permits a wider application but at a significantly lower efficiency.

The program expand_pt can also be used to extend the IPTA analysis to obtain the residual phase, deformation rate, and differential height using
the local reference point.