snaphu_man1.html

phase unwrapping algorithm for SAR interferometry

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<pre>     snaphu -f configfile -i wrappedfile 1024 -o mstfile
</pre>Read the unwrapped data in ``mstfile'' and use that as the initialization to the modified network-simplex solver:</td></table><p>

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<pre>     snaphu -f configfile -u mstfile 1024 -o unwrappedfile
</pre>Note that in the previous two examples, the output file name in the configuration file is overrided by the one given on the command line. The previous two commands together are in principle equivalent to the preceding one, although round-off errors in flow-to-phase conversions may cause minor differences</td></table><p>

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Unwrap the interferogram as above, but use the MCF algorithm
for initialization:</td></table><p>

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<pre>     snaphu -f configfile wrappedfile 1024 --mcf
</pre>Unwrap the interferogram once again, but first flatten it with the unwrapped data in ``estfile,'' then reinsert the subtracted phase after unwrapping:</td></table><p>

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<pre>     snaphu -f configfile wrappedfile 1024 -e estfile
</pre>The following assumes that the wrapped input interferogram measures deformation, not topography. Unwrap the interferogram with the given correlation data:</td></table><p>

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<pre>     snaphu -d wrappedfile 1024 -c corrfile
</pre>Unwrap the input interferogram by minimizing the unweighted congruent L2 norm:</td></table><p>

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<pre>     snaphu -p 2 -n wrappedfile 1024
</pre>Unwrap the interferogram as a three-by-four set of tiles that overlap by 30 pixels, with the specified configuration file, using two processors:</td></table><p>

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<pre>     snaphu wrappedfile 1024 -f configfile \
          --tile 3 4 30 30 --nproc 2
</pre></td></table><p>
<a name="HINTS AND TIPS"></a>
<h2>HINTS AND TIPS</h2>

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The program may print a warning message about costs being
clipped to avoid overflow. If too many costs are clipped,
the value of COSTSCALE may need to be decreased in a
configuration file (via the <b>-f</b> option). If the
program prints a warning message about an unexpected
increase in the total solution cost, this is an indication
that too many costs are clipped. It is usually okay if just
a few costs are clipped.</td></table><p>

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In topography mode, if the unwrapped result contains too
many discontinuities, try increasing the value of LAYMINEI
or decreasing the value of LAYCONST. The former determines
the normalized intensity threshold for layover, and the
latter is the relative layover probability. If there are too
many discontinuities running in azimuth, try decreasing the
value of AZDZFACTOR, which affects the ratio of azimuth to
range costs. If the baseline is not known, take a guess at
it and be sure its sign is correct. Specify the SAR imaging
geometry parameters as well as possible. The defaults assume
ERS data with five looks taken in azimuth.</td></table><p>

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In deformation mode, if the unwrapped result contains too
many discontinuities, try increasing the value of
DEFOTHRESHFACTOR or decreasing the value of DEFOCONST. If
the surface displacement varies slowly and true
discontinuities are not expected at all, DEFOMAX_CYCLE can
be set to zero. This behavior is also invoked with the
<b>-s</b> option. The resulting cost functions will be
similar to correlation-weighted L2 cost functions, though
the former are not necessarily centered on the wrapped
gradients. Congruence is still enforced during rather than
after optimization.</td></table><p>

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The program can be run in initialize-only (<b>-i</b>) mode
for quick down-and-dirty MST or MCF solutions.</td></table><p>
<a name="SIGNALS"></a>
<h2>SIGNALS</h2>

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Once the iterative solver has started, <b>snaphu</b> traps
the interrupt (INT) and hangup (HUP) signals. Upon receiving
an interrupt, for example if the user types Ctrl-C, the
program finishes a minor iteration, dumps its current
solution to the output, and exits. If a second interrupt is
given after the first (caught) interrupt, the program exits
immediately. If a hangup signal is received, the program
dumps its current solution then continues to execute
normally.</td></table><p>
<a name="EXIT STATUS"></a>
<h2>EXIT STATUS</h2>

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Upon successful termination, the program exits with code 0.
Errors result in exit code 1.</td></table><p>
<a name="FILES"></a>
<h2>FILES</h2>

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The following files may be useful for reference, but are not
required. They are included in the program source
distribution and may be installed somewhere on the
system.</td></table><p>

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<i>snaphu.conf.full</i></td></table><p>

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Template configuration file setting all valid input
parameters (though some may be commented out).</td></table><p>

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<i>snaphu.conf.brief</i></td></table><p>

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General-purpose template configuration file setting the most
important or commonly modified input
parameters.</td></table><p>

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In addition to parameters read from configuration files
specified on the command line, default parameters may be
read from a system-wide configuration file if such a file is
named when the program is compiled.</td></table><p>
<a name="BUGS"></a>
<h2>BUGS</h2>

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The <b>-w</b> option has not been tested
exhaustively.</td></table><p>

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Extreme shadow discontinuities (i.e., abrupt elevation drops
in increasing range due to cliffs facing away from the
radar) are not modeled that well in the cost functions for
topography mode.</td></table><p>

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Abrupt changes in surface reflectivity, such as those of
coastlines between bright land and dark water, might be
misinterpreted as layover and assigned inappropriate
costs.</td></table><p>

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The algorithm's behavior may be unpredictable if the costs
are badly scaled and excessively clipped to fit into their
short-integer data types.</td></table><p>

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There is no error checking that ensures that the network
node potentials (incost and outcost) do not overflow their
long-integer data types.</td></table><p>

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Automatic flow clipping is built into the MST
initialization, but it can give erratic results and may loop
infinitely for certain input data sets. It is consequently
turned off by default.</td></table><p>

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Dedicated programs for specific Lp objective functions may
work better than <b>snaphu</b> in Lp mode. Note that snaphu
enforces congruence as part of the problem formulation,
however, not as a post-optimization processing
step.</td></table><p>
<a name="REFERENCES"></a>
<h2>REFERENCES</h2>

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C. W. Chen and H. A. Zebker, ``Two-dimensional phase
unwrapping with use of statistical models for cost functions
in nonlinear optimization,'' <i>Journal of the Optical
Society of America A</i>, <b>18</b>, 338-351
(2001).</td></table><p>

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C. W. Chen and H. A. Zebker, ``Network approaches to
two-dimensional phase unwrapping: intractability and two new
algorithms,'' <i>Journal of the Optical Society of America
A</i>, <b>17</b>, 401-414 (2000).</td></table><p>

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C. W. Chen and H. A. Zebker, ``Phase unwrapping for large
SAR interferograms: Statistical segmentation and generalized
network models,'' <i>IEEE Transactions on Geoscience and
Remote Sensing</i>, <b>40</b>, 1709-1719
(2002).</td></table><p>
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