Provided by: snaphu_2.0.5-1_amd64 bug

NAME

       snaphu - phase unwrapping algorithm for SAR interferometry

SYNOPSIS

       snaphu [options] [infile] [linelength] [options]

DESCRIPTION

       snaphu  is a statistical-cost network-flow algorithm for phase unwrapping.  Given an input
       interferogram and other observable data,  snaphu  attempts  to  compute  congruent  phase-
       unwrapped solutions that are maximally probable in an approximate a posteriori sense.  The
       algorithm's solver routine is based on network optimization.  By default,  snaphu  assumes
       that  its  input  is  a  synthetic  aperture  radar  (SAR) interferogram measuring surface
       topography.  Deformation measurements are assumed if the  -d  option  is  given.   Smooth,
       generic data are assumed if the -s option is given.

       This  man page documents only the syntax and usage of snaphu.  Its theoretical foundations
       are discussed in the references cited below.

       The most common input parameters may be given  on  the  command  line,  while  many  other
       twiddle  parameters  are  handled  via the -f option and configuration files.  At the very
       least, the name of a wrapped-phase input file and  its  line  length  must  be  specified.
       Range  should  increase  towards  the right in the interferogram, and the flat-earth phase
       ramp should be removed from the input interferogram before snaphu is run.  For deformation
       interferograms, phase variations due to topography should be removed as well.

       Except  for  the  input  file  name and the line length, all input parameters take default
       values if not specified.  However, these parameters should be customized whenever possible
       since  the  accuracy  of the solution depends on how well the statistics of the estimation
       problem are modeled.  To avoid poor-quality solutions, users are  strongly  encouraged  to
       provide  their  best  estimates of the relevant problem parameters.  Parameters are set in
       the order in which they are given on the command line, so multiple configuration files  or
       options may be given, with later values overriding earlier ones.

       Allowable  file  formats  are  detailed  below.   The default format for the input file is
       COMPLEX_DATA,  but  any  of  the  described  formats  may  be  used.   If  either  of  the
       ALT_LINE_DATA  or  ALT_SAMPLE_DATA  formats  are used, the magnitude and phase (in radians
       from 0 to 2pi) of the interferogram should be in the first  and  second  channels  of  the
       file,  respectively.  If the FLOAT_DATA format is used, the input file should contain only
       the phase of the interferogram (in radians from 0 to 2pi); the  magnitude  may  be  passed
       with the -m option.

OPTIONS

       -a ampfile
              Read brightness data from the file ampfile.  The file should contain the amplitudes
              (not powers) of the two individual SAR images  forming  the  interferogram  if  the
              formats  ALT_SAMPLE_DATA (default) or ALT_LINE_DATA are used.  It should contain an
              average of those two  images  if  the  FLOAT_DATA  format  is  used.   If  (1)  the
              amplitudes  of  both  images are available, (2) the interferogram magnitude is also
              available, and (3) the -c  option  is  not  used,  then  a  coherence  estimate  is
              automatically  formed  from  the available data.  The number of looks used for this
              estimate can be set in a configuration file.  If no amplitude  or  power  data  are
              specified,  then  the  magnitude  of the input interferogram is used as the average
              amplitude, and no coherence estimate is formed.  Note that  the  magnitude  of  the
              interferogram  is  not  equal  to  the  average  amplitude  of the SAR images.  The
              amplitude data  should  be  in  the  same  system  of  units  used  for  the  input
              interferogram, and also coregistered to it.

       -A pwrfile
              Similar  to  the  -a  option,  except  the data in the specified file is assumed to
              represent the powers of the two individual SAR images.

       -b Bperp
              For topography mode, use Bperp (decimal value, in  meters)  as  the  value  of  the
              perpendicular  component of the interferometric baseline.  The sign is defined such
              that Bperp is negative if the unwrapped phase increases  with  the  elevation.   By
              default,  repeat-pass  or  ping-pong  mode  is assumed; for single-antenna-transmit
              data, the value of Bperp should be halved, or  the  transmit  mode  should  be  set
              accordingly  in  a  configuration  file (see the -f option).  The baseline value is
              only used in topography mode.

       -c corrfile
              Read correlation data from the file corrfile.  The correlation data should  be  the
              same  size  as,  and  registered  to, the input interferogram.  Consequently, a raw
              correlation estimate may need to be upsampled if it incorporates  more  looks  than
              the  interferogram.   If the -c option is not given, a coherence estimate is formed
              from the available data if possible.  Otherwise, a  uniform  default  coherence  is
              assumed   for   the  entire  interferogram.   If  the  ALT_LINE_DATA  (default)  or
              ALT_SAMPLE_DATA formats are used, the correlation data should be in the second data
              channel  of the file; the first channel is ignored.  The FLOAT_DATA format may also
              be used.  The correlation values should be between zero and one, inclusive.

       -C configstr
              Parse the string configstr  as  if  it  were  a  line  from  a  configuration  file
              containing a keyword-value pair (see the -f option).  Configuration lines generally
              have whitespace between the keyword and the value, so configstr will  usually  need
              to  be  surrounded  by quotation marks on a command line so that the shell does not
              split it into separate arguments (snaphu itself does not require or allow quotation
              marks,  however).  The syntax for how quotation marks are handled is defined by the
              shell.  Multiple instances of the -C  option  may  be  used  in  order  to  specify
              multiple  configuration  inputs.   Later instances of the -C option take precedence
              over both earlier instances of the -C option and the  configurations  specified  by
              earlier instances of the -f option.

       -d     Run  in  deformation mode.  The problem statistics and resulting cost functions are
              based  on  the  assumption  that  the  true  unwrapped  phase  represents   surface
              displacement rather than elevation.

       -e estimatefile
              Flatten  using the unwrapped phase estimate in the file estimatefile.  The estimate
              is subtracted from the input interferogram before unwrapping, and is inserted  back
              into the solution just before the output is written.  The estimate also affects the
              cost functions used, since subtracting a constant from a random variable shifts the
              probability  density function of the random variable.  If the formats ALT_LINE_DATA
              (default) or ALT_SAMPLE_DATA are used, the unwrapped estimate (in  radians)  should
              be  in  the  second  data  channel  of the file; the first channel is ignored.  The
              FLOAT_DATA format may also be used.

       -f configfile
              Read configuration parameters from file configfile.  The file  is  parsed  line  by
              line  for  key-value  pairs.   Template  configuration  files are included with the
              snaphu  source  code:  snaphu.conf.full  contains  all   valid   key-value   pairs;
              snaphu.conf.brief contains the most important parameters.  Lines not beginning with
              alphanumeric characters  are  treated  as  comment  lines.   Command  line  options
              specified  after  -f  will override parameters specified in the configfile and vice
              versa.  The -f option may be given  multiple  times  with  different  configuration
              files, with parameters in later-specified files overriding those in earlier ones.

       -g maskfile
              Grow  a  connected  component mask for the unwrapped solution and write the mask to
              the file maskfile.  A connected component is a region of  pixels  in  the  solution
              that  is  believed to have been unwrapped in a relative, internally self-consistent
              manner according to the statistical costs used.  Regions that are  smaller  than  a
              preselected threshold are masked out.  Parameters for this option can be set in the
              configuration  file.   The  connected  component  file  is  composed  of   unsigned
              characters  by  default,  with  all  pixels of the same value belonging to the same
              connected component and zero corresponding to masked pixels.

       -G maskfile
              Grow a connected component mask (see the -g  option)  for  the  input  data  array,
              assuming  that  it  is  already unwrapped, and write the mask to the file maskfile.
              Statistical cost functions are computed for forming the mask, but a  new  unwrapped
              solution is not computed.

       -h, --help
              Print a help message summarizing command-line options and exit.

       -i     Run  in  initialize-only mode.  Normally, snaphu uses either an approximate minimum
              spanning tree (MST) algorithm or a minimum cost flow (MCF) algorithm for generating
              the  initialization  to  its  iterative, modified network-simplex solver.  If -i is
              given, the initialization is written to the output and the  program  exits  without
              running the iterative solver.

       -k     Keep  temporary tile outputs.  If this option is specified when snaphu runs in tile
              mode, the temporary directory where tile outputs are stored will be left  in  place
              rather  than  deleted.   The tile-mode initialization of the -S option will also be
              left in place rather than deleted.

       -l logfile
              Log all  runtime  parameters  and  some  other  environment  information  into  the
              specified  file.  The log file is a text file in the same format as a configuration
              file.

       -m magfile
              Read interferogram magnitude data from the specified file.  This option  is  useful
              mainly  if  the  wrapped-phase  input  file  is given as a set of real phase values
              rather than complex interferogram values.  The interferogram magnitude is  used  to
              form  a  coherence  estimate  if appropriate amplitude data are given as well.  The
              default file format is FLOAT_DATA.  If the formats ALT_LINE_DATA or ALT_SAMPLE_DATA
              are used, the magnitude should be in the first data channel of the file; the second
              channel is ignored.  If the COMPLEX_DATA format is used, the phase  information  is
              ignored.  Areas where the magnitude is zero are treated as masked areas (see the -M
              option).

       -M bytemaskfile
              Read a byte mask from the specified file.  The mask file should be the same size as
              the input array to be unwrapped.  The mask should have the binary (not ASCII) value
              0 where pixels of the input array are to be ignored during the primary optimization
              stage of the program.  Values elsewhere should be binary 1.  Masking is not applied
              until after the initialization stage of snaphu.  Masked areas are treated as  areas
              in  which  the  solution  phase  value  is  irrelevant  to  the solution cost.  The
              magnitude of the interferogram is set to zero in masked areas in the  output  file.
              Areas  with  zero  magnitude in the input data are treated as masked areas as well.
              Areas near the edges of the input may also be masked via options in a configuration
              file.

       -n     Run  in  no-statistical-costs mode.  If the -i or -p options are given, snaphu will
              not use statistical costs.  Information from a weight file (-w option)  will  still
              be used if given.

       -o outfile
              Write  the  unwrapped  output  to  a  file  called  outfile.   If  the file formats
              ALT_LINE_DATA (default) or ALT_SAMPLE_DATA are used, the unwrapped phase is written
              into the second data channel, while the interferogram magnitude is written into the
              first channel.  The format FLOAT_DATA may also be used.

       -p value
              Run in Lp-norm mode with p=value, where value is a nonnegative decimal.  Instead of
              statistical  cost  functions, the program uses Lp cost functions with statistically
              based weights (unless -n is also given).  Solutions  are  still  always  congruent.
              Moreover,  congruence  is  enforced  within  the  solver  routine,  not  as a post-
              optimization processing step.  Therefore, if p=2, for example,  least-squares  cost
              functions  are  used,  but  the  solution  will  probably be more accurate than one
              generated from a transform-based least-squares algorithm.

       -q     Run in quantify-only mode.  The input data are assumed to be unwrapped already, and
              the  total cost of this solution is calculated and printed.  The unwrapped phase is
              wrapped assuming congruence for the cost calculation.  Round-off errors  may  limit
              the  precision  of  the  quantified  cost.   See  the  -u option for allowable file
              formats.

       -s     Run in smooth-solution mode.  The problem statistics and resulting  cost  functions
              are  based  on  the  assumption  that the true unwrapped phase represents a generic
              surface with no discontinuities.  This is the same as  deformation  mode  with  the
              DEFOMAX parameter set to zero.

       -S     Do  single-tile  re-optimization after tile-mode initialization.  If this option is
              specified, snaphu will run in tile mode to generate an unwrapped solution, which is
              then  used  as  the  initialization to a single-tile optimization that produces the
              final unwrapped output.  The tile-mode initialization may itself be initialized  by
              the  MST  or  MCF  algorithms  (or  an input unwrapped phase file) as normal.  This
              option is equivalent to running an instance of snaphu in tile  mode,  then  running
              another  instance  of  snaphu taking the tile-mode output as an unwrapped input via
              the -u option.  Tile parameters must be specified when  using  this  option.   This
              approach  is often faster than unwrapping an interferogram as a single tile from an
              MST initialization, especially if multiple processors are used.

       -t     Run in topography mode.  The problem statistics and resulting  cost  functions  are
              based on the assumption that the true unwrapped phase represents surface elevation.
              This is the default.

       -u     Assume that the input file is unwrapped rather than wrapped.  The  algorithm  makes
              iterative improvements to this solution instead of using an initialization routine.
              The input file may be in the formats ALT_LINE_DATA  (default)  or  ALT_SAMPLE_DATA;
              the  interferogram  magnitude should be in the first data channel and the unwrapped
              phase should be in the second data channel.  The  format  FLOAT_DATA  may  also  be
              used.

       -v     Run  in  verbose mode.  Extra information on the algorithm's progress is printed to
              the standard output.

       -w weightfile
              Read external, scalar weights from file weightfile.  The weights, which  should  be
              positive  short  integers, are applied to whichever cost functions are used.  There
              is one weight value for each arc in  the  network,  so  weightfile  should  be  the
              concatenation  of  raster horizontal-flow and vertical-flow arc weights.  Thus, for
              an N row by M column interferogram, weightfile would consist of a rasterized  (N-1)
              by  M  array followed by a rasterized N by (M-1) array of short integer data.  This
              option is not well tested.

       --aa ampfile1 ampfile2
              Amplitude data are read from the files specified.  The data from the two individual
              SAR  images  forming the interferogram are assumed to be separately stored in files
              ampfile1 and ampfile2.  These files should  be  in  the  format  FLOAT_DATA.   This
              option is similar to the -a option.

       --AA pwrfile1 pwrfile2
              Similar to the --aa option, but power data are read from the specified files.

       --assemble
              Assemble  the  tile-mode  temporary  files from a previous tile-mode run of snaphu,
              possibly with  different  secondary  optimization  parameters,  to  produce  a  new
              unwrapped  solution.   The tile directory name must be specified with the --tiledir
              option.  Most configuration options (from the command line  and  any  configuration
              files)  must  be  specified  similar to the previous run, including the output file
              name from which the names of temporary tile files are derived.  The previous output
              file may hence be overwritten by the new output file.  This option is useful if the
              user wishes to modify tile-assembly parameters without  unwrapping  the  individual
              tiles over again.

       --copyright, --info
              Print the software copyright notice and bug report info, then exit.

       --costinfile costfile
              Read statistical cost arrays from file costfile.  This file should be in the format
              written by the --costoutfile option.  The cost file does not control whether snaphu
              runs  in  topography,  deformation, or smooth-solution mode; the latter two must be
              specified explicitly even if costfile was generated while running in those modes.

       --costoutfile costfile
              Write statistical cost arrays to file costfile.  This option can be used  with  the
              --costinfile  option  to  save the time of generating statistical costs if the same
              costs are used multiple times.

       --debug, --dumpall
              Dump all sorts of intermediate arrays to files.

       --mst  Use a minimum spanning tree (MST) algorithm for the initialization.   This  is  the
              default.

       --mcf  Use  a minimum cost flow (MCF) algorithm for the initialization.  The cs2 solver by
              Goldberg and Cherkassky is used.  The modified network-simplex solver  in  L1  mode
              may  give different results than the cs2 solver, though in principle both should be
              L1 optimal.

       --nproc n
              Use n parallel processes when in tile mode.  The program forks a  new  process  for
              each  tile so that tiles can be unwrapped in parallel; at most n processes will run
              concurrently.  Forking is done before data are read.  The standard  output  streams
              of child processes are directed to log files in the temporary tile directory.

       --piece firstrow firstcol nrow ncol
              Read  and  unwrap only a subset or part of the input interferogram.  The read piece
              is the nrow by ncol rectangle whose upper left corner is the pixel at row  firstrow
              and  column  firstcol  (indexed  from  1).   All  input  files  (such as amplitude,
              coherence, etc.) are assumed to be the same size as  the  input  phase  file.   All
              output files are nrow by ncol.

       --tile ntilerow ntilecol rowovrlp colovrlp
              Unwrap  the  interferogram  in  tile  mode.   The interferogram is partitioned into
              ntilerow by ntilecol tiles,  each  of  which  is  unwrapped  independently.   Tiles
              overlap  by  rowovrlp  and  colovrlp  pixels in the row and column directions.  The
              tiles are then segmented into reliable regions based on the cost functions, and the
              regions are reassembled.  The program creates a subdirectory for temporary files in
              the directory of the eventual output file  (see  the  --tiledir  and  -k  options).
              Tiles can be unwrapped in parallel (see the --nproc option).

       --tiledir dirname
              Use  dirname as the name of the directory in which temporary tile-model outputs are
              written and/or read.  The directory is created if it does  not  exist,  and  it  is
              removed at the end of the run unless the -k or --assemble options are specified.

FILE FORMATS

       The formats of input files may be specified in a configuration file.  All of these formats
       are composed of raster, single-precision (float, real*4, or complex*8) floating-point data
       types  in the platform's native byte order.  Data are read line by line in row-major order
       (across then down, with the column index varying faster than the row  index).   Regardless
       of  the file format, all input data arrays should have the same number of samples in width
       and depth and should be coregistered to one another.  Note  that  weight  files  and  cost
       files  have  their  own formats.  The allowable formats for other data files are described
       below.

       COMPLEX_DATA
              Alternating floats correspond to the real  (in-phase)  and  imaginary  (quadrature)
              components of complex data samples.  The specified line length should be the number
              of complex samples (pairs of real and imaginary samples) per line.

       ALT_LINE_DATA
              Alternating lines (rows) of data correspond to lines of purely real data  from  two
              separate  arrays.  The first array is often the magnitude of the interferogram, and
              the second may be unwrapped phase, coherence, etc.  This is also  sometimes  called
              hgt, rmg, or line-interleaved format.

       ALT_SAMPLE_DATA
              Alternating  samples  correspond  to  purely real samples from two separate arrays.
              This format is sometimes used for the amplitudes of the two SAR images.

       FLOAT_DATA
              The file contains data for only one channel or array, and the data are purely real.

EXAMPLES

       Unwrap a wrapped topographic interferogram called ``wrappedfile''  whose  line  length  is
       1024  complex  samples  (output  will be written to a file whose name is compiled into the
       program):

           snaphu wrappedfile 1024

       Unwrap the same file as above, but use brightness information from the  file  ``ampfile,''
       set  the  perpendicular  baseline  to  -165  m at midswath, and place the output in a file
       called ``unwrappedfile'' (coherence data are generated  automatically  if  ``wrappedfile''
       contains complex data and ``ampfile'' contains amplitude data from both SAR images):

           snaphu wrappedfile 1024 -a ampfile \
               -b -165 -o unwrappedfile

       Unwrap  the  interferogram  as  above,  but  read  correlation  information  from the file
       ``corrfile'' instead of generating it from the interferogram and amplitude data:

           snaphu wrappedfile 1024 -a ampfile -c corrfile \
               -b -165 -o unwrappedfile

       The following is equivalent to the previous example, but input parameters are read from  a
       configuration file, and verbose output is displayed:

           cat > configfile
           # This is a comment line which will be ignored
           AMPFILE      ampfile
           CORRFILE     corrfile
           BPERP        -165
           OUTFILE      unwrappedfile
           EOF (ie, Ctrl-D)

           snaphu -v -f configfile wrappedfile 1024

       Unwrap  the  same  interferogram,  but  use  only  the  MST  initialization  (with  scalar
       statistical weights) and write the output to ``mstfile'':

           snaphu -f configfile -i wrappedfile 1024 -o mstfile

       Read the unwrapped data in ``mstfile'' and use that as the initialization to the  modified
       network-simplex solver:

           snaphu -f configfile -u mstfile 1024 -o unwrappedfile

       Note  that in the previous two examples, the output file name in the configuration file is
       overridden 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

       Unwrap the interferogram as above, but use the MCF algorithm for initialization:

           snaphu -f configfile wrappedfile 1024 --mcf

       Unwrap the interferogram once again, but first flatten  it  with  the  unwrapped  data  in
       ``estfile,'' then reinsert the subtracted phase after unwrapping:

           snaphu -f configfile wrappedfile 1024 -e estfile

       The  following  assumes  that  the  wrapped  input interferogram measures deformation, not
       topography.  Unwrap the interferogram with the given correlation data:

           snaphu -d wrappedfile 1024 -c corrfile

       Unwrap the input interferogram by minimizing the unweighted congruent L2 norm:

           snaphu -p 2 -n wrappedfile 1024

       Unwrap the interferogram as a three-by-four set of tiles that overlap by 30  pixels,  with
       the specified configuration file, using two processors:

           snaphu wrappedfile 1024 -f configfile \
               --tile 3 4 30 30 --nproc 2

HINTS AND TIPS

       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 -f 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.

       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.

       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 -s 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.

       The program can be run in initialize-only (-i) mode for quick down-and-dirty  MST  or  MCF
       solutions.

SIGNALS

       Once  the  iterative solver has started, snaphu 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.

EXIT STATUS

       Upon successful termination, the program exits with code 0.  Errors result in exit code 1.

FILES

       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.

       snaphu.conf.full
              Template  configuration file setting all valid input parameters (though some may be
              commented out).

       snaphu.conf.brief
              General-purpose template configuration file setting the most important or  commonly
              modified input parameters.

       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.

BUGS

       The -w option has not been tested exhaustively.

       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.

       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.

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

       There  is  no  error  checking  that  ensures that the network node potentials (incost and
       outcost) do not overflow their integer data types.

       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.

       Dedicated programs for specific Lp objective functions may work better than snaphu  in  Lp
       mode.   Note  that snaphu enforces congruence as part of the problem formulation, however,
       not as a post-optimization processing step.

       A tree pruning capability is built into the code and can be enabled from  a  configuration
       file, but this functionality is experimental and not well tested.

REFERENCES

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

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

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

                                                                                        snaphu(1)