Gamma DIFF&GEO: Reference Manual

gc_map_inversion

NAME
gc_map_inversion - Geocoding lookup table inversion.

SYNOPSIS
gc_map_inversion <gc_map> <width_in> <gc_out> <width_out> [nlines_out] [interp_mode] [n_ovr] [rad_max] [nintr]

<gc_in>	lookup table containing pairs of real valued output coordinates (fcomplex)
<width_in>	width of gc_map
<gc_out>	inverted geocoding map corresponding to the new range and azimuth pixel number)
<width_out>	width of output geocoding lookup table gc_out
[nlines_out]	number of lines of output data file (default=0: all lines up to highest value)
[interp_mode]	resampling algorithm flag (default = 0) (0: 1/dist, 1: nearest neighbor, 2: SQR(1/dist), 3: const, 4: Gaussian)
[n_ovr]	interpolation oversampling factor (default = 2)
[rad_max]	rad_max maximum interpolation search radius (default = 4*n_ovr: 8)
[nintr]	number of points required for interpolation when not nearest neighbor (default = 4)

EXAMPLE
gc_map_inversion 8560_8059.utm_to_rdc_fine 1422 8560_8059.rdc_to_utm_fine 1696

Inverts the (refined) geocoding lookup table. By default the least squares fit interpolation algorithm is used.

INSTALLATION
Source code gc_map_inversion.c in ./src. For compilation adjust and use Makefile: Executable version gc_map_inversion in ../bin

AVAILABILITY
Uses ISP and DIFF type definition files typedef_ISP.h, typedef_DIFF.h.

DESCRIPTION
gc_map_inversion inverts the input geocoding lookup table to create a a lookup table that maps data in the reverse direction. If the input data is a table that gives the range/Doppler SAR image coordinates for every point on a uniform grid in the MAP coordinate system, gc_map_inversion will calculate the real-valued MAP coordinates at every point in the range/Doppler SAR sample grid.

The reverse lookup table is generated using the same algorithm as is used in geocode, with the exception that input data are the pixel grid coordinates in the map geometry ( [0,0] [0,1]... [1,0] [1,1]... ) rather than an input image values.

The interpolation of the irregularly sampled coordinates in gc_map_inversion is implemented using either nearest neighbor or convolutional interpolation methods. Different weighting functions are selectable for the convolutional interpolation including 1/distance, 1/distance**2, and Gaussian.

The combination of gc_map_inversion and geocode_back achieves similar results to using geocode alone. The reverse lookup table produced by gc_map_inversion can be efficiently applied to multiple data sets. For example, the simulated image and resampled DEM produced by gc_map is in the map coordinate system. To transform these into SAR coordinates, the geocode_backcan be applied twice, using the same reverse lookup table.

OPTIONS
The output grid can be oversampled by a factor n_over such that all data that can be used for the interpolation can be stored. The size of the search region for interpolation is given by rad_max while the number of points used for interpolation is nintr

SEE ALSO
typedef_ISP.h, typedef_DIFF.h.

NOTES
None.