Gamma DIFF: Reference Manual

extract_gcp

NAME
extract_gcp - extract GCPs from a DEM in range-Doppler coordinates

SYNOPSIS
extract_gcp <DEM_rdc> <OFF_par> <GCP> [nr] [naz] [mask]

<DEM_rdc>	(input) DEM in range-Doppler coordinates
<OFF_par>	(input) ISP offset/interferogram parameter file
<GCP>	(output) ground control point file (text format)
[nr]	(input) number of GCP points in range (default=24)
[naz]	(input) number of GCP points in azimuth (default=24)
[mask]	SUN raster/BMP format mask file, regions flagged with 0 are excluded

EXAMPLE
extract_gcp 8560_8059.dem 8560_8059.off 8560_8059.gcp 32 32 8560_8059.mask.ras

Selects 32 x 32 ground control points from DEM which was resampled to SAR coordinates. The file 8560_8059.mask.ras is a SUN raster file format file used to determine from which areas to select GCP data.

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

AVAILABILITY
Uses ISP type definition file typedef_ISP.h.

DESCRIPTION
extract_gcp uses a digital elevation model (DEM) which was resampled to slant range geometry (registered to interferometric image files) to automatically extract a large number of ground control points (GCPs).

Automated extraction of ground control points based on a DEM resampled into range/Doppler coordinates improves the processing efficiency substantially. The user can specify a SUN raster or BMP format mask file to prevent GCPs being selected from specific regions with deformation. An image of the scene (i.e. the unwrapped phase) in slant range coordinates can be edited using programs such as GIMP, xv, or Photoshop to create the mask file. Select black (value = 0) as the region fill color in regions that to be excluded. The edited image must be saved and entered as command line argument to extract_gcp.

A pre-requisite for the use of extract_gcp is the availability of an appropriate DEM in SAR slant range/Doppler coordinates. The steps required to obtain a precise mapping of the DEM data requires matching the actual image with a simulated SAR image derived from the DEM. This simulated image is created using a 2 step process. First a lookup table is calculated containing the SAR range/Doppler coordinates for every point in a DEM covering the region as well as a simulated SAR image in DEM coordinates with program gc_map. This simulated SAR image is resampled into SAR range-Doppler coordinates with program geocode. The offsets between the simulated and actual images are estimated using the programs init_offsetm, offset_pwrm, and offset_fitm. The geocoding lookup table is than corrected using program gc_map_fine).

OPTIONS
extract_gcp may also be used to automatically extract values for the validation of an interferometricly generated height map.

SEE ALSO
typedef_ISP.h.

DIAGNOSTICS
All messages are generally self-explanatory.