Gamma DIFF: Reference Manual

offset_list_fitm

ANSI-C program: offset_list_fitm.c

offset_list_fitm calculates fine registration polynomials for geocoding from control point list.

SYNOPSIS
offset_list_fitm <cp_list> <DIFF_par> <DEM_par> [lookup_table] [lt_type] [type1] [type2] [coffsets] [poly_order] [interact_flag} [trans_list]

<cp_list>	(input) list with registration control point (CP) coordinates: CP_nr ref_col ref_row col2 row2
<DIFF_par>	(input/output) DIFF&GEO parameter file (for output of fine registration polynomial)
<DEM_par>	(input) DEM/MAP parameter file
[lookup_table]	(input) geocoding lookup table (required to convert between map pixel numbers and SAR pixel numbers, fcomplex)
[lt_type]	lookup table type 1: map_to_rdc, 2: rdc_to_map (default=1)
[type1]	reference coordinate type (default=1: SAR (col,row) pixel numbers, 2: map (col,row) pixel numbers, 3: map (East,North) coordinates)
[type2]	image 2 coordinate type (default=1: SAR (col,row) pixel numbers, 2: map (col,row) pixel numbers, 3: map (East,North) coordinates)
[coffsets]	(output) list of culled registration offsets (ASCII file)
[poly_order]	polynomial order parameter (1,3,4,6, default: 4)
[interact_flag]	interactive culling of input data (1=YES, 0=NO, default=NO)
[trans_list]	transformed coordinate list using model

EXAMPLE
offset_list_fitm i1_i2.cp_list i1_i2.diff_par i1_i2.utm.dem_par i1_i2.utm_to_rdc 1 1 3

Derives the 6-parameter regression polynomial based on the registration control point list (with SAR (col,row) pixel numbers indicated for the reference image 1 and UTM map coordinates indicated for the image 2), to be used for the refinement of the lookup table i1_i2.utm_to_rdc (which is lt_type=1 as it was generated using gc_map or gc_map_grd).

offset_list_fitm i1_i2.cp_list i1_i2.diff_par i1_i2.utm.dem_par i1_i2.rdc_to_utm 2 1 3

Derives the 6-parameter regression polynomial based on the registration control point list (with SAR (col,row) pixel numbers indicated for the reference image 1 and UTM map coordinates indicated for the image 2), to be used for the refinement of the lookup table i1_i2.rdc_to_utm (which is lt_type=2 as it was generated using gc_insar).

DESCRIPTION

offset_list_fitm computes range and azimuth registration offset polynomials from manually determined control point coordinates which are provided in the cp_list ASCII file. The cp_list contains for each control point the control point number and the coordinates of the control point in the reference image and in image 2. These coordinates can be provided as SAR image pixel numbers, map pixel numbers, or map coordinates. An example for such a cp_list with reference coordinates provided as MAP image pixel numbers (col,row) and UTM map coordinates (Easting, Northing) may look as follows:

1	413	240	256250	824250
2	812	120	254345	825450
3	984	782	257255	825080
...	...	...	...	...
21	840	724	257650	829800

Such an cp_list is manually edited, supported by visualization tools to display the SAR image and other digital products.

It is recommended to first transform a SAR image to the map geometry, using the initial geocoding lookup table, which makes the geometries of a reference map and the SAR image very much alike - allowing to much facilitate the control point identification.

The lookup table type allows to distinguish between lookup tables pointing from map to SAR geometry (such as *.utm_to_rdc, as derived using gc_map or gc_map_grd) and lookup tables pointing in the opposite direction (such as *.rdc_to_utm, as derived using gc_insar, i.e. when geocoding based on INSAR derived heights). In the first case all coordinates are transformed to SAR pixel numbers; the offset polynomials are in the SAR image geometry (in SAR pixel number units). In the second case all coordinates are transformed to map pixel numbers; the offset polynomials are in the map geometry (in map pixel number units).

Based on the control point coordinates the program determines registration offsets. The offset data are culled before fitting the offsets using the least-squares method. Points can be rejected if offset values deviate more than an indicated multiple of the standard deviation of the fit.

This offset estimates culling can be done either automatically or interactively. The program queries the user in the interactive mode for the SNR and maximum range and azimuth errors. Offset data above the threshold (culled data) are then used to determine a new global fit to the estimates. Typically in the first round of culling, the points 3 or more standard deviations away from the global fit are rejected.

The formats and definitions of the individual parameters in the are ISP offset/processing parameter file are found in the ISP and DIFF&GEO type definition files typedef_ISP.h typedef_DIFF.h and in the description of the DIFF/GEO parameter file and in the description of the create_diff_par and create_diff_par programs. The coordinates of the image section selected to be processed are relative to the image 1.

The 6-parameter polynomials used to model the range and azimuth offsets are of the type:
range_offset = A0 + A1*range + A2*azimuth + A3*range*azimuth + A4*range*range + A5*azimuth*azimuth
azimuth_offset = B0 + B1*range + B2*azimuth + B3*range*azimuth + B4*range*range + B5*azimuth*azimuth

The polynomial order can be indicated on the command line. Valid orders are 1,3,4. The corresponding polynomials are (r: range pixel number, az: azimuth pixel number):

Order	Polynomial Model
1	`A0`
3	`A0 + A1r + A2az`
4	`A0 + A1r + A2az + A3raz`
6	`A0 + A1r + A2az + A3raz + A4r^2 + A5az^2`

In the case of noisy data the fit a polynomial with less terms may lead to a more stable fit. In such a case the use of a 3 parameter polynomial may be preferred over the use of the 4 or 6 parameter models. A possible approach is to first derived a 3 parameter polynomial and to reject estimates which are far away from this fit. Then in a second step the culled offsets (coffs) are used to derive the least squares fit for the 4 or 6 parameter polynomial.