Gamma ISP: Reference Manual

gcp_ras

ANSI-C program: DISP/src/gcp_ras.c .
This program is located in the DISP package.

NAME
gcp_ras - Ground Control Points (GCP) selection from SUN raster or BMP format images.

SYNOPSIS
gcp_ras <ras> <GCP> [mag] [win_sz]

ras	(input) SUN raster .ras or BMP .bmp format image
GCP	(output) GCP data file (text format)
mag	zoom magnification factor (default=3) )
win_sz	zoom window size before magnification (default=96)

EXAMPLE
gcp_ras 1352_1610.flt.ras 1352_1610.gcp

DESCRIPTION
gcp_ras reads and displays an image in SUN raster or BMP format. It can be used to select ground control points for precise estimate of the interferometric baseline for repeat track interferometry (RTI). Heights of the GCPs are derived from topographic maps or GPS ground measurements. In this application the reference image is generally of a flattened interferogram

The data entered using gcp_ras are stored in a text format file that can be edited using a text editor if necessary. The file format consists of columns of numbers containing the GCP number, x (cross-track) pixel coordinate, y (along-track) line number, and the entered data. Each GCP has a single line entry in the file.

Using the program is simple. Different mouse buttons perform different functions. Pressing the left button of the mouse allows viewing the area around the cursor in a zoom window. GCP points are selected by pressing the right mouse button. A red cross appears at the selected point and the data entry window becomes active (able to accept input). It is permitted to move the initial point location by right clicking another point. Only when Once a desired point is selected, enter the height for that point in the data entry window. Hitting the enter key writes the coordinates and entered data to the GCP file. When finished with data entry click the close button to exit. If you have been working on a GCP list you can append additional GCP points by simply rerunning the application. The previously entered points will be drawn on the image

The heights, not the true horizontal position is required in the calculation of the interferometric baseline. At least 20 control points at various altitudes are recommended to obtain a reasonable baseline estimate.

The next step in using the ground control points is to extract values of the phase corresponding to the GCPs from an unwrapped interferogram. This is done using program gcp_phase. The output of gcp_phase is text format file with an additional column containing the unwrapped phase values. This file in turn is used as input to program base_ls to estimate the baseline.

SEE ALSO
typedef_ISP.h, gcp_phase, base_ls.