avgc - geometric correction for unregistered AVHRR datasets

SYNOPSIS

avgc  [ parameter=value ... ]  [ inputfile outputfile ]
avgc  [ parameter=value ... ]  [ inputfile ... directory ]

Parameters are include_vars, gc_delta_line, gc_delta_samp.

DESCRIPTION

avgc geometrically corrects AVHRR datasets to compensate for the curvature of the earth. Because the AVHRR sensor scans at a constant rate, pixels near the ends of scan lines are up to 5 times wider than pixels at the center of scan lines. avgc redistributes the pixels along each scan line so each pixel is approximately square, about 1.1 km on a side. This involves replicating pixels near the ends of a given scan line, while discarding a few pixels at the center of the scan line.

Geometrically correcting full 2048-sample AVHRR scan lines results in output lines containing 2560 samples.

avgc does NOT correct for the rotation of the earth. To do so would result in a pair of thin triangular strips of missing data on the sides of the geometrically corrected output. As a result of not correcting for earth rotation, output pixels are actually parallelograms with approximately equal sides. The slope of these parallelograms is approximately cos(latitude)/10, which does not result in a great deal of distortion.

avgc allows subsampling on output. Input to avgc must be full resolution AVHRR data. avgc does not require full scan line input. However, input cannot be subsampled or re-oriented (e.g., rotated). Input can be calibrated or uncalibrated, sharpened or unsharpened. See avcal(1) and avsharp.

avgc is provided as a faster alternative to registration. See fastreg.

PARAMETERS

include_vars

List of variables to subsample. If the list is prefixed by a minus sign, then all variables except those listed will be subsampled. Wildcard characters * and ? may be used. The default is to inlude all input variables in the output.

gc_delta_line, gc_delta_samp

These parameters control subsampling on output. For example, if gc_delta_line is 1, then there is one output line for every input line. If gc_delta_line is 2, then there is one output line for every two input lines. The valid range for gc_delta_line and gc_delta_samp is [1, 4]. The default for each value is 1.

EXAMPLES

\fC% avgc
in/out files   : char(255) ? n12.93273.1533 n12.93273.1533.avgc
include_vars   : char(255) ? [] *ch3
gc_delta_line  : int       ? [1]
gc_delta_samp  : int       ? [1]
cabo%

SEE ALSO

fastreg, avcal, avsharp, avin, avhrr


Last Update: $Date: 1998/05/28 19:45:48 $