Gamma LAT: Reference Manual

looks

ANSI-C program: looks.c

NAME
looks - Equivalent number of looks (ENL) estimation for (homogeneous) test areas (polygon regions).

SYNOPSIS
looks <data> <width> <polygon> > [outfile]

<data>	(input) intensity file (float)
<width>	number of samples/row
<polygon>	polygon data file
>	> sign to redirect standard output
[outfile]	output filename (default=stdout)

EXAMPLES
looks 1352_1610.pwr 2500 1352_1610.poly1 > 1352_1610.poly1.ENL

Estimation of ENL for file 1352_1610.pwr using polygon regions defined in 1352_1610.poly1.

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

AVAILABILITY
Uses type definition file typedef_ISP.h.

DESCRIPTION
looks estimates of the equivalent number of looks (ENL) from the data.

The ENL estimation is based on the statistics of homogeneous test areas within a real valued image (usually the SAR backscatter intensity). Therefore, such homogeneous test areas are first selected using the program polyras. Ideal test areas are large and homogeneous. The two estimators used assume that the pixel variation is due to speckle random noise only. Other variation will result in an underestimation of the ENL.

Once the polygons are determined the ENL can be estimated. The program looks extracts the values for the test areas and calculates the arithmetic mean, the standard deviation, and the geometric mean.

Two different methods are used to estimate the ENL.

The first method is based on the Method of Moments. The ENL is calculated by
ENL = SQR(arithmetic_mean/standard_deviation)
This method is well adapted if no a priori knowledge about the pixels distribution is available.

The second method is based on Maximum Likelyhood estimation. The estimator is uses the arithmetic and geometric means. For the exact functions we refer to Bruniquel J. and A. Lopes, Multi-variate optimal speckle reduction in SAR imagery, Int. J. Remote Sensing, Vol. 18, No. 3, pp. 603-627, 1997. It is found that the second method is a more accurate way to estimate the ENL in a test area of limited size.

The estimated ENL are written to standart output. This allows to either display it on the screen or to redirect it to an ascii-file.

The ENL is of interest to monitor the effect the speckle reduction achieved with filtering and averaging. Notice that the avaraging of two pixels will double the ENL only if the two looks are uncorrelated.

OPTIONS
none.

SEE ALSO
typedef_ISP.h, polyras .

DIAGNOSTICS
All messages are generally self-explanatory.

NOTES
none.