Statistical combination operator algorithm

The Statistical combination operator is used to create a virtual line by applying a statistical operation to operand lines.

Algorithm terms
Before you begin
Statistical combination operator algorithm

Algorithm terms

Term	Definition
Line segment	The join between between adjacent time/depth co-ordinates of a line. In Echoview, line segments have a line status.
Operands	User selected lines used as input line data for the Statistical combination operator. The algorithm uses the Divergence limit to determine eligible line segments from the operands.
Statistic	A statistical operation applied to Operands. Available Statistics are: Minimum depth - the minimum depth from the line segments is chosen for the output line. Maximum depth - the maximum depth from the line segments is chosen for the output line. Median depth - the median depth of line segments is chosen for the output line. Mean depth - the linear average depth of line segments is calculated and is the result for the output line. Note: The median depth of an odd number of lines is determined using the median of the (odd number of) depths. Where there is an even number of depths (lines) the median is the arithmetic mean of the two middle depths.
Choice operation	The Statistics: Minimum depth, Maximum depth or Median depth.
Combination operation	The Statistic: Mean depth.
Divergence limit	The allowable distance that a line segment can deviate from the mean of all the eligible line segment depths.

Before you begin

A typical use for the Statistical combination line operator is to combine sounder-detected bottom lines (or Echoview-generated lines) from a range of frequencies. When using multiple frequency data, frequency dependent effects need to be accounted for.

Simmonds and Maclennan1 discuss the implications of the increase of absorption coefficient with frequency in sea water and fresh water. For sea water, a factor of ten increase in frequency results in an approximate factor of ten increase in acoustic absorption coefficent. Such increases in absorption coefficient lead to an increase in TVG and an increase in the deterioration of the signal to noise ratio. It follows that each frequency has some range limit beyond which the acoustic signal can not be detected.

An example of this is seen for higher frequencies used in surveys in deep water. It is common for the range limit of the data to occur before the bottom depth. As a result sounder-detected bottom lines become erratic as data is swamped by noise (beyond the range limit).

To address this problem and to allow you to pre-process lines for the Statistical combination line operator, Echoview offers a Crop operator. This operator allows you to apply range limits to lines.