PRC_ABC and PRC_NASC

PRC_ABC and PRC_NASC enable you to use region-based classification on your data, integrate the regions in the cells and then estimate biomass with respect to your region classifications. All without requiring the use of virtual variables.

PRC_ABC and PRC_NASC represent the contribution of the region-cell intersection (RC) to the ABC or NASC respectively of the cell (C) that contains the region cell intersection (RC). They are only meaningful and are only output when integrating by regions by cells, i.e. when the analysis domain is a region-cell intersection.

Hint: "PRC" is a mnemonic for "Proportion that the Region contributes to Cell".

Screen shot of part of the AutoOpen.EV Sv pings echogram with regions achieved by using a 2D school detection and then applying a region classification rule.

Three regions are shown:

A (selected, yellowish) region with a region class of "school detection 1"
The two other regions are classified with a region classification rule to output the "Really big fish" region class. This region class is assigned when there are less than 220 samples in the school region.

A cell is marked in red with an inset that displays greater detail of the regions that contribute to the cell.

The grid units are 40m distance by 40m depth.

PRC_ABC
PRC_NASC
Additivity (includes a worked example for two regions that fill a cell)

PRC_ABC (PRC Area Backscattering Coefficient - m2m-2)

Let ABCC be the ABC of a domain C defined by a cell.

Let ABCRC be the ABC of a domain RC defined by a region-cell intersection.

Then PRC_ABC is defined as:

where:

PRC = Number of pings in the domain RC

PC = Number of pings in the domain C

PRC_NASC (PRC Nautical Area Scattering Coefficient - m2nmi-2)

PRC_NASC is defined in the same way as PRC_ABC above and accordingly:

Notes:

ABCRC and NASCRC are not output by Echoview.
Changes to the Exclusions and Analysis settings on the Analysis page of the Variable Properties dialog box can reduce the Thickness_mean and hence the volume that NASC is valid over. Changes to the Threshold settings of the Data page of the Variable Properties dialog box may change data values in the analysis domain. Such changes can affect the Echoview values for Sv_mean and/or Thickness_mean and the final NASC value. For more information refer to About samples and echo integration.
Care is required when using Exclusions settings, stray bottom signal (associated with an Exclude below line) can introduce large errors to the calculated NASC.

Additivity

The powerful feature of PRC_ABC and PRC_NASC, is that it is possible to meaningfully add all PRC_ABC values in one interval or cell. The same holds true for PRC_NASC. If a cell is completely divided into i regions, such that none of the regions overlap and all of the samples in that cell fall within one of the regions, then the sum of all the PRC_ABCi is equal to the ABC of the cell. Similarly, the sum of all the PRC_NASCi is equal to the NASC of the cell.

Note: The Regions page of the Cell integration dialog box displays PRC_NASC for the cell.

Worked example

Diagram of a cell (heavy outline) on an echogram with region-cell intersections RC1 and RC2.

For case where the region cell intersections completely fill the cell, the additivity assertion can be expressed as:

(1)

(2)

=
Height mean or Thickness mean of an area of interest. In this case RC1, RC2 and the CELL.

The Height can be express in terms of the samples, pings and pixel height within the area.

The Height mean can be express in the following way with CELL being the area of interest:

=
Linear Sv mean of the area of interest.

The linear Sv mean can be expressed in the following way with RC1 being the area of interest:

Pings =
Number of pings in the area of interest.

Ns =
Number of samples in the area of interest.