Multibeam Target Detection

This operator generates multibeam targets from multibeam data. Targets are created from groups of adjoining data points that are not thresholded. Each resulting target has a range and major-axis angle corresponding to the geometric center of the group.

Operand 1

Multibeam magnitude Multibeam Sv Multibeam TS Multibeam unspecified dB

You can choose target detection settings, target linking settings and a bottom elimination method on the Multibeam target detection page of the Variable Properties dialog box.

You can choose the target properties that are calculated by this operator on the Target page of the Variable Properties dialog box.

For more information refer to About target properties.

Hint:  You may wish to remove background noise before detecting targets. One method for doing this is to create a virtual variable that identifies background objects using the Mean of n pings operator and then subtract this variable from its operand using the Minus operator.

See: Multibeam target detection algorithm for more information.

Notes:

• Another way of detecting individual targets in multibeam data is to use the By ping school detection algorithm.
• Detected targets are displayed by default with an outline indicating the group of data points included in the target and the input data that contributes to the target. You can choose not to display the outlines or the input data by unchecking the Show target sample outlines option and the Show input data under targets option on the Echogram Display page of the Variable Properties dialog box.
• Valid targets are those that have positions in front of the transducer face. There may be situations where data occurs behind the transducer face e.g. when roll is applied. In such cases targets with positions behind the transducer face are invalid.
• Use the Multibeam target overlay operator to display specified multibeam data under multibeam targets.
• About multibeam targets provides an overview of the issues and features for multibeam targets.

Settings

The Multibeam Target Detection Variable Properties dialog box pages include (common) Variable Properties pages and these operator pages:

Operands page

Filter Targets page

Target page

Target Detection page

The Filter Targets page is used to filter the multibeam targets for display. The Target page is used to specify target properties to calculate when targets have been identified.

The settings on the Target Detection page are used to specify target detection and target linking criteria and eliminate bottom data from the target detection process. The settings are applied to each multibeam ping (a 2-dimensional echogram).

Definitions for classification

Target linking term	Description
Candidate	A cluster of above-minimum-threshold samples. A Minimum threshold may be specified on the Data tab of the Variable Properties dialog box. Other thresholds are appled elsewhere. Advanced settings specify the linking distance for non-contiguous samples and minimum candidate dimensions.
Seed	A candidate with an area greater than the specified Minimum seed area on the Multibeam target detection page of the Variable Properties dialog box.
Satellite	A candidate with an area greater that the specified Minimum satellite area and less than the specified Minimum seed area.
Target	A cluster of one or more candidates that satisfy the specified seed-satellite linking and candidate retention options. Advanced settings specifiy minimum cluster length and height. Note: Targets with a target area (cm2) less than the Minimum satellite area are rejected.

Under Bottom elimination choose one of the options:

Settings	Description
None	Choose this option if you do not want to eliminate bottom data or there is no bottom data in the echogram.
Specified maximum range	Choose this option if you want to exclude data outside a specific range. Enter the range in meters.

Under Target Linking the settings for a target linking algorithm are available:

Settings	Description
Link target clusters	Select Link target clusters when you want to apply seed-satellite linking to detected targets. For more information refer to the Target linking algorithm.
Minimum seed area	Specifies a minimum seed area (cm2).
Minimum satellite area	Specifies a minimum satellite area (cm2).
Link distance	Specifies the linking distance for targets. The distance between the target of interest and the candidate must be less than the Link distance. For more information refer to Target linking algorithm - Linking distance.
Link satellite clusters	Select Link satellite clusters when you want to apply satellite-satellite linking to detected targets.
Retain unlinked targets	Select Retain unlinked targets when you want to retain unlinked satellites as detected targets.

When you click Advanced the Target Detection settings are displayed:

Settings	Description
Maximum horizontal linking distance (beams)	Maximum horizontal distance (in beams) allowed between two beams being linked to form a detected target. Note: The two linking distances form the vertical and horizontal semi-axes of an ellipse. The ellipse is, in effect, moved around the boundary of a target candidate. If any part of any other target candidate falls within the ellipse, a link is created between the candidates.
Maximum vertical linking distance (samples)	Maximum vertical distance (in samples) allowed between two samples being linked to form a target.
Minimum candidate length (cm)	Minimum length allowed for a target candidate.
Minimum candidate height (cm)	Minimum height allowed for a target candidate.
Minimum target length (cm)	After linking is completed, targets shorter than this length will not be included in detections.
Minimum target height (cm)	After linking is completed, targets shorter than this height will not be included in detections.

Notes:

• The default settings of the target detection algorithm use the maximum beam width of the sector plot, rather than the actual beam width of individual beams. This affects the horizontal linking distance setting.

• Warning: The target detection algorithm uses a nominal value to represent the parallel and non-diverging width of a typical beam in the sector plot. This nominal value is not a 3 dB beam angle. These advanced settings have been provided for experimental purposes only and changes to these settings may not produce the expected results.

Target page

Under Multibeam target properties the following calculations are available:

Setting

Description

Multibeam target properties

Select or clear the target properties that you want the Multibeam target detection algorithm to calculate. Available target properties are: Target area Target area edge midpoints Target compactness Target compactness edge midpoints Target length Target length across beams Target perimeter Target perimeter edge midpoints Target intensity variation Target range extent Target orientation Target elongation Target elongation across beams Notes: To achieve better performance with the Multibeam target detection operator, clear the target properties that you don't require for analysis. For more information about primary, optional and manual target properties refer to Target properties from multibeam data.

Under Target strength source the following settings are available:

Settings	Description
Mean Sv	Choose this option to set the detected target TS value to the mean Sv (or mean TS, mean magnitude, or mean unspecified dB, depending on the input operand data type) of the samples in the target.
Maximum Sv	Choose this option the set the detected target TS value to the maximum Sv (or mean TS, mean magnitude, or mean unspecified dB, depending on the input operand data type) of the samples in the target.

Algorithms

• Multibeam Target Detection
• Target Linking

Multibeam Target Detection algorithm

The multibeam target detection algorithm and the output of the target linking algorithm work as follows. For each ping:

1. Create a new multibeam boolean ping with the same ping geometry as the input operand.
2. Fill this new ping with true/false values as follows:
   1. false if
      the operand's data value is less than the Minimum threshold setting on the Data page of the Variable Properties dialog box, or
      the operand's data value is greater than the Maximum threshold setting on the Data page of the Variable Properties dialog box, or
      the data point's range is greater than the maximum range defined under Bottom elimination on the Multibeam_target_detection_page of the Variable Properties dialog box, or
      
   2. otherwise true
3. Using the settings on the Target page of the Variable Properties dialog box, create a new multibeam target for each island of neighboring true values, calculating from the data points they represent:
   1. the Target strength using the setting specified under Target strength source on the Target_page of the Variable Properties dialog box.
   2. the geometric center, which determines the minor-axis and major-axis angular position of the multibeam target.
   3. the estimated target length.
   4. the estimated target length across beams.
   5. the estimated target range extent.
   6. the estimated target perimeter length.
   7. the estimated target area.
   8. the estimated target elongation.
   9. the estimated target elongation across beams.
   10. the estimated target coefficient of variation of intensity.
   11. the estimated target compactness.
   12. the estimated target thickness.
   13. the estimated target perimeter edge midpoints.
   14. the estimated target area edge midpoints.
   15. the estimated target compactness edge midpoints.
   16. the estimated target orientation.
4. Create a Multibeam targets ping containing all the targets thus identified.
5. If Link target clusters is selected on the Multibeam_target_detection_page of the Variable Properties dialog box, apply the Target linking algorithm to the identified targets.
   1. otherwise go to Step 7.
6. Using the targets returned by the Target linking algorithm and the settings on the Target page of the Variable Properties dialog box, calculate from the data points contained in the targets:
   1. the Target strength using the setting specified in under Target strength source on the Multibeam_target_detection_page of the Variable Properties dialog box.
   2. the geometric center, which determines the minor-axis and major-axis angular position of the multibeam target.
   3. the estimated target length.
   4. the estimated target length across beams.
   5. the estimated target range extent.
   6. the estimated target perimeter length.
   7. the estimated target area.
   8. the estimated target elongation.
   9. the estimated target elongation across beams.
   10. the estimated target coefficient of variation of intensity.
   11. the estimated target compactness.
   12. the estimated target thickness.
   13. the estimated target perimeter edge midpoints.
   14. the estimated target area edge midpoints.
   15. the estimated target compactness edge midpoints.
   16. the estimated target orientation.
7. Create a new Multibeam targets ping containing all the identified targets and discard the multibeam boolean ping.

Notes:

• At the multibeam boolean pings stage, the identification of targets is sensitive to rounding and precision, both of which can change between Echoview versions.
• The initial identification of targets considers sample group perimeter, sample group bounding box and the path between sample groups. This part of the detection is sensitive to large path size together with very a small group perimeter, and can lead to incorrectly excluded samples.

Target Linking algorithm

The Target linking algorithm applies simple classification and linking rules to detected targets passed from the Multibeam target detection algorithm.

• Classification
• Linking distance
• Outcomes
• Algorithm

Classification

The Definitions for classification create potential targets from candidates, seeds, satellites.

Linking distance

Target linking term

Description

Link distance

Value specified by Link distance on the Multibeam target detection page of the Variable Properties dialog box. For a specified target, the potential targets to link to are found at distances less than the Link distance. A 'closest point in the target' technique is applied to find potential targets to link to. The technique creates points at the top and bottom mid-points of each beam and then determines the closest points. This technique avoids problems associated with linking large targets to small targets.

Outcomes

Summary of outcomes for Multibeam target detection target linking selections.

The selection on the Multibeam Target Detection page	Applies	Returns these targets to the Multibeam target detection algorithm
Link target clusters	Seed-satellite linking.	Newly linked seed-satellite targets. Unlinked seed targets. If Retain unlinked targets is selected, then unlinked satellite targets are returned. Otherwise unlinked satellite targets are discarded.
Link target clusters and Link satellite clusters	Seed-satellite linking, then satellite-satellite linking.	Newly linked seed-satellite targets. Unlinked seed targets. Linked satellite-satellite targets. If Retain unlinked targets is selected, then unlinked satellite targets are returned. Otherwise unlinked satellite targets are discarded.

The Target linking algorithm works in the following way.

For the identified targets in each ping from the output of step 4 of the Multibeam target detection algorithm, given the Seed list and Satellite list sorted in descending area:

1. For each seed:
   1. check if there is a satellite within the specified Link distance, if so add the satellite to the seed and remove the satellite from the Satellite list.
   2. repeat until no satellite is within the Link distance.
2. If Link satellite clusters is enabled, then take the largest satellite and temporarily treat it as a seed. Add any satellite that's within the Link distance, to the temporary seed. If the temporary seed area is greater than the Minimum seed area, move it to the Seed list otherwise it stays on the Satellite list.
3. Repeat 1 and 2, until no seed is enlarged.
4. Add all seed and satellite targets to the ping.
5. If Retain unlinked targets is enabled, add any target smaller than the Minimum satellite area.

Notes:

• If there are no satellites and no seeds, the multibeam ping is empty. Return to the Multibeam detection algorithm.
• Only candidates classified as satellites can be linked to seeds. Seeds can never be linked to other seeds.

See also

About virtual variables
Operator licensing in Echoview