Detecting other boundary lines

The Threshold offset algorithm detects in a specified direction, relative to a nominated (base) line, for the first occurrence of a boundary value. The boundary may represent water column features like: transducer ring down, bubbles around the transducer, shadow zones or deadzones.

Prepare an echogram

1. Remove noise from an Sv, TS, Unspecified dB variable.
2. The specification of bad data regions, exclusion lines on the Analysis page and use of the Processed data operator can also clean echogram data.
2. Create and edit a (base) line to use with the Threshold offset operator.
3. Consider the most suitable base line relative to the boundary of interest.
4. Determine a boundary value for the feature of interest.
5. It may be helpful to use a single color gradient color scheme to enhance the contrast between water and the feature.

Create and configure the Threshold offset operator

A feature boundary may be approached from a base line that is an edited line pick that represents the acoustic bottom. The detection is configured for Detect minimum threshold.

A feature boundary may be approached from a base line positioned in empty water with the detection configured for Detect maximum threshold.

Open an echogram. On the Line and Surface tool select New Virtual Line.

-OR-

On the Dataflow window, use the right-click menu and select New, Virtual Line.

1. On the New Virtual Line dialog box, select Threshold offset and enter a name for the virtual line.
2. Click OK.
3. On the Operands page of the Line Properties dialog box specify a prepared acoustic variable for Operand 1 and prepared line for Operand 2.
4. On the Threshold Offset page:
   1. Select Detect minimum threshold or Detect maximum threshold.
   2. Under Threshold (dB) enter a feature boundary value.
   3. Select or clear Allow detection from echogram extents. Use the setting if the line (Operand 2) extends past Operand 1.
   4. Select or clear Apply line-relative smoothing to the echogram data. The decision to apply smoothing (blur convolution) depends on the nature of the echogram data.
   5. Under Minimum accepted distance (m) enter a minimum distance between the line operand and the Threshold offset line.
   6. Under Maximum detection distance (m) enter a distance to restrict the boundary search with respect to the line operand.
   7. Select a Direction of search.
5. You may want to specify the display and visibility of the Threshold offset line. This can be done on the Display and Visibility pages.
6. Click OK.

Notes:

• The suitability of the Threshold offset operator is discussed under About the Threshold offset operator: Notes.

See also

About the Threshold offset operator
Detecting a vegetation boundary