Detecting a vegetation boundary

The Threshold offset algorithm looks in a specified direction, relative to a nominated (base) line, for the first occurrence of a vegetation boundary value. Bottom vegetation can occur near the acoustic bottom line. Floating vegetation may occur near the surface (0 meters).

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 vegetation of interest.
3. Determine a boundary value for the vegetation of interest.
4. It may be helpful to use a single color gradient color scheme to enhance the contrast between water and vegetation.

Create and configure the Threshold offset operator

A vegetation 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 vegetation 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 vegetation 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 distance for the vegetation boundary search. For more information refer to Threshold offset Minimum line separation.
   6. Under Maximum detection distance (m) enter a distance to restrict the vegetation boundary search with respect to the line operand.
   7. Select a Direction of detection.
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.
• Specify Vegetation settings on the Analysis page for an acoustic variable before exporting Vegetation analysis data.

See also

About the Threshold offset operator
Detecting other boundaries
Exporting a vegetation analysis