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 representing the acoustic bottom (configure for Detect minimum threshold), or from a base line positioned in empty water (configure for Detect maximum threshold).

Option 1 — Line & Surface tool

Open an echogram. On the Line and Surface tool select New Virtual Line/Time Series…

1. On the New Virtual Line dialog box, select Threshold offset.
2. Click OK.

-OR-

Option 2 — Dataflow Toolbox

From the Dataflow Toolbox, drag the Threshold offset operator to the Dataflow window.

Then configure the operator:

1. On the Operands page of the Time Series Properties dialog box, select a prepared acoustic variable for Operand 1 and a prepared base line for Operand 2.
2. On the Threshold offset page:
   1. Select Detect minimum threshold or Detect maximum threshold.
   2. Under Threshold (dB), enter the vegetation boundary value.
   3. Select or clear Allow detection from echogram extents (useful if the base line (Operand 2) extends past Operand 1).
   4. Select or clear Apply line-relative smoothing to the echogram data (a blur convolution) depending on data characteristics.
   5. Under Minimum accepted distance (m), enter the minimum line separation. See Threshold offset for details.
   6. Under Maximum detection distance (m), enter a limit for the search relative to the base line.
   7. Choose the Direction of detection (Above or Below operand line).
3. Optionally adjust display and visibility on the Display and Visibility pages.
4. 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