Line picking

Line picking is the process of picking lines from acoustic variables using algorithms. This is typically done to detect bottom lines in variables. A bottom line may be a basis for biomass estimation or for bottom classification. Be aware that the Backstep for these analyses is different.

Before you pick an editable line, you should access the Single beam tab of the Lines and Surfaces page of the EV File Properties dialog box and check the line picking algorithm and other settings that will affect line picking.

When a line pick is complete, you may want to repick or recalculate or edit the line. Verification may be simpler when you display an echogram relative to a specified reference line.

To pick a new editable line over the entire variable:

1. Display a single beam echogram
2. On the Shortcut menu select Pick New Line
3. Specify the Destination line, Line visibility and Span gaps
4. Click OK

Note: This method can be used to create a new editable line from a Source other than a Line pick.

• -OR-

1. Display a single beam echogram
2. Click the Line and Surface tool  , then click New Editable Line
3. Select the Line pick from current variable option
4. Select Create new line and enter a name for the line
   - OR -
   Select Overwrite existing line and select a line to overwrite
5. Click OK

To replace part of an existing line by applying a line picking algorithm to the data in a selection:

1. Display a single beam echogram.
2. Click the Line and Surface tool , then click the required line to make it the active line.
3. Make a selection using one of the echogram selection tools.
4. On the Shortcut menu (right-click), click Repick Line.

To pick a virtual line:

1. Select a Line pick operator when you follow the instructions to create a virtual line.

Notes:

• If there is high surface noise on the echogram the Line pick algorithm may not pick the bottom as you would expect. If this is the case, you will get better results by setting the Start depth and Stop depth on the Single beam tab of the Lines and Surfaces page of the EV File Properties dialog box.
• If there are breaks in the bottom return the line pick may look irregular. You can use the Line and Surface tool to edit these parts of the line pick. Alternatively, you can delete parts of the line by setting the Line status to None for selected areas.
• Line picking uses a default Backstep value of 0 meters which is ideal for bottom classification. On the other hand, line picking for biomass estimation normally requires the Backstep value to be -0.50 meters so that none of the bottom is included in integration. If a line pick is to be used for both purposes, you will need to create an additional virtual line that copies the line pick and adds an offset of -0.50 meters.

See Line picking algorithms and Best bottom candidate settings for more detailed information.

See also

Line picking algorithms
Multibeam bottom detection algorithm
Creating, renaming and deleting lines
About setting variable properties
Best bottom candidate description of settings