Step 2 - Excluding bad data
After determined the basic settings you have to exclue the bad data from
the original data.
The bad data is mainly the bubble layer at the surface and data below the
sea bed.
Others will be the some spike error data that is caused by the ship or
the bubble and non biological echo like the the trace of the CTD cast.
Also there are sometimes interfered signals caused from another echosounder
or ADCP.
To exclude the bad data that exist for all data, create a line in Echoview.
Select the EV file properties and choose "Line", click "New"
For a fixed depth, just enter the depth and name it.
For the bottom line there are two options.
One is using the exsiting bottom line, created by the original echosounder.
Select the specific line that is already created at the "Exisiting
line".
Make this line a few meters shallower than the original line to exclude
the seabottom data.
The other is creating a new bottom line by using echoview function called
"Linepick".
Choose "Linepick from the current echogram" from the pulldown
menu of the Line icon under New line.
After a little while, you will have a new bottom line indicated by bold
green line.
(Line pick will take minutes when your dealing with a large amount of data
or a low spec computer.)
Exclude the surface and the bottom data by setting the line on the Analysis
properties, Exclue above line (Surface) and Exclue below line (Bottom).
You can do this almost automatically, but don't forget to check the bottom
line manually.
This is because even after the line pick, the line may partially include
the bottom.
And if so, it may cause a huge error.
For the other bad data that exist randomly, check the data one by one and
choose the bad area with polygon selecting tool and define the area as
"Bad regions"