Bottom Echo Bitmap
This operator creates a bitmap (multibeam Boolean variable) with a "false" value for each input value that is determined to contain energy from the bottom echo, and a "true" value for all the remaining input samples. "No data" values are translated to "false".
You enter settings for this operator on the Bottom Echo Bitmap page of the Variable Properties dialog box.
Echoview accepts a single operand of the following data types as input:
- Multibeam magnitude
- Multibeam Sv
- Multibeam TS
- Multibeam unspecified dB
Refer to the Bottom echo bitmap algorithm for more information.
Warning: When the Surface to use a bottom intersects a no data value and the Set below bottom samples to false is selected a "true" value is returned. This may be contrary to your expectations of this operator. In this situation, Echoview support advises you to create a 3D bottom surface that does not intersect with no data values.
Settings
The Bottom Echo Bitmap Variable Properties dialog box pages include (common) Variable Properties pages and these operator pages:
Operands page
Bottom Echo Bitmap page
|
Setting |
Description |
|
Surface to use as bottom |
Choose a surface which defines the bottom. The surface must intersect with the echogram data for it to be of any use, and should ideally be:
|
|
Threshold (in units of operand) |
Specify the threshold value above which signal is considered to be bottom and below which it is considered not to be bottom. |
|
Set below bottom samples to false |
Choose whether to set all samples below the chosen surface to false, or only those which are above the chosen threshold. Samples above the chosen surface will be set to false if they are above the chosen threshold |
Warning: When the Surface to use as bottom intersects a no data value and the Set below bottom samples to false is selected a "true" value is returned. This may be contrary to your expectations of this operator. In this situation, Echoview support advises you to create a 3D bottom surface that does not intersect with no data samples.
Algorithm
The Bottom echo bitmap operator is used to produce a bitmap in which all samples attributed to bottom echo signal are set to 0 and all others to 1. This can then conveniently be used to mask another variable to eliminate the bottom echo trace.
The algorithm requires that a surface S be specified as the intersection between each ping and this surface is where the bottom echo trace is expected. A threshold value T must also be specified above which the bottom echo signal is expected be. An option B to exclude also signals below (or beyond) the bottom echo trace is also available. These are all specified on the Bottom echo bitmap page of the Variable Properties dialog box.
In the resulting bitmap samples which are attributed to bottom echo signal are set to false, and the remaining samples to true.
The bitmap is generated in as follows:
-
Define a minimal bottom echo trace.
In each beam of each ping:-
each sample is set to true
-
if the beam intersects with the specified surface S then,
the first sample below S (nearest sample with greater depth) is designated 'bottom echo' and set to false if it has a value greater than the specified threshold T
-
-
Capture edge samples which do not intersect the surface.
Repeat for each beam of each ping until no new beams qualify:-
if the beam does not intersect the surface S but a neighboring beam has a false value then
the last sample in the beam is designated 'bottom echo' and set to false if it has a value greater than the specified threshold T
-
-
Expand the bottom echo trace to the threshold limit.
In each beam of each ping:-
all samples adjacent to false samples are also set to false if they have a value greater than the specified threshold T
-
all samples at greater range than a false sample are also set to false if the option B is selected
-
Warning: When the Surface to use as bottom intersects a no data value and the Set below bottom samples to false is selected a "true" value is returned. This may be contrary to your expectations of this operator. In this situation, Echoview support advises you to create a 3D bottom surface that does not intersect with no data values.