Bottom Echo Bitmap H-mode - Furuno
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 always translated to "false".
You enter settings for this operator on the Bottom Echo Bitmap H-mode Furuno 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
Only operands of ping mode 3 (H-mode) are accepted as input.
Refer to the Bottom echo bitmap H-mode Furuno algorithm for more information.
Settings
The Bottom Echo Bitmap H-mode - Furuno Variable Properties dialog box pages include (common) Variable Properties pages and these operator pages:
Operands page
Bottom Echo Bitmap H-mode - Furuno page
|
Setting |
Description |
|
Start depth |
Specify the minimum depth at which you expect to find the bottom. The operator will not examine any data above this depth (these samples will remain true) and only examine samples below this depth. |
|
Stop depth |
Specify the minimum depth at which you expect to find the bottom. The operator will not examine any data above this depth (these samples will remain true) and only examine samples below this depth. |
|
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. |
Algorithm
The Bottom echo bitmap H-mode 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. This operator is specific to H-mode variables, Bottom echo bitmap operator is more generally applicable.
The algorithm is based upon the expectation that a vertically oriented conical H-mode beam will capture the bottom echo trace in the form of a ring at constant range (this assumes a relatively flat bottom). Objects like fish schools on the other hand will appear only in one part of the echogram. The bottom can thus be distinguished as a peak in the average of signal strength at a constant depth (i.e. over all beam).
The bitmap is generated as follows:
The start depth D0, stop depth D1 and threshold T are all specified on the Bottom echo bitmap H-mode - Furuno page of the Variable Properties dialog box.
For each beam of each ping:
For each distinct sample depth d between (and including) D0 and D1, determine the average signal strength sb over all beams.
- Starting at D0 and scanning downwards, find the first sample for which sb > T - this is s0
- Starting at D1 and scanning upwards, find the first sample for which sb > T - this is s1
- In the resulting bitmap:
- all samples above s0 are set to true
- all samples below s1 are set to true
- all samples between s0 and s1 are set to false if the sample value s > T and true otherwise.
In the resulting bitmap all samples designated 'bottom echo' are false and the remaining samples are true.
Note: This operator assumes that the H-mode beam is pointing vertically downwards (transducer elevation, azimuth and rotation should all be zero).
See also
About virtual variables
Operator licensing in Echoview
Bottom Echo Bitmap