About sector plots and beams
Multibeam data, by definition, provides data from a number of beams. Typically these beams define a sector (see About ping modes). This is the view customarily displayed on a multibeam echogram.
Such a sector is defined by:
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A number of beams, called Number_of_beams in Echoview
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A sector angle, called Sector_angle in Echoview
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A stop range, called Range_stop in Echoview
Also, if data near the transducer is not collected the sector can be further defined by (thus becoming effectively an annular segment):
- A start range, called Range_start in Echoview
And, if the spacing and/or width of the individual beams is not uniform:
- The angle that each beam makes from a reference beam, seen in the status bar on multibeam echograms
(the reference may be the first beam, or the beam closest to the central axis of the transducer, depending upon the ping mode)
Some data may further be logged with a subset only, of the beams the echosounder can produce. Such a subset of beams is described as above (by Number_of_beams and Sector_angle) but the full sector is additionally described by:
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The number of beams in the full sector, called Full_sector_beam_count in Echoview
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The angle subscribed by the full sector, called Full_sector_angle in Echoview
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The number of the first beam within the full sector, called First_beam_number in Echoview
This is illustrated graphically in the following illustrations:
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Figure 1
Schematic of a typical multibeam sector plot, illustrating the defining parameters and beam numbering in Echoview. The Range_start is 0 m.
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Notes:
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Figure 2
Schematic of a multibeam sector plot with non-zero start range, illustrating the defining parameters and beam numbering in Echoview.
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Notes:
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Figure 3
Schematic of a multibeam sector plot beam subset, illustrating the defining parameters and beam numbering in Echoview.
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Notes:
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The displayed beam width is determined by the Sector_angle and Number_of_beams on sector plots with uniform beam spacing. This is distinct from the beam width, which is not displayed on the sector plot.
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Sector_angle, Number_of_beams, Range_start and Range_stop are exported with all multibeam data exports and used in multibeam text data files.
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Full_sector_angle, Full_sector_beam_count and First_beam_number are exported with all multibeam data exports for information only.
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Beam number is visible on the status bar
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Full sector beam number cannot be seen anywhere in Echoview.
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In this example: Number_of_beams = 8
Full_sector_beam_count = 13
First_beam_number = 3
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Figure 4
Schematic of a simple multibeam sector plot (with only 5 beams) illustrating a non-uniform beam spacing. Beam 3 is used to illustrate the various angles.
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Notes:
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The displayed beam width is determined by the angles that individual beams (their axes) make to a reference beam - in this case the first beam (beam 0). The angles midway between the beam axes are the boundaries of the displayed beam, and the first and last beam are displayed symmetrically about their axes respectively.
- The axis of each beam is not necessarily at the center of the displayed beam. Beam 3 is a good example.
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Sector_angle, Number_of_beams, Range_start and Range_stop are exported with all multibeam data exports and used in multibeam text data files. Note: The individual angles that each beam makes with the first beam is not currently exported.
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Beam number and the angle between it and the first beam are visible on the status bar
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See also
About multibeam echograms
About ping modes
About beam geometry
Resample multibeam pings