MBMAX64 Filters—Over/Under and
Median
by Joe Burnett
There are many filters within the MBMAX64 editor (64-bit HYSWEEP® EDITOR). Their main
function is to remove the majority of the obvious and extraneous bad soundings, so that you
don’t have to. Of all the filters in the MBMAX64 editor, the Over/Under and Median filters are
my favorite.
There are a couple of terms that need to be understood when filtering:
Ping: An individual swath of data points (single sweep) from a multibeam echosounder.
Gate: A value that is added and subtracted from a specific value (ie. Median) that will
create a range of values to be used by a specific filter.
Flagging: When a filter determines that a sounding exceeds its parameters, it ‘flags’ that
sounding with a yellow X, so you can see the soundings that will be removed/deleted when
the filter is applied. Once you apply a filter, those soundings that were flagged with yellow X’s
will be changed to red X’s. These red X soundings are always saved in the edited format files
(HS2 and HS2x) and can be restored to the data set at any time. The red X soundings are
only removed from the final data set, when you save it to an XYZ file.
•
•
OVER/UNDER FILTER
The Over/Under filter is fairly straight-forward in
its approach to flagging soundings for deletion.
It looks at each ping individually and finds
sounding points that either overhang or
undercut previous or next soundings (in
sequential Beam # order) in that ping.
FIGURE 1. Overhang
FIGURE 2. Undercut
There is nothing fancy about this filter, but it works very well.
January / 2014
1
MEDIAN FILTER
With the introduction of the new 64-bit HYSWEEP® EDITOR, there are many new features
that the 32-bit editor did not have. One of these new features is the Median filter.
The median is the middle number of a sorted (sequential) set of numbers.
The Median filter allows you to be more selective about how the filter picks the soundings to
be flagged for deletion.
FIGURE 3. Configuring the Median Filter
Enter the number of sequential pings to be
used.
• Enter. how many sequential beams from
each ping will be used
• Set the size of the gate above/below the
Median value.
Tip: Always set the Gate Size to larger than the
expected value, as you only want the filter to flag
soundings that obviously do not belong in the
final data set.
•
It is your job to accurately determine and remove the smaller anomalies in the collected
soundings.
The Median filter takes the first set of sequential pings (P) and the first set of sequential
beams (B) in each ping, finds the median value of these (PxB) soundings, then applies the
Gate Size to this median value and creates a range of values for the filter. The filter then
looks at all soundings in this (PxB) group, and flags any of the soundings that do not fall
within the gate range for deletion.
Using the settings in Figure 3, 3 sequential pings and 5 sequential beams in each ping will be
used.
In the following example, the median value of these 15 soundings is 61.1.
(5.9,5.9,6.1,6.3,6.5,60.9,61.0,61.1,61.1,61.3,61.8,61.8,61.8,62.2,62.2)
FIGURE 4. The Original Set of 15 Soundings
Next, the Gate Size (6.0 in this example) is
applied to the median value of 61.1.
The range of values to be used by the median
filter, goes from 55.1 to 67.1.
2
FIGURE 5. Soundings Outside the Gate Range
Are Filtered
The filter now looks at all 15 soundings in this
group of soundings, and any of the 15
soundings that do not fall within that range,
will be flagged for deletion.
After filtering the first group of soundings, it will move across to the next 3x5 group of 15
soundings and perform the filtering on them, and continue following this procedure for the
rest of the file and all files that are currently selected.
FIGURE 6. Filtering the File
MORE FILTERING TIPS
Tip: When using filters, enable one (1) filter at a time and look at all of the soundings that it
has flagged as points for deletion. If you disagree with what it has flagged as points for
deletion, modify the filter parameters (if possible) and view the flagged points again. Repeat
this procedure until you agree with what the filter has flagged, then apply it to the selected
(highlighted) files or all of the currently loaded files.
Tip: If you enable and apply the Over / Under filter first, you will find that the Spike filter will
more closely work like the Median filter. Most of the filters will perform at a more refined
degree, if the Over / Under filter is enabled and applied as the first filter.
January / 2014
3
FIGURE 7. Applying the Over/Under Filter
Go to the next filter you would like to use and repeat the above procedures.
FIGURE 8. Applying the Median Filter
Repeat the above procedures with all of the filters that you want to use and apply.
WHY THE OVER/UNDER AND MEDIAN FILTERS ARE MY
FAVORITE FILTERS
Well… if you set up the Median filter carefully and let the Over/Under do its thing, these two
filters can remove between 80% to 90% (or more) of all the erroneous sounding points that
need to be removed, allowing you to more quickly and meticulously remove the remaining
erroneous data, using the manual editing tools.
4