Sidescan Mosaicking

Whereas sidescan track review allows the display and review of a single sonar track log, sidescan mosaicking allows any number of sonar track logs, and parts of sonar track logs, to be combined into a single, large image. Sidescan swath segments can be combined and configured to produce a single, composite sidescan mosaic, which can then be saved for further use within ReefMaster or other GIS applications.

 

Detail from the image export of a large sidescan mosaic. This section contains three parallel, overlapping tracks.

 

The process of creating a sidescan mosaic involves selecting regions of sidescan from one or more tracks, adding these to the mosaic and then configuring the individual segments to produce a finished mosaic.

Once a mosaic has been produced, it must be exported in one of the available image formats before it can be used within ReefMaster (as an overlay in Map Projects, or for tracing in a User Map), or used in further GIS applications (such as Google Earth).

 

Sidescan Mosaic Fundamentals

Creating a new Sidescan Mosaic

The Sidescan Mosaic asset type appears between Waypoints and Images in the Asset Library, and is only visible if the sidescan mosaic module has been purchased and activated:

 

 

New sidescan mosaics can be created using the New Asset button and opened for editing by double-clicking the mosaic name in the list, in the same way as any other ReefMaster asset (see the Asset Library for more details).

 

A new mosaic can also be created whilst adding tracks or track parts - see below.

 

Sidescan Segments

Sidescan mosaics are simply a collection of one or more tracks (or parts of tracks) that contain sidescan sonar information. Each track (or track part) that is added to a sidescan mosaic is known as a sidescan segment, and each segment can be edited independently to configure properties such as port and starboard ranges, brightness, contrast, opacity and z-order (the drawing order of the segment relative to other segments).

 

A large sidescan mosaic composed of over 20 tracks and a number of track parts. Individual segments are clearly visible.

 

 

Adding Tracks and Track Parts to a Sidescan Mosaic

Creating a sidescan mosaic involves choosing regions of sidescan to combine together to produce a finished image.

 

Adding whole tracks to a sidescan mosaic

To add a whole track to a sidescan mosaic, right-click the required track in the Asset Library and select the menu option Add Track to Mosaic (multiple tracks can be added to a mosaic by selecting the required tracks in the asset library track list, before activating the right-click menu). Choose either the New Mosaic option, which creates a new sidescan mosaic, or select an existing sidescan mosaic from the list provided:

 

Adding parts of a track to a sidescan mosaic

It can make the subsequent editing of sidescan mosaics much easier if the useful areas of sidescan data are selected before they are added to the mosaic.

 

To add just a part of track to a sidescan mosaic, open the required track for editing and place the track edit window into sidescan mode. Examine the sidescan swath to locate a region of interest, right-click the swath edge and choose the Add to Mosaic menu option. The selected range will be added to the chosen mosaic:

 

Adding a segment range to a mosaic from the track edit window

 

Note that:

 

Only the sidescan data within the specified start/end times is added to the mosaic.

All other settings, including port and starboard ranges, brightness, contrast etc. will be copied with the segment, but can still be edited within the mosaic. This includes increasing the port or starboard ranges.

Any number of ranges can be added to a mosaic from a track, including ranges that overlap in time.

The entire track can be added if the visible range covers the whole track.

 

Editing Sidescan Mosaics

Editing sidescan mosaics consists of adding sidescan swaths to a Sidescan Mosaic and editing individual swath segments to produce a finished mosaic, which can then be exported as an image file.

 

 

The sidescan mosaic edit window contains a graphical editing area where all of the component segments are displayed.

When a sidescan mosaic is the being edited, the Edit panel contains the editable properties of the sidescan mosaic. These properties are split into two areas:

 

(1) Mosaic Level Properties

 

(2) A list of segments

Every segment within the mosaic is shown as a row in the Segment List. The row contains controls for the swath limit and z-order (see below) as well as a menu for each segment that can be shown by clicking the right mouse button. Selecting a segment in the list will also select that segment in the graphical display (3), where it is shown with a red outline. Selecting a segment in the graphical area (by clicking the segment outline), will also select that segment in the list.

 

(4) Palette selector

The palette for the mosaic can also be selected by using the palette selector in the graphical area.

 

(5) Segment swath control

A swath control is shown at the bottom of the graphical edit window, which controls the currently selected segment.

 

Mosaic Properties

Mosaic properties include the mosaic name, palette and auto-gain options, which apply to all segments within the mosaic.

Other properties include brightness, contrast and minimum-curve and maximum-speed filters, which can be set at the mosaic level, and can also be set for  individual segments.

 

 

(1) Mosaic Name

 

(2) Palette selector

The selected palette always apply to the whole mosaic.

 

(3) Auto-gain

The auto-gain option applies to the whole mosaic.

 

Setting properties in the mosaic versus setting properties for individual segments

Some properties can be set for both the mosaic as a whole, and for individual segments. The way in which these settings are handled when settings in the segment and mosaic differ varies depending on the property:

 

(4) Contrast and Brightness: Mosaic level contrast and brightness values are simply added to any changes made at the segment level. This means that relative brightness and contrast changes are preserved if they have been altered for an individual segment, but the overall brightness and contrast can be varied at the mosaic level.

(5) Minimum-curve and Maximum-speed filters: If values for either of these filters are different between a segment and the mosaic, then the most restrictive of the two values is used. For example, if a segment has a minimum-curve radius of 30m specified against 20m in the mosaic, then the 30m value will be used for the segment. Conversely, if 30m is specified in the segment and 50m specified for the mosaic, then the 50m value will be used. See Sidescan Mosaic Track Review for more details on the minimum-curve and maximum-speed filters.

 

(6) Apply filters button

The Apply Filters button splits segments along boundaries defined by the current filter settings, which include the minimum-curve and maximum speed filters, as well as the currently defined segment start and end postions. This option is not available in the track review screen, where sidescan is limited to a single segment.

 

Splitting a segment cuts that segment into two or more pieces, which can then be edited individually. This is very useful because it allows individual segment properties, such as port and starboard ranges, and z-order (drawing order) to be changed for only part of a sonar swath (see below for more on splitting segments).

 

The following series of images demonstrates how a sonar swath can be split into separate segments using the Apply Filters function:

 

 

An entire track has been added to a new sidescan mosaic, named "apply_filters" for the purposes of this demonstration. The track has a large number of curves, which cause a lot of overlap. Notice that the outline of the segment is drawn around the entire swath - we do not have any control over the drawing order of individual overlaps within the segment.

 

 

Setting a Min Curve radius of 33 feet removes many of the more curved parts of the swath, and leaves a number of isolated straight sections of track. Note that a single outline still surrounds the entire segment, and that we still do not have any control over the individual straight sections of sonar within the segment.

 

 

After clicking the Apply Filters button (circled) the segment is split according to the current filter settings. Note that all filter settings are considered during this operation, including start and end of the swath range. Each straight section of track is now contained within its own segment, and options such as port and starboard ranges and z-order can be configured individually for each segment. Also note that the segment list now contains a large number of segments.

 

Note that:

Apply filter operations are destructive. Areas that were hidden by the current filter settings will be removed by the apply filter operation. Note that this applies only to the segment in the current sidescan mosaic - data from the original track is never lost.

The Apply Filters operation can be undone by right-clicking in the graphical edit area and selecting the option undo apply filters.

 

Editing segments

Individual segments can be edited via the swath control bar and the right-click menu, which can be activated by holding the mouse pointer over a segment edge in the graphical edit area, or a segment row in the segment list, and clicking the right mouse button.

 

The Swath Control Bar

The swath control bar is shown at the bottom of the graphical edit area, and controls the currently selected segment:

 

 

(1) Move forward or back

The selected segment can be moved forwards or backwards by clicking the up or down arrows. Segments that are higher in the draw order are drawn on top of segments that are lower.

 

(2) Swath start and end

The range slider controls the visible start and end times of the current segment - see Sidescan Mosaic Track Review for more details.

 

The Segment Menu

All other segment editing operations are carried out using the segment menu, which is activated using the right mouse button whilst holding the mouse pointer over a segment in either the graphical edit area or the segment list:

 

 

 

(1, 2) Port and starboard range, opacity, brightness, contrast

Port and starboard range, brightness, contrast and opacity work in the same way as they do in the Sidescan Mosaic Track Review, where they are described in detail.

 

(3) Minimum curve and maximum speed filters

The minimum curve and maximum speed filters work for an individual segment in exactly the same way as for an entire mosaic - see above for more details.

 

(4) Move forward or back

Move the segment forwards or backwards in the drawing order.

 

(5) Split segment

The Split Segment function splits a segment along the boundaries defined by the start and end positions in the sonar swath control into two or three resulting segments. The existing range becomes one segment, any segment part prior to the start position of the existing range becomes another, and any part after the end position of the current range becomes another:

 

Single segment with start and end sub-range specified

 

The result of the split segment operation is three segments

 

The split segment operation is very useful when attributes such as draw order or the port and starboard ranges need to be edited for just a portion of a segment.

 

(6) Show or hide segment

If a segment is visible, it can be hidden by selecting the Hide Segment option. Hiding a segment prevents that segment from being drawn during image exports, or when zoomed in sufficiently to see sonar imagery. It does prevent the outline of the segment from being displayed when zoomed out. When the segment menu is opened for a hidden segment, the Hide Segment option is replaced by Show Segment. 

 

(7) Delete segment

Remove the segment from the mosaic. This operation can be undone.

 

(8) Edit track

Open the segment source track for editing.

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