Tips for Building Sidescan Mosaics

Tips on building sidescan mosaics

Start with good data

This point cannot be over emphasised; good looking sidescan mosaics must start with good sonar data:

·Keep tracks straight when running over areas of interest.

·Create parallel runs with overlapping effective sidescan coverage. Monitor the sidescan return on your unit to determine the effective range for usable sidescan data. For example, if you are reading good data out to 30m (100ft), then make your runs 25m apart so that you have a little overlap to play with when arranging your mosaic.

·Keep speed low, but not so low that the boat starts to wander. Somewhere between 3-5 knots is probably about right, but take guidance from the quality of sidescan that you are viewing on your unit as you record the track. Recording at too low a speed can affect the straightness of the track and, subsequently, the calculated heading. Too high a speed introduces noise into the sidescan recording.

·Make turns only when you are well past the area of interest.

·Consider recording a new track for each straight run over areas of interest. This can improve the performance of the auto-gain feature.

·Record your data in good weather conditions.

·Make sure to use the correct transducer offset when importing sidescan data into ReefMaster.

·If you are targeting a specific object or structure, pass to the side of the target, not directly overhead. Because of the nature of the slant-range correction required to

Review tracks and add useful segments of track individually

Each segment can be edited on its own within a sidescan mosaic, and editing segments to adjust overlaps, drawing order and relative brightness is the main part of creating a good looking sidescan mosaic.

It can be easier to select required portions of track during the sidescan review phase, and to add these bit-by-bit to the sidescan mosaic.

For example, this section of track has reasonable quality data, but overlaps itself in a few places:

Raw track data with lots of overlap

Instead of adding the entire track and cutting it down in the mosaic view, select ranges of track that do not self-intersect and add each of these to the mosaic in turn:

Using the sidescan swath control to select ranges to be added to a mosaic

A number of segments in the mosaic - each of this can be edited individually, if required

Once the segments are in the mosaic, they can be edited; overlaps can be reduced using the port/starboard ranges, and better data can be brought forward so that it is drawn on top of lower quality returns:

The sidescan mosaic after a little editing

Split the ReefMaster window layout to show the target mosaic alongside the track you are editing and use the Mouse Tracer tool to cross-reference sidescan locations between the mosaic and track windows

ReefMaster has a modern user-interface which lets you arrange edit windows however you like within the application. When building a mosaic from individual tracks, it can be very helpful to arrange the window layout so that the target mosaic and track windows are shown alongside each other. If you have a dual monitor display you can go one better and detach one of the edit windows from the main ReefMaster application and locate it on your second monitor:

The Mouse Tracer reflects the current mouse pointer position into all open windows, as a small grey circle (highlighted above, right-side panel). Use the mouse tracer to help locate the relative position of sonar data between the track and mosaic screens.

Use the zoomed-out segment view to adjust overlaps quickly

The quality of the sonar return typically deteriorates at the edges of the swath. When we have a set of overlapping track segments, one of the most common operations is to adjust the port and starboard ranges of each segment to minimize the overlap, and retain as much as possible of the closer sonar returns.

This operation can be carried out very quickly when viewing the mosaic at lower zoom level, that shows the overlap between segments very clearly:

10 parallel tracks, showing significant areas of overlap

overlap reduced by adjusting port and starboard ranges using the right-button menu

Adjusting overlaps in this way is a quick way to start improving a mosaic

Split segments as required to limit edits to certain ranges

When configuring segments, the same properties are not always appropriate for the full extent of the swath. For example, when configuring overlaps it might be desirable to limit the port or starboard range for the overlapping part of the segment whilst keeping the full range where there is no alternative data. This can be done very easily by splitting the segment, and applying different port/starboard ranges to the resulting segments:

Segment on the left has been split, and the starboard range reduced where it overlaps another segment

Splitting segments is also very useful when you want to bring forward (or push back) areas of sidescan in the draw order.