Quick Start Guide to Map Corrections

This guide will show you how to correct navigation in your sidescan data using ground truth points identified on maps, charts or bathymetric surfaces.

Towed sidescan imagery can be difficult to navigate precisely. A good way to reduce the positional uncertainty in the imagery is to introduce a ground truth data set such as a bathymetric surface, air photo, or chart, against which the sidescan navigation can be compared and corrected.

The method relies on identifying targets in the ground truth data that can also be seen in the sidescan imagery. At each target, a map correction adjustment is made to the sidescan navigation so that the sidescan image precisely aligns with the position of the target in the ground truth data.

For more information about the individual commands, please refer to the User Guide found in the Help Menu of SonarWiz.

By following this procedure, you will learn how to:

• Identify targets which will serve as Map Correction points.

• Adjust sidescan navigation using one or more Map Corrections.

Instructions

1. Load a ground truth dataset into SonarWiz as either a Basemap or Grid. Ground truth data should show the accurate positions of targets you can also see in the sidescan imagery. Good ground truth data include high resolution bathymetric data, air or satellite imagery that clearly shows the seafloor, or in some cases historic seafloor imagery.

2. Identify a group of targets in different sidescan track lines that are the same object or geologic feature but because of navigation issues or missing offsets, appear in different locations in the plan view. In this example, the dataset has unknown lever arm offsets, and the boulder shows up in 3 different locations on each track line.

Open

Each of the highlighted images in red are a single boulder displayed in multiple locations.

3. Open the Map Corrections tool in the Maps tab of SonarWiz.

Open

4. Select Add in the Map Corrections window. This creates an empty Map Correction Point.

In the Sidescan Files in Project area:

Toggle OFF all the correct check boxes
Toggle OFF all the show check boxes.

5. Toggle ON a Basemap or Grid file that displays the object you are using as a reference and select Add in the Points section. Your cursor in the Map Display will change to a + icon.

6. Using the + cursor select the location of the object in the map or grid file. See the location and file appear in the Points section.

7. In the Sidescan Files in Project box, Check the Show file box. If the sidescan file overlaps the ground truth point, Click Add in the Points section, and capture the target in the Plan View.

8. Repeat step 7 for each sidescan file in the area of your ground truth target. In the example, there are 3 sidescan files where it appears:

9. Once all of the overlapping sidescan lines have been captured, you can check how effective the correction will be by turning ON the “Correct” check boxes next to each track.

You should see an improvement in the positioning of the adjusted lines.

You can add as many map correction points as you want to the project. When two or more corrections affect the same track, the pings are adjusted using a distance weighted algorithm. For the best results, we recommend applying at least one correction per file in the project and spacing them out across each file if you have viable objects in the data.

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