Finding the Difference

Chesapeake Times, Vol 16 | January 2024

A few tools in SonarWiz – both old and new, will provide the user with a simple way for analyzing change differences, both visually and in numerical form.  Whether you want to monitor the difference in a pre and post dredge survey, monitor environment change over a sand bar or calculate the needed material to fill in the void, these functions can help.  The ability to both visualizes and calculate the volume difference is needed.

As an example, let’s take a small survey area.  This contained about 15 lines.  Importing the data and cleaning up the flyers would produce an image like this.  Nothing too exciting, but zoom in enough, we’ll see the actual sounding points for the survey.   On the big picture, the single point really doesn’t do much.

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Next, we’ll create a few grids with the data. Grids of 1 x 1m, with an output of minimum, maximum, average.  As an exercise to show the change difference, I used the minimum and maximum grid values to highlight the change in the grid. Once in a grid, we can now show the data as a sun illuminated image in the main map starts to highlight the bottom topography.

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A new tool in SonarWiz is the ability to draw cross sections across grid data.  This is separate from the cross-section tool we use with raw bathy files.  With the creation of the new cross section tool, we added an enable/disable menu, allowing the user to easily turn off grids from this view, and not the main explorer window.  For the display, there is color by lines and depth, filled histogram view and can sync the cursor in the main map view.

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Visually, we can see the two surfaces have changed, but getting the exact value will be important for projects.  With the two grids, we create a volume grid which will determine the volume change between the two.  In this case, we see around 31k cubic meters difference across the survey area.

Volume Calculation Results:
Cell Dimensions:       1.000 m x 1.000 m
Cell Area:             1.000 m^2
Total Area:            69699.000 m^2
Total Area Decreasing: 69663.000 m^2
Total Area Increasing: 36.000 m^2
Net Volume Change      30734.457  m^3

To note – when comparing data for differences, we’re not going to do a point-to-point comparison, that just doesn’t work. The point data is a discrete point on the seabed and getting a second point exactly in the same spot isn’t practical. The computed grid will encompass a series of points (such as all those in a 1m x1m cell). There is some averaging going on, but taken as a whole, and with a small size cell for comparison, the difference between the two surfaces will be determined.  The tools in SonarWiz – both visually with the cross-section tool and mathematically, with the volume grid difference will allow you to get the job done.