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Using MATLAB to Understand Currents

Posted by on March 23, 2012

I’m always looking for ways to connect my Engineering background with my many outdoor interests. This may be the geekiest thing that I’ve attempted yet. As a graduate student, I have spent countless hours siting at my computer struggling to accurately model the interaction of photons with tissue using MATLAB. This amazing software package is a super calculator capable of doing everything from making difficult calculations to plotting data to probably figuring out the meaning of life if you let it run long enough. One day I was looking at the nautical chart of Plymouth MA that I have hanging on my office wall. The chart is covered in soundings which mark the depth of the bay and I wondered to myself about how cool it would look in 3D! There was only one way to know…by making it happen in MATLAB.

I started with the nautical chart of Plymouth Bay on the left. The soundings mark the depth of the bay at low tide in feet. I divided the chart into 484 blocks; 22 columns by 22 rows and marked the depth of the closest sounding at the corner of each block. This produced a three dimensional dataset that I entered into MATLAB to produce the false color image on the right. In the image, red and yellow represent land above sea-level , green represents mud flats, light blue represents shallow water and dark blue represents deeper water.

Comparison of Nautical Chart with MATLAB False Color Image of Plymouth Bay

The false color image really shows us a lot about what is going on at the bottom of Plymouth Bay. You can easily see land features like Plymouth Long Beach and Saquish.  Also, the main channel that runs along Saquish to Bug Light is clearly visible in dark blue. Unfortunately, the grid size that I used (sampling rate) was too coarse to resolve narrower channels such as the Plymouth Channel which is clearly missing from the false color image.

As cool as the false color image is I was super psyched about the 3D image shown below. This image really provides a sense of the topology of the ocean floor within Plymouth Bay and can help to serve as a teaching tool for the discussion of tidal currents.

3D Depth Profile of Plymouth Bay

Tidal currents typically exist in and around channels (as illustrated by the dark blue troughs in the 3D image). As the water flows out of the shallow bay it’s funneled into these deeper channels thus concentrating the current in these areas. One can think of these deep channels as underwater rivers that carry the tide in and out. Paddling in these areas can be both a savior and a burden depending on the motion of the current relative to your intended course. So pay attention to the soundings on those trusty nautical charts!

Happy Paddling!

-Kayak Dave


One Response to Using MATLAB to Understand Currents

  1. leandro

    Would you mind giving the code?

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