SEAMANSHIP
Understanding Bathymetry
BATYMETRIC NAVIGATION: LINE OF SOUNDING METHOD
Even vessels with the most austere complement of navigation electronics will usually have a depth sounder. Rarely, however, do we recognize the full utility of this unassuming tool, especially for navigation in the absence of GPS.
I first learned the art of navigation aboard U.S. Navy submarines, years before GPS was available to military or civilian mariners (no, really, it wasn’t that long ago). The technique we’ll be working with here was part of our routine navigation, just one of the ways in which submariners use depth sounders. It’s an awfully handy tool and works equally well for small cruising powerboats or sailboats. This technique demands a committed navigator, at least for as long as it takes to derive a fix. It is simply not possible to maintain a proper helm and lookout while navigating with a depth sounder. Later in this series we will explore satellite-free navigation techniques that can be used safely while single-handing a small vessel, but for now, I am assuming that navigation is the primary job of one of your crew.
Most commercially available depth sounders provide a speed through the water reference that is independent of GPS speed over ground. If your depth sounder does not provide this data, you must still know your speed through the water by some means (various methods for obtaining this will be the subject of a future article).
In addition to a depth sounder and standard plotting tools, such as a paper chart with printed depth contours, you will need something akin to tracing paper. For this exercise I used a piece of white tissue wrapping paper, just to demonstrate that really any semi-transparent paper will suffice.
Bathymetry, as per Merriam-Webster, is defined as, “The measurement of water depth at various places in a body of water; also: the information derived from such measurements.”
For simplicity, throughout the exercise I am assuming that the depth sounder transducer is at the sounding datum of the chart. In practice, you will need to correct for both the tidal height and your transducer depth.
Example: My vessel is on course 120° True at a speed through the water of 10 knots. I am using a dead reckoning (or DR) interval of six minutes, simply because this is one tenth of an hour and makes the math in our example easier ( Figure 1, page 36). Because my speed is 10 knots, each six minute interval happens to be one nautical mile. Once I begin my sounding run, I must maintain my course and speed for the duration.
Note that the sounding contours we are about to pass over are plentiful and varied. This is important. A very flat bottom, or a very uniform slope with parallel contour lines, will not yield a useful fix.
I cut out a piece of tracing paper that is large enough to cover the area of soundings over which I’ll be passing. I then redraw my dead reckoning track on the paper, down the middle (heading is not relevant for this part). If I lay my tracing paper DR over my chart DR, they should match exactly ( figures 2,3, 4, 5).
Now I begin my run of soundings. I happen to begin at time 06:00:00. The charted sounding is about 25 fathoms, and my depth sounder reads 22 fathoms, but we cannot yet tell much from this.
I begin recording the times that I actually cross each depth contour. Based on my speed and the scale of the chart, I choose
to round to the nearest half-minute, but this is simply a matter of preference.
At time 06:02:30 I cross my first depth contour ( figure 7), 30 fathoms, getting deeper. An elapsed time of 150 seconds at 10 knots is 0.42 nautical miles, so I draw a 30 fathom tic mark .42 miles from my 06:00:00 DR on my tracing paper ( figure 8, page 39).
I cross the 40 fathom curve two minutes later at 06:04:30, .75 miles from my 06:00:00 DR. Note that I calculate all of these back to the start of my run, rather than between individual sounding lines. This prevents rounding errors and other systemic errors from compounding. For this sort of intricate dead reckoning, I always prefer a nautical slide rule for my time/speed/distance calculations ( figure 6, page 39), but any such method is fine. Based on our DR, we expect to see our soundings dip down to about 49 fathoms and then come shallow again, crossing back to the 40-fathom curve and barely crossing up and over the 30-fathom curve as well. However, instead we see the depths continue deeper, crossing the 50-fathom curve and getting deeper still to about 55
fathoms, and then sloping back up across the 50-fathom curve again, and then 40 fathoms, and again at 30 fathoms by time 06:18:00.
When I again lay my tracing paper over my original DR, I see that my sounding tic marks do not match my DR at all. I slide my tracing paper until I find a fit, which is nearly perfect. This is the track I was actually on. Note that my original 06:00:00 DR was also in error: It, too, was just derived from dead reckoning, and was an arbitrary starting point for my soundings run.
Using my divider point, I poke a hole through the tracing paper where my line of soundings ends—in this case at the 30-fathom curve at time 06:19:00—3.2 nautical miles from my starting point, onto the chart beneath it ( figure 13). This is my 06:19:00 bathymetric fix. I plot the fix on the chart, label the time of the fix, and update my dead reckoning to it. Note that the symbol used for the bathymetric fix is a triangle ( figure 14), the same as a radar fix and for the same reason: While it is utilizing electronics to derive the fix, it is using real-world information as opposed to a mathematical abstraction that is derived by GPS.
That’s all there is to it. In Part 2 (which will be featured in PassageMaker’s Channels e-newsletter) we will demonstrate a method using this same data to derive a bathymetric fix in a completely different way. Good watch!