Danger Bearings

One of the most basic principles of piloting underway is keeping track of the bearing to a prominent landmark or light as a way to set limits on your location. Perhaps the most notorious example of this is the 24 year old AB Lookout telling the 39 year old Third Mate in charge of the Exxon Valdez that the red light on Buoy 6 was on the wrong side of the bow. In fact, she told him twice. The grounding and subsequent tragic oil spill that followed affected the lives of thousands of people for more than a decade. The circumstance of the incident is illustrated below. 

Figure 1. Glacial ice from Columbia Glacier had stretched across the shipping lanes, so the southbound ship was given permission to temporarily leave the lanes to go around it by the green route, which was not uncommon. But the officer in charge failed to turn in time and wandered off course as noted by the red line. The grounding occurred at 0009 AST (Alaska Standard Time). The Lookout reported the light was on the wrong bow, implying the ship was not where it was supposed to be, at 0001 AST. 

Figure 2. Lookout reported the red Buoy 6 light was on the starboard bow at 0001 AST and should have been on the port side of the bow, but this observation was not acted upon.

Normally we would not rely on buoy locations for crucial navigation (because they can move), but this was a very important buoy whose position was carefully monitored, nevertheless, it has since been replaced with a light on a beacon fixed to the reef. The chart above is one for  the time of the accident in 1987 that we found in the NOAA archives. Other details are from the official NTSB report.

Returning back to the task at hand... A single bearing to a target does not tell us exactly where we are, but it does tell us a line we are on that goes through that target, which is a line of position (LOP). If we sail off that line, the bearing to the target changes. We still do not know precisely where we are, but we know from the change in bearing what side of the original line we are on.  This is the principle of the danger bearing.

For example, if a lighthouse bears 090 at the moment, I can draw that line on the chart. Start at the lighthouse and draw a line in the opposite direction, 270. As I proceed away from that line the bearing will get smaller or larger. We may have to look at a chart and compass rose to picture this numerically, but if the bearing gets larger, i.e., 090 goes to 095, goes to 100 etc, then I am sailing north of that line. If on the other hand, the bearings get smaller, i.e., 090 goes to 085, goes to 080 etc, then I am proceeding south of that original line, as shown in Figure 3. 

How fast the bearing changes depends on how far away the target is located. This type of navigation reckoning is generally done with fairly close objects, within a  mile or so, not with distant landmarks on the horizon, whose bearings barely change as we move.  If an object is 1 mile off, you will get a 6º bearing shift for every 0.1 miles off the line you move.

Figure 3. How bearings to a point tell us if we are to one side or the other of a reference bearing.

Now we can see how to apply this concept to a "danger bearing."  Suppose we want to transit the area from A to B in Figure 4, and we know there is a north setting current that might push us onto the underwater rocks. We have other ways to guard against this especially if we have radar, but it always pays to use all methods possible, plus we might not have radar, nor any electronic aids, so these basic methods remain crucial.

Figure 4. A danger bearing set up to guard against getting pushed on to the rocks. We need to stay below the 074 bearing to the light which means we monitor the bearing of the light and  it must stay below 074.  If the bearing slips to say 076 and 077 we have been pushed across the line

Now we can take a short look at how this basic concept can be made more complex than it really is. We look at this same example as presented in Bowditch (American Practical Navigator.)

Figure 5. Danger bearing from Bowditch. NMT stands for "Not More Than." Y is a bearing that is less than 074, and Z is a bearing that is more than 074.  The goal is to state that we must sail in an area where the bearing must be kept less than 074, which is stated in the negative as we want a bearing that is not more than 074.

Headed in other directions the instructions could be different, as shown below.

Figure 6. The Navy way of specifying the limits on a danger bearing in the other direction.  Here everywhere on the safe side of the line, the bearing to the light is greater than 286, so the danger bearing is specified as Not Less Than 286.

Looking back briefly to the Exxon Valdez, the danger bearing concept could have been applied more digitally as shown in Figure 7. 

Figure 7. Applying the danger bearing concept to the Exxon Valdez incident, we see that a guide of NMT 219 on the buoy light could have kept the ship on the safe side of the buoy and off the reef. We keep in mind as well that bearings taken with a ship's gyro repeater can be very accurate.

We have these extended notes on this topic because periodically this concept is a stumbling block in our navigation course. This can come about because of lack of experience with numerical bearings, complicated by the use of a negative phrase, i.e., "not less than" versus "more than."  

Another stumble comes up when someone mistakenly assumes their location or heading affects the choice of the danger bearing. These do not enter into the decision.

To avoid these snags when working an exercise or in actual navigation practice, use these steps:

1) Identify the hazard.

2) Find a prominent landmark or light (target) from which you can draw a line that spans the region where you might be exposed to getting pushed into the hazard—think of it as a boundary line you do not want to cross.

3) Then measure the bearing along that line toward the target. That is your danger bearing.

4) Now we need to figure how to specify the bearing limits to look out for.  Imagine your boat on the safe side of that line at a couple places along your intended passage, such as the first three positions in Figure 8. Then measure the bearing to the target from each of these positions. You will see the bearing getting smaller (or bigger in other cases) as you approach the danger bearing.  If it is getting smaller as you approach the line, the limit is NLT (not less than) the danger bearing. If it is getting bigger as you approach the line from the safe side, them the limit is NMT (not more than) the danger bearing. 

Figure 8. A danger bearing of NLT 220 T, showing a vessel entering the channel getting set across the line and quickly turning back across the line into safe water. Consider the first 3 positions as sample points to check that the bearing is getting smaller so the limit is Not Less Than.  

Another way to punctuate the fact that a danger bearing does not depend on where we are or what route we follow is to think of the role of sector lights set up to warn us of dangers, as shown below. If the light is white, you are in safe water; if the light is red you are in dangerous waters—you have crossed the danger bearing line.

Figure 9. The sector light at Gay Head providing a danger bearing for the area around Nomans Land. This is a echart display (qtVlm) that shows the light coverage and range when the cursor is on the light. In an ENC display white lights are yellow. Here we have a white light indicated by the yellow tint on the chart, along with the red tint for the red sector. Both have nominal ranges of about 20 nmi. The inside yellow ring indicates that there are back up emergency white lights for these but the ranges are not known.

Sailing north at night on the west side of Nomans Land there are no lights to warn of it. In an east flowing current this could be a hazard so a red light sector has been provided to warn us of this.  If that red sector were not available, which (by chance) runs due north at 000, we could use that bearing to Gay Head light as a danger bearing meaning here we want to keep it to the right of 000, which is on the small numbers side or north, and not slide into waters where the bearings are to the left of 360 on the large numbers side.  As in a lot of navigation scenarios, we may have to improvise... in this case we must improvise how to remember the bearings when a danger bearing is right at or near 000.  The simple NLT or NMT does not work well here!

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Added Apr 27, 2022.

In response to an interesting question from Andrew in the comments: the answer is yes, there is still warnings in the Coast Pilot about ice over the full region at times, but almost certainly it must be less, which brings up your second question. How much has Columbia Glacier receded in this period, ie 1987 to 2022. See the new chart below to show that the Glacier has moved back well over 7 nmi.

This picture is made using item (6) from starpath.com/getcharts.