Making landfall after an offshore voyage continues to be a notable event, even with today’s excellent electronic aids to navigation. The horizon may have been empty for days when a navigator starts to look for land to break the clean line between sea and sky. There is the search, the wait, the inevitable lift from first seeing land. Long showers, fresh food, clean clothes, and new scenes await. More important, navigation that is “good enough” offshore must improve quickly when entering pilot waters.
In these days of accurate and immediate electronic position-fixing with GPS and loran, making landfall may seem anticlimactic. In clear weather, when returning to a waypoint measured earlier with the same system, it is. Watching the navigation receiver display the bearing to an entrance buoy’s waypoint, and seeing the distance wind down, gives a degree of precision that is quite welcome.
The buoy should be visible at around two or three miles in daylight and perhaps twice as far at night. If it doesn’t pop into view nearly on schedule, something is wrong. The receiver may be off line or blocked by interference, or the destination waypoint may be entered incorrectly. Sometimes the navigation data is correct, but land in the background or a cluster of boats makes it difficult to pick out the buoy.
In clear weather a navigator changes easily from electronic to visual methods. He needs to spot the entrance, find landmarks, and follow the turns in the channel by finding their aids to navigation well in advance. He uses the buoys to tell where the ship or boat is going (as opposed to heading), to start turns, and to estimate the current.
Add a bit of rain or fog, and it becomes most important to spot that first buoy or to plot a good radar fix. It’s rather easy to do while a navigation receiver shows accurate and up-to-date positions, but it remains essential to change to low-visibility piloting methods. Navigators seldom steer up a channel with GPS or loran; they watch buoys, follow known courses at a specific speed, calculate when to turn, check the depthfinder, and use the radar intensively. This is profoundly different from offshore navigation. When going to an unfamiliar port, electronic systems require special caution. A GPS receiver will give positions within 100 meters 95% of the time, and within 300 meters 99.99% of the time, with respect to the WGS-84 coordinate system. This sounds great, and it is. But there are additional errors due to chartsfrom 50 to 100 yards on many of them. Resetting the GPS receiver to the local chart datum is essential, but significant errors may remain. Remote islands, surveyed years ago with astronomical methods, may have latitude and longitude shown on their charts that differ from WGS-84 by several miles. Now that’s a serious error, one that can easily lead to a grounding. There is a growing group of people who have found out the hard way that modern electronics and electronic charts can lead a navigator astray. Navy ships, passenger ships, yachtsall have followed the siren song of electronic navigation to unplanned stops on charted shoals, shoals that have been there a very long time. Verify electronic positionsIt is essential to compare GPS positions with the local chart positions before relying on GPS or DGPS in inshore waters. When approaching land, it is necessary to make landfall, plot a visual or radar fix, and then compare that with the navigation receiver position (figure 1). The traditional methods of approaching landfall cautiously are just as necessary today, when going to a place that doesn’t have verified positions taken by a GPS or loran receiver.
Unfortunately, people tend to embrace new and jazzy technology long before they understand its limitations. What if the latest electronic receiver dies in mid-voyage? Or maybe it has trouble receiving signals in the few hours before reaching land. It may have encountered local interference. Near the turn of the century, sunspot activity will be severe, and we can expect periods of interference to GPS. It’s bad to rely too heavily on a system that may not be available when approaching a landfall.
When approaching land, it is important to determine the distance at which land or a lighthouse should come into view. This depends on the height of the land or the light, the boat’s “height of eye” above the sea, the clarity of the atmosphere, and the intensity of the light.
First, find the maximum distance dependent on the heights. This is called the geographic range and is found by adding the distance from the light to the horizon, and the distance from eye level to the horizon. The distance from the boat to the light must be equal to or less than the sum of two distances for the light to be above the horizon.
To find the distance (in nautical miles) from the boat’s height of eye to the horizon, use the following formula from Bowditch (1995):
Distance to horizon = 1.17 times the H (in feet)
Thus, if eye level is seven feet above the water, the horizon is 3.1 miles distant. Earlier versions of Bowditch used slightly different constants for estimates of atmospheric refraction, giving distances that differ by a tenth of a mile or so. Table 12 in Bowditch also gives the distance to the horizon.
NOTE: Many charts give heights in meters. To find the distance to the horizon in nautical miles, use the following formula:
Distance to Horizon = 2.12 times the H (in meters)
Suppose a boat with eye level seven feet above the waterline is approaching a lighthouse that has its optic 200 feet above the sea. The horizon is 1.17 x 200 or 16.5 miles from the light. Now we have to calculate the effects of our own height of eye of seven feet: 1.17 x 7 = 3.1 miles (figure 2).
Thus, the light should be visible at a maximum geographic range of 3.1 + 16.5 or 19.7 nautical milesif it is sufficiently bright and if the air is clear. This distance isn’t exact, due to variations in atmospheric refraction and tidal height, but it is a useful guideline. Table 13 in Bowditch has entries for both height of eye and the light’s height, to give geographic range without calculation.
When making landfall in daytime and approaching a hill 200 feet high, it won’t be visible at 19.7 miles, as a light would be. The land must extend well above the horizon in order to be visible. To show a visible target at 20 miles, at least 1/4 of the hill should be above the horizon. Calculate for 150 feet rather than 200 to get a realistic estimate of its maximum geographic visibility (figure 3).
Geographic visibility is only half the story. The light must be intense enough to be seen. Large lighthouses usually show lights intense enough to be seen beyond their geographic range. Smaller lights, those that can be seen at up to 10 miles or so, usually are high enough that they could be seen farther if their lights were more intense. A red light on a buoy, 12 feet above the water, could be seen about 12 miles from a 50-foot height of eye, but it isn’t bright enough. Usually a red or green light on an offshore buoy can be seen about three to five miles.
The clarity of the airdescribed by the term visibilityis equally important. If the daytime visibility is 10 miles, expect to see the hill at about 10 miles, even though it is high enough to be seen at a greater distance.
The Coast Guard calculates the distance of reliable visibility for each light when the visibility is 10 miles. This is called nominal range, and is shown in the Light List. First find the geographic range, as we have described above. Then compare this with the nominal range from the Light List. The light should be visible at the shorter of the two ranges, in 10-mile visibility.
What if the visibility is greater or less than 10 miles? A light visible 15 miles in 10-mile visibility will be seen at a lesser distance if the visibility is five miles, more if the visibility is 15 miles. The Light List and Bowditch contain graphs to predict the distance at which a light is visible under various conditions of visibility. The graph shows nominal range on the left; find the nominal range and move horizontally to the appropriate curve of visibility. Then move vertically and read the luminous range at the top. The luminous range is the distance at which the light is clearly visible in prevailing visibility.
This may all sound confusing, but remember that the visibility refers to the distance at which naturally lighted objects can be seen in daytime. A powerful light can be seen farther at night than a painted surface can be seen in daylight.
A city can be seen from well over the horizon at night, particularly if there is an overcast. The lights of the city shine on the clouds or mist, creating a loom of light that is visible for 20 or 30 miles. The loom of city lights often is visible well beyond the horizon. A single powerful light on a lighthouse also may have a loom visible a few miles beyond its geographic range. The loom of a light is a bit different from the loom of the many lights from a city in that it is concentrated just at the horizon rather than higher up on low clouds.
Having found the distance at which the land or a light should be visible, a navigator usually draws an arc on the chart at that distance from the landfall. Then he can determine the ETA to the arc of visibility. It is important to recognize the error in the DR when predicting the time to sight land. Suppose the boat has traveled 30 miles since the last fix, which had an accuracy of plus or minus 2 miles. DR growth is often about 15% of the distance, or 4.5 miles. It can be less than 15%, but it’s a good idea to avoid assuming the best performance. When combining the estimated error of the fix and the DR, square each error, add the two squared numbers, and then find the square root. The estimated error of the new DR is thus:
(2 x 2 + 4.5 x 4.5), or 4.9 miles. Round it to 5.0.
To find the time for earliest sighting, take the distance between the last fix and the sighting circle, subtract five miles, and calculate the time to run that distance. Add five miles to the distance and calculate the latest time at which the light should be in sight (figure 4).
Check the depth of water near the arc marking the radius of visibility. The depth is a valuable check on the DR in areas where the depth decreases regularly closer to shore. In areas where the land rises quickly from a relatively uniform bottom, or where the depth varies irregularly, this technique isn’t as useful. But when approaching a coast with gradually changing depth, depth readings are valuable.
If land or a light isn’t in sight and the soundings indicate that the boat is close enough, something is wrong. Maybe there is a band of fog near the coast, or maybe the boat is off to the side of the planned track. It’s critical to solve the problem or at least avoid getting in too close. Sometimes, on the other hand, growth on the transducer keeps the depthfinder from getting echoes from the bottom as deep as it does normally. But depthfinders work best in shallow water, and help to avoid groundings when the boat’s position is uncertain.
Identify aids to navigation
Having made landfall, it is most important to identify the aid to navigation or the land. Often a light has a distinctive number of flashes to distinguish it from other lights along the same stretch of coast. If it has a single flash, use a stopwatch to time it. The next light down the coast may also have a single flash, but will have a significantly different interval between flashes. In daytime, the land may have a recognizable pattern of hills or of manmade structures that are visible from a few miles offshore.
If the first aid to navigation in sight is an offshore buoy, positive identification is most important. There are a number of buoys well offshore, and it is quite possible to mistake one for another when making landfall. The cruise ship Royal Majesty was on a routine trip from Bermuda to Boston guided by GPS coupled to an electronic chart in June of 1995. The chief mate sighted the first buoy of the Boston traffic lane “BA” right on schedule, in daylight. Unfortunately, the GPS wasn’t providing correct information to the electronic chart, and the buoy he saw wasn’t, “BA,” The ship was actually passing the Asia Rip buoy “AR” well to the west. It was too far away to see the lettering, but the buoy came in sight at about the right time.
That’s not enough for a positive identification. The ship’s officers didn’t bother to check the depthfinder or the loran, nor did they notice that they didn’t sight the “BB” buoy, nor were they concerned when they saw Sankaty Head Light. A navigator always finds the correct position, eventually; it’s much less trouble if he does it before the ship comes to a shuddering halt.
Landfall and positive identification of aids to navigation, and correlating fixes from electronic navigation position systems with visual or radar information, remain critical to safe voyaging. Assumptions based on one or a few sources of information sometimes turn out to be correct. On the other hand, they may notand the result can be dramatic. A good navigator has a healthy suspicion of unverified data, and he always takes steps to make certain of the position before heading near shallow waters.