Line ’Em Up: Navigating  With Natural Ranges - Soundings Online

Line ’Em Up: Navigating  With Natural Ranges

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Natural ranges can be used to monitor a track line or to indicate when it is safe to make a course change. By monitoring the first range (red), a vessel can stay in the deeper water west of the Bar Islands and avoid the ledges to port. The second range (black) indicates when it is safe to turn west and avoid those same ledges. The third range (blue) indicates the point at which a vessel can turn north and, by keeping Sheep Island to starboard, pass clear of the ledges south of the island.

Natural ranges can be used to monitor a track line or to indicate when it is safe to make a course change. By monitoring the first range (red), a vessel can stay in the deeper water west of the Bar Islands and avoid the ledges to port. The second range (black) indicates when it is safe to turn west and avoid those same ledges. The third range (blue) indicates the point at which a vessel can turn north and, by keeping Sheep Island to starboard, pass clear of the ledges south of the island.

Among the tricks and techniques in the navigation toolbox, few are as foolproof as the natural range. Also known as a transit, a natural range exists when two charted objects — one closer and one farther away — visually align and appear to meet, forming a line of position, or LOP.

Captain Daniel S Parrott photo

Capt. Daniel S. Parrott

Whereas a bearing requires a compass, and raises questions of accuracy and skill, a natural range is indisputable. When the two charted objects visually form a line, you are on that line and nowhere else — end of story. Natural ranges are so reliable that, in an effort to imitate nature, ports around the world for centuries have erected lighted and unlighted ranges to guide vessels. These consist of two navigation aids, one closer and one farther away, the farther one being higher than the nearer one. When the two structures or lights line up, you’re in the channel.

Natural ranges abound. When the shoreline edges of two islands align, it’s a natural range. It’s much the same when an isolated rock lines up with a ledge or a point of land. Natural ranges might include unnatural objects, such as the end of a jetty lining up with a water tank, church spire, lighthouse, radio tower or any combination of distinct, charted features. So long as the features can be identified on the chart and in the real world, they are useful allies in confirming your whereabouts.

Many boaters use ranges that do not appear on charts, such as a flagpole that lines up with a particular tree or a chimney. This tactic falls into the category of local knowledge.

Having a reliable, hands-free LOP at a glance is handy, but my favorite way to use a natural range is for setting a waypoint. A waypoint can be situated in any number of places in a given vicinity. However, if a natural range is available, I prefer to locate my waypoint on it so that I can verify my approach by eye. This is not to be confused with eyeball navigation; natural ranges are a systematic means for cross-referencing information from other sources, such as a chart plotter.

I should note that an LOP formed by a range is not a fix. Without more information, you cannot say exactly where you are. But you will have an excellent idea of where you are not, and sometimes that is enough.

When a natural range is dead ahead, like artificial ranges erected for port entrances, the relationship between the two features can be used to monitor whether you are being set to the left or right of track. For instance, if two features that form a natural range begin to overlap or pull apart, it indicates that you are sliding off course and must correct for it.

A natural range can be used to form a line of position, but when two are visible at the same time, they can form a fix.

A natural range can be used to form a line of position, but when two are visible at the same time, they can form a fix.

Like any navigational tool, natural ranges have limitations. In darkness or fog, visual references may not be available, so don’t build your world around them if those conditions might prevail. However, even in darkness or fog some natural ranges may show up on radar.

As I mentioned, the edge of a shoreline can form one side of a natural range. But depending on the tidal range and the slope of the shore, the visual shoreline may differ significantly from the charted shoreline as the tide comes and goes. If there are navigational constraints, this difference could matter. Also, ranges lose accuracy over great distances. A distant island that lines up with an even more distant headland miles away will not be as definitive as two objects in close proximity to each other and to you.

Some coasts don’t lend themselves to natural ranges. A long, bald stretch of uninterrupted coastline may not offer much in the way of natural ranges. The opposite can occur where there are so many features that there is a risk of misidentifying an islet, ledge or rock when trying to establish a natural range.

I have, on occasion, admitted buoys into my inventory of available ranges, but in a noncommittal way. Buoys can be off-station, and the state of the tide can affect their positions. Also, to the extent that all charts can contain inaccuracies, the accuracy of a natural range can be affected if the positions of charted features are questionable.

Despite a handful of limitations, natural ranges provide high-quality information, as well as a satisfying way to optimize a chart. Don’t use natural ranges in place of other navigational methods but, rather, as a tool that augments and verifies other information. Along with being inherently reliable, natural ranges can be foreseen. They lend themselves to the notion that navigation begins at the dock.

Next time you plan a route using a chart, look for natural ranges, and try to picture opportunities to use them, not only in the moment but also to enhance your situational awareness of what is coming.

This article originally appeared in the January 2017 issue.