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Author Topic:   S57 Charts: Light Sectors
jimh posted 04-09-2011 10:22 AM ET (US)   Profile for jimh   Send Email to jimh  
This discussion in not particularly electronic in nature, but the discussion deals with navigation charts, particularly electronic navigation charts.

A very interesting feature of modern S57 electronic navigation charts is their ability to display light sectors, that is, the portion of an arc in which a light is visible or obscured. In the process of browsing some S57 charts of the Great Lakes, I happened to notice that many lights were obscured in certain sectors. This data is often not included on paper charts or on electronic versions of paper charts (called BSB or RNC charts).

An example of an aid to navigation with a obscured lighted sector is the GRAYS REEF LIGHT in Northeast Lake Michigan. A sector of the light is obscured so that the light is not seen by vessels approaching in that arc. The arc is chosen so as to avoid leading vessels to approach the light from an area of shoal water and hazards. This makes good sense.

In the case of the GRAYS REEF LIGHT, the light is shown over an arc from 350° clockwise to 260°. Correspondingly, the light is obscured in the arc from 260° to 350°. On the electronic chart this feature is clearly conveyed by a representation of a red circle whose circumference is broken in the region where the light is obscured.

Now to the interesting part, which I had never noticed before. The S57 electronic charts have the ability to show information about objects such as aids to navigation, and the NOAA chart has information about GRAYS REEF LIGHT. The electronic chart description says:

Landmark (tower)
"Grays Reef Light"
Type: light support
Visibility: visually conspicuous
Info: Square

Light, red, flashing, 10s, night light
Range: 15.0Mi, Height: 82ft
Pattern: 01.0+(09.0)
Sector from: 170° True, To: 80° True

Radar Transponder
Wavelength: 0.03-X

Fog Signal (horn)
Signal Sequence: 02.0+(02.0)+02.0+(24.0)
Info: Operates year round

Notice the description of the light sector:

"Sector from 170° True To 80° True"

This did not make any sense to me on first encounter. The bearings given did not correspond to the arcs shown on the chart. I began to wonder if perhaps there was an error in the representation of the light on the chart. The data, after all, is actually in vector form, and its representation depends on the skill of the chart viewing program.

My first step in investigating the apparent discrepancy was to get the NOAA LIGHT LIST and see what it had to say. I had the LIGHT LIST V7 on my laptop in PDF format, so it only took me a few seconds to find the listing, which said:

"Obscured from 080° to 170°."

There is a slight difference in the two descriptions. The electronic chart described where the light was shown, and the LIGHT LIST described where the light was obscured. However, neither matched what I was seeing on the chart. This led me to investigate a bit further to find the definition of "light sector" in the LIGHT LIST. Here is the applicable text:

"Light Sector: The arc over which a light is visible, described
in degrees true, as observed from seaward towards
the light. May be used to define distinctive color difference
of two adjoining sectors, or an obscured sector."

The definition is the crucial element to understanding the description. The visible or obscured region of the light is described in terms of its bearing when observed from seaward! In other words, the bearings are 180-degrees different than as seen from the light. Once I found this, you might say, the light went on (in my head).

If the light can be seen when bearing 080°: from seaward, this means the bearing from the light would be 080 + 180 = 260°. And if visible when bearing 170°: the reciprocal bearing would be 170 +180 = 350°. Now, finally, the description of the light matched the representation of the light on the chart.

ConB posted 04-09-2011 08:10 PM ET (US)     Profile for ConB  Send Email to ConB     
I'm wondering if I forgot that light sector stuff or never knew it.

What does the light list say about White Shoal just north of Grays Reef?

Why do Grand Traverse Light and Mission Point light in Grand Traverse Bay not have sectors? They assume you would already be aground if you saw the light?


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