Wreck Symbols on Electronic Navigational Charts (ENC)

The International Hydrographic Organization (IHO) describes light symbols as the most complex electronic navigational chart (ENC) symbols in their own published standard for the symbols called IHO Pub S-52, Annex A, Presentation Library. Anyone can download Pub S-52, but the Presentation Library costs 500 euros! Draft copies found online have many errors, and can lead to hours of wasted time with no productive results.

But the IHO does not give themselves all the credit they deserve regarding complex symbols. Let's take a look at the rules for wreck symbols on ENC, for example.

There are six wreck symbols presented below with the official IHO Symbol Explanations, followed by our notes on the required attributes, which are explained in more detail later in the post. Five of the six are essentially the same wreck symbols used on paper charts, but the complexity comes into play because now we know the rules that determine which symbol is used for which category of wreck, and this new specificity is both a virtue and a challenge to those who must display the proper symbols or write books on their meanings. Plus we have the all new concept of isolated danger symbol unique to ENC.

This type of symbol inquiry is good practice working with ENC objects and attributes, which will become more important to mariners as we learn to live without traditional paper charts, relying on the ENC as the only official nautical charts.

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Symbol Name: SY(WRECKS01) 

IHO Symbol Explanation: wreck showing any portion of hull or superstructure at level of chart datum. 

Attributes: VALSOU not given;  CATWRK = 4 or 5  or WATLEV = 1, 2, 4, or 5. This symbol means there is no sounding given for the wreck and some part of it is showing at all stages of the tide.

The IHO reference to "chart datum" means "sounding datum," which is always zero tide height on all ENC from any nation. 

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Symbol Name: SY(WRECKS04)

IHO Symbol Explanation: non-dangerous wreck, depth unknown.

Attributes: VALSOU not given; CATWRK = 1;  and WATLEV = 3. In other words, no sounding given, it is charted as not dangerous, and it is always underwater.

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Symbol Name: SY(WRECKS05)

IHO Symbol Explanation: dangerous wreck, depth unknown.

Attributes: VALSOU not given; CATWRK = 2;  and WATLEV = 3. In other words, no sounding given, charted as dangerous, and always underwater. 

Some symbol reference books imply that "dangerous" or "non-dangerous" is determined by the location of the wreck relative to the safety contour, but that is not the case. Dangerous or non-dangerous is coded into the ENC by the Hydrographic Office that made the chart, using rules they set. As noted below, NOAA charts all wrecks known to be shallower than 20.1 m as dangerous, keeping in mind that these are wrecks whose exact soundings are not known.

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Wrecks can also be plotted as a generic hazard (meaning rock, wreck, or obstruction) with one of these symbols when the value of sounding (VALSOU) of the wreck is known.

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Symbol Name: SY(DANGER01)

IHO Symbol Explanation: underwater hazard with a defined depth.

Attributes: VALSOU less than or equal to the mariner's choice of Safety Depth. The known  sounding is then printed in the center of the symbol. Black if less that the safety depth; gray if deeper.

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Symbol Name: SY(DANGER02)

IHO Symbol Explanation: "underwater hazard with depth greater than 20 metres." [SIC]

Attributes:  VALSOU greater than the mariner's choice of Safety Depth. 

The official IHO Symbol Explanation given above, taken from the latest edition Presentation Library, is not correct. There is a detailed Conditional Symbology Procedure (CSP) explaining when to use this symbol, and it is based on the Safety Depth, not on a fixed 20 meters depth. Both the US and the UK Chart No. 1 booklets include the incorrect reference to 20 meters. Consequently, some navigation apps (ECS) also do not make this depth distinction correctly, so the symbols in those apps do not change from blue to clear at the correct sounding. It is not a major effect navigationally, but reflects the complexity of the symbol.

The known  sounding is then printed in the center of the symbol. Black if less that the safety depth; gray if deeper.

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Wrecks can also be plotted as an isolated danger, depending on its location relative to the navigator's choice of requested safety contour.

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Symbol Name: SY(ISODGR01)

IHO Symbol Explanation: isolated danger of depth less than the safety contour.

Attributes:  This is a complex, but valuable symbol unique to ENC. It automatically replaces hazard symbols depending on the depth and location of the hazard. It warns us of hazards (based on our own definition of safe depth) that are located in deeper water where we would not expect them. 

Specifically, if a wreck is outside of the displayed safety contour and it has a sounding less than the requested safety contour—or its sounding is not given—then the wreck symbol is replaced with the isolated danger symbol—depending on several other properties of the wreck.  That procedure applies to all hazards (rocks, wrecks, and obstructions). 

Most ENC users are familiar with that role of the isolated danger symbol, but not so many realize that the reference sounding is the requested safety contour, not the displayed safety contour, and this is not at all clear in the IHO Symbol Explanation.

We have in practice two safety contours. We have the one we requested, say 8 m, and we have the one displayed on the screen, which might be 10 m, because only contours native to the ENC can be assigned as the displayed safety contour. This special contour is then made bold and it separates two prominent water colors, and also triggers various alarms when crossed. If our requested contour is not in the ENC, the next deepest contour is selected for display.

For example, we request a safety contour of 8 m, but there is none in the ENC, so the active safety contour displayed is at 10 m.  On the deep side of the 10 m safety contour there is a wreck with a sounding of 7 m. This is shallower than our requested 8 m and outside the displayed safety contour at 10 m, so this one will be replaced by an isolated danger symbol.

If we then change our requested safety contour to 6 m, the displayed safety contour will stay at 10m, but now our wreck is deeper than our requested safety contour, so it will not be replaced with an isolated danger symbol.

I might stress that this symbol depends on a value of the safety contour; whereas the distinction between DANGER01 and DANGER02 above (blue or clear inside a dotted oval) depends on the value of the safety depth. Some nav apps (ECS) do not follow the IHO and IMO guidelines of having a user selected safety depth in addition to the safety contour, so they then use the same value for both symbols.

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Those are all of the possible symbols for a wreck. Any wreck on the chart will be one of those symbols. The tricky part is how does a specific nav app (electronic charting system, ECS), decide which symbol to show? This is not such an easy question. The rules (outlined briefly above) are spelled out in the S-52 Presentation Library, which in turn depend on the specific attributes of the object WRECK. These attributes are encoded into the ENC using rules from another IHO standard called S-57.

The attributes of the object WRECK that determine how it should be plotted are:

WATLEV, water level effect 

VALSOU, value of sounding  

CATWRK, category of wreck

EXPSOU, exposition of sounding

Every WRECK must have a WATLEV, plus it must have either a VALSOU or a CATWRK. You can review these attributes at caris.com/s-57.

WATLEV describes the visibility of the wreck as the tide changes.   The options are:

ID    Meaning

1 partly submerged at high water  

2 always dry

3 always under water/submerged

4 covers and uncovers

5 awash

6 subject to inundation or flooding   

7 floating

A wreck with WATLEV = 3, always submerged, with no sounding given, will have one of the traditional wreck symbols we are used to from  traditional paper charts, WRECKS04 or WRECKS05.

VALSOU is a single number, the depth of the water over the wreck when the tide is 0.  This can be a positive number, such as 3.5 m, meaning when the tide is 0, the top of the wreck is 3.5 m below the surface, or it could be -3.5 m, meaning when the tide is 0, the top of the wreck is 3.5 m above the water. Negative soundings are drying heights. Depending on the range of the tide and the location of the object, it could be underwater at all tide levels, or it could cover and uncover with the tide, or it could be always visible to some extent regardless of tide height. A drying height sounding is shown underlined on the screen. We see wrecks that cover and uncover with known drying heights along or in the foreshore.

A known VALSOU means the wreck will be shown as one of the the three danger symbols shown above, and not the type of wreck symbol we were accustomed to on traditional paper charts.

The VALSOU relative to the mariner's choice of Safety Depth determines the symbol DANGER01 vs DANGER02, regardless of other attributes.

CATWRK can have a direct influence on the symbol used. The options are:

 ID Meaning

1 non-dangerous wreck

2 dangerous wreck

3 distributed remains of wreck

4 wreck showing mast/masts

5 wreck showing any portion of hull or superstructure

Each nation making ENC have to establish how they are going to define a wreck as dangerous or not.  It is not spelled out in the IHO S-57.  NOAA's own Chart Manual, Vol 3, Section 6.3.2 on ENC production states that all NOAA ENC will encode any wreck as dangerous if it is known to be shallower than 20.1 m. They do not need to know its actual sounding, only this limit.

EXPSOU has a more subtle effect on the symbol. The options are:

ID Meaning

1 within the range of depth of the surrounding depth area

2 shoaler than the range of depth of the surrounding depth area

3 deeper than the range of depth of the surrounding depth area

This attribute only affects whether or not a wreck symbol (or any hazard) can show up as an isolated danger symbol. If the exact sounding of a wreck is not known, but it is known that the depth of the wreck is deeper than the shallowest contour of the depth area it is in (ie EXPSOU = 1) then this wreck will not show as an isolated danger symbol. The goal is to avoid the unnecessary display of isolated danger symbols.

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This last detail (some hazards not showing up as danger symbols when similar ones do) is not crucial in practical use of ENC because the first thing we learn is we must cursor pick any object that might be crucial and the pick report will tell us all about the object and its attributes. Furthermore, isolated danger symbols are an all new concept in ENC that we are not familiar with on paper charts, so it would be rare to even know something is unique about any specific example.

Also we note the commonality of all hazard symbols on ENC. For most encounters it does not matter at all if we are avoiding a rock, wreck, or obstruction, and indeed more often than not they have the same symbols.

Here is a graphic summary of the wreck symbols

1.   Top of the wreck is 3 m above the water when the tide is zero. The underline means drying height.

2.   Sounding not known, but some part of the wreck must show when the tide is zero.

3.   Same as 2, but can be in deeper water if tall enough to show when tide is zero, i.e., if it is 5 m tall it could be in a sounding of 4 m.

4.   Sounding to the top of the wreck is 5 m, which is less than the safety depth so the sounding is printed black. 

5.   Encoded in the ENC as dangerous wreck, with no sounding given.

6.   Same as 5.

7.   Wreck located on the deep side of the displayed safety contour with a sounding less than the requested safety contour, which is usually same as safety depth, or the sounding is not given.

8.   Sounding to the top of the wreck is 15 m, which is more than the safety depth so the sounding is printed in gray. Black vs gray on the sounding color is a property of the sounding itself, not the wreck. It changes at the safety depth for all soundings on the screen.

9.   Encoded in the ENC as a non-dangerous wreck, with no sounding given.

Note this display uses the 2-color option, but using the 4-color option does not affect the wreck symbols.

A couple last details about the object Sounding (SOUNDG). 

A wreck with known sounding can also have an attribute Technique of sounding (TECSOU), and value 6 means "Swept by wire drag," so the the sounding is accurate.  When TECSOU=6,  the sounding gets underlined with a horizontal bracket, as shown below, which is in a sense a different wreck symbol, but it is actually the sounding symbol, not the wreck, that is different. These are fairly common in some areas. 

In contrast to that precsion, we have the opposite condition of a sounding or position that is uncertain, in which case we would see this wreck plotted this way,

A circle around any sounding, not just those on a wreck, means the value of the sounding is uncertain. There is an attribute QUASOU, quality of the sounding, that applies to wrecks and soundings in general, which can take on values of:

1 depth known

2 depth unknown

3 doubtful sounding

4 unreliable sounding

5 no bottom found at value shown

6 least depth known

7 least depth unknown, safe clearance at value shown

8 value reported (not surveyed)

9 value reported (not confirmed)

10 maintained depth

11 not regularly maintained.

Any value 3, 4, 5, 8, or 9 will trigger the circled sounding, which is SY(SOUNDC2).

Likewise, an uncertain position of the wreck, can also trigger SY(SOUNDC2).  The attribute QUAPOS, quality of the position, can have values of:

1 surveyed

2 unsurveyed

3 inadequately surveyed

4 approximate  (the old PA from printed charts)

5 position doubtful (the old PD from printed charts)

6 unreliable

7 reported (not surveyed)

8 reported (not confirmed)      

9 estimated

10 precisely known

11 calculated.

Any value not equal to 1, 10, or 11 will also trigger the low accuracy sounding symbol SY (SOUNDC2).

If there is such a QUAPOS value for the wreck, then an additional low accuracy symbol (?) will be attached to the wreck symbol as shown. This is symbol SY(LOWACC01). These can important annotations to the symbols, being the ENC equivalent of the "PA" (position approximate) or "PD" (position doubtful) labels that were very common on paper charts.  Very few non-ECDIS nav apps can show these symbols. Turn on Low-accuracy symbols in qtVlm to see them in action.

One thing we miss in the new NOAA Custom Charts (NCC) is the lack of these low accuracy indicators. The information is programmed into the ENC that the printed NCC are based upon, but the present version of the NCC app does not print them.