Squall Forecasts

It is likely known that we can get GRIB formatted wind and pressure forecasts from numerical weather models such as GFS. But it is probably less known that we can get usable squall forecasts as well. We get this from the output parameter composite reflectivity (REFC), often called “simulated weather radar,” which is effectively what it is. Once we are in an area of squalls, we can indeed watch them and maneuver around or with them using our marine radar, but it is often valuable to know when they are likely, how severe they might be, and how they will move.

It was not that long ago that navigators beat themselves up chasing atmospheric instability parameters such as CAPE (convective available potential energy) and CIN (convective inhibition) and LI (lifted index) hoping to piece together a usable probability of squalls and their severity—with, I venture to guess, much the same success I had, minimal at best. Now we have a new generation of navigators who can skip all of that, and let the models do the stability analysis, and report it to us as a nice weather radar image right on our chart screens. It is in a sense like new navigators now never having to struggle with the Table 2 tide and current secondary-station corrections, which were, thank goodness, discontinued in 2021.

We can see what live weather radar looks like nationwide at radar.weather.gov. Our textbook Modern Marine Weather has an extended section on the interpretation of REFC.

Figure 1. Sample weather radar. From Modern Marine Weather.

Figure 2. Unofficial guidelines for relating dBZ to squall intensity. From Modern Marine Weather.

The units of reflectivity (Z) are complex and logarithmic (see noaa.gov/jetstream/reflectivity), so they have been simplified to decibels as dBZ. There is no official scale for squall wind intensity, but we made a rough correlation with thunderstorms (rain based) in Figure 2, which has proven practicable. We thus anticipate severe squalls for dBZ values above 40 or so. Squall conditions are most severe with fastest onset where the dBZ gradient is steep, meaning color change from blue to red is narrow.

Besides the global model GFS, the regional model HRRR also provides REFC. Gribs of both models are available by email request from Saildocs. REFC is also included in the high-res NAM models. A sample is shown in Figure 3.

Figure 3. REFC display from the NAM-Puerto-Rico model downloaded and displayed in LuckGrib (luckgrib. com). Forecasted squall winds of 32 kts, gusting to 36, in an area with REFC about 62 dBZ.

This is just a 4-hr forecast, but the general information would have been known earlier. These data are best in the regional models with higher resolution and more frequent updates. The GFS and NAM are only updated every 6 hr, but the HRRR is updated hourly, so it can be useful for near-live squall forecasting in local waters.

You can test these forecasts by looking at the actual weather radar for the same region and time, as shown in Figure 4.

Figure 4. Sample weather radar at about the same time and place as the NAM forecast in Figure 3.

To practice with this, look at the national radar map to find squalls (Florida has the most) and then compare to an HRRR REFC forecast for the area. The hourly updates of HRRR extend out 18 hr, except those run at the synoptic times (00, 06, 12, 18 UTC) that extend out to 48 hr.

Note in passing that the HRRR forecasts for all parameters might be your best local weather forecast available, especially in remote parts of the country.

Barometers and Marine Navigation

Even in the age of high-speed internet at sea, remarkable weather model forecasts, and satellite wind measurements, our knowledge of the correct atmospheric pressure, and how it changes with time, remains the key to safe, efficient routing decisions. Pressure data are also the most direct means of evaluating the model forecasts that we ultimately rely on for routing.

Productive barometer use in navigation is a relatively new concept—it was actually used more effectively in the 1700s than in the 1900s! The Barometer Handbook explains its interesting history and its role in marine navigation. The major change came when accurate, affordable digital barometers started to find their way on to boats. Now we have many options. Chances are the barometer in your phone is the most accurate barometer on the boat, and the easiest to use with a good app. Several options for mobile devices and computers are listed at starpath.com/marinebarometer, which also includes a link to an extensive set of barometer resources.

Phone barometers are typically accurate to better than ± 2 mb right out of the box, and it is relatively easy to improve on that with online resources given in the link above. The goal would be to get its accuracy down to < 1 mb, which is the effective standard used in the buoy and ship reports shown on surface analysis maps. Map pressures and forecasts give the pressures to a precision of 0.1 mb, so we can make comparisons on that level, keeping in mind the overall uncertainty.

Unlike aneroid barometers, modern sensor accuracies do not vary much (just a few tenths) over the full pressure range we expect at sea—940 mb to 1040 mb, always hoping to avoid the two ends! Thus setting it to the right pressure at any value is effectively calibrating it over the full range. One fast way to calibrate in US coastal waters is to make regular comparisons with NOAA stations accessed through tidesandcurrents.noaa.gov. Procedure: (1) Go to the site and click your state. (2) Turn on Barometric pressure on the right. (3) Zoom in to find two pressures to interpolate between. (4) Consider this to be the correct sea level pressure (SLP) at the moment, compare this to your barometer reading, and record the difference in a logbook. These data are updated every 6 min. 

Remember your pressure will be lower than the sea level value even if your barometer is spot on because you are at some height above sea level. Precise corrections are in the resources cited above, but you can compute the correction with the jingle "Point four four per floor," which means the pressure drops 0.44 mb for each 12 ft above sea level. Correct your reading for your height before comparing the two.

With a calibrated barometer we are ready to tackle some weather applications. Many ocean sailing routes are going around Highs because there is no wind in the middle of the Highs. We may be following a rule of thumb, such as stay two isobars (8 mb) off the central pressure, or we might be following a computed route that often takes us dangerously close to the High. In any event, knowing how the High is moving is crucial information. With a good barometer you can tell if the pressure is rising or falling very quickly, because the instruments can dependably show steady changes of just a few tenths of a mb. 

When interpreting any pressure change, we need to keep several things in mind. The pressure will go up if the High is indeed moving toward us, or if it is not moving, and we are sailing toward it. It can also go up if neither one of us is moving, but the High is just building. So, we need to watch our track on the chart compared to the isobars on the chart from the model forecast we are using to properly interpret changes detected. At lower latitudes, we also must correct for the semidiurnal variation of the pressure caused by a tidal effect in the atmosphere. It is a variation of about ± 1.7 mb, with two highs and two lows daily. Check out a pressure plot from any ndbc.noaa.gov station in the tropics to see the pattern.

A good barometer is especially valuable sailing in waters prone to tropical storms, because the standard deviation of the pressure is very low in these waters—typically 2 mb or so. When sailing there for some time, you will know the mean ambient pressure for that time and place (after correcting for semidiurnal variation), which might be about 1013 mb. Then when you observe the average pressure drop to 1009, you know this is almost certainly the approach of a tropical storm, even if the wind or clouds have not signaled it. A drop of 2 standard deviations has only a 2.3% chance of being a statistical variation of the pressure. This does not work at higher latitudes because the standard deviations are much larger.

As a general guideline to the interpretation of pressure drops at any latitude, we suggest the rule "4-5-6" meaning any change of 4 or 5 mb over a 6 hr period is fair warning that bad weather might be headed your way. Not guaranteed, just a guideline to practice with to see how well it works for you. Drops of much less than that do not usually signify anything, and much more than that often puts you past the realm of forecasting. It is there. With a good barometer we can monitor this guideline precisely.

Beyond those couple examples of pressure as forecaster, a key role of the barometer these days is for evaluating numerical forecasts. Remember, there will always be a model forecast, and they are not marked good or bad. It is up to us to evaluate the forecast in every way we can before setting routes based upon it. We would also do this with the wind speed and wind direction, but both have several corrections to apply, plus they rely on instruments that are difficult to calibrate accurately. With the barometer we can know before we leave the dock that our barometer is spot on, and then we are just comparing two numbers. 

For this evaluation, we need to log the measured pressure at least at every synoptic time (00, 06, 12, 18 UTC). We then look back over our track on the screen to where we were at the synoptic time and compare our pressure to what the forecast says. If the pressures agree within a mb, we have a hopeful sign the forecast could be right, but we learn more if they notably do not agree. Then we know the forecast is wrong on some level. With practice we can likely piece together, including using the wind data, how it might be wrong—i.e., too early, or too late; isobars rotated, Low or High deeper than forecasted, and so on. The barometer gives us one clean, indisputable data point to use.

Six-minute pressure reports from tidesandcurrents.noaa.gov. If you were in Salisbury, MD your correct SLP would be (1018.6 + 1017.4)/2 = 1018.0

USCG License Exams Come of Age — Tide Wise

 In 2020 NOAA announced that this was the last year they were going to authorize an annual set of tables for tides or for currents. The tables were called:

Tide Tables

2020 East Coast of North and South America Including Greenland

2020 Europe and West Coast of Africa Including the Mediterranean Sea

2020 Central and Western Pacific Ocean and Indian Ocean

2020 West Coast of North and South America Including the Hawaiian Islands

Tidal Current Tables

2020 Atlantic Coast of North America

2020 Pacific Coast of North America and Asia.

Prior to 2021, these were "the official sources." All other third-party printed or electronic  presentations of  US tide and current data, readily found along the waterways and cybersphere, were derived from these, sometimes mixing up actual locations or confusing standard times and daylight times. 

These are what we called "The Tide Tables" or "The Current Tables" that were either required or recommended to be on all vessels. These tables included daily data for numerous Reference Stations and then a Table 2 that included corrections to be applied to thousands of Secondary Stations.

That ended in 2021. And despite the fact that some third party companies still print these tables including the Table 2 data that they reproduce from the 2020 tables, the data are not valid. Hundreds of those secondary stations have been discontinued and values for many others have changed.

But more to the point at hand, up till just recently, the USCG license exams still tested on the Table 2 procedures using the old Table 2 data, which has been totally wrong for nearly 5 years now. Many schools around the country still teach this method as well.

The USCG has now corrected that and their new exams treat tides and currents in the modern, correct manner, which is outlined below.  This greatly simplifies this important part of navigation.  We wrote several notes on this in the past:

No More Tide and Currents Table 2 — Navigation Students Celebrate!  

and

NOAA Discontinues Tide and Current Books — What Do We Do Now?

You can review these for background and in the second one for step by step procedures for most efficient access to the new data, including how to make your own set of annual tables

Another aspect of the simplicity (progress) is that tide and current questions are now essentially the same for entry level OUPV license exams as they are for unlimited ocean master.

Here is an example.

The diagrams included are:

The solution to #36 is fast. Go to the time on the graph and read the speed, then note that the harmonic directions are given in the figure titles. 

That is the right answer to the test question, but not at all the guaranteed answer on the water. These currents are treated as pure reversing, with two directions only, but in practice they rotate, flowing with some strength in an ellipse of directions. The direction given is just the average direction around the time of peak flow at the long axis ends of the ellipse.

Question #35 asks about rotary currents, which is interesting in that the discontinued annual current tables did have a Table 5 listing details of rotary currents along the coasts. But like Table 2, much of the Table 5 data were not considered unreliable, so they have been discontinued. If we want coastal tidal currents, we should use the OFS model forecasts, which are very good. Nevertheless, we can answer this question by looking up any coastal station along the East Coast and seeing how long it takes between successive floods or ebbs, and it will be about 12 hr.

The tide problem, #37, is just as direct. We are between two stations that have different heights but we are only asked for time, and that is the same for both. If they had ask for height the answer would, presumably,  be (0.51+0.29)/2 = 0.4 ft.

(Tide heights are pretty uniform over large areas of open water, so the tide values are more likely to be correct out on the water than the current values — assuming the atmospheric pressure is about normal, and the wind has not been strong over the past 12h, and there is no unusual river run off, all of which can throw the water level predictions off a foot or two.)

Likewise for the other end of the license exam spectrum, unlimited master.

This the same as #36, but we have to adjust the time. Starting at 0130 we must travel 15 nmi at 10 kt which takes 1.5h so we get there at 0300.

Question #6 for unlimited master is the same as the #37 for OUPV.

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So the summary is the USCG exams now follow the existing procedures for tides and currents, which is tremendously easier and faster than it was before. It is a pity that the invalid Table 2 data (for tides and currents) are still being published by third party printers, but we should just know this, and move on.

Our main resource for all tide and current data is now www.tidesandcurrents.noaa.gov.  Please refer to the article above (What do we do now?) for the exact steps for the most efficient use of the NOAA site. The best procedure is not intuitive. What might seem an intuitive approach can lead to other types of data that you likely do not want. We want the types of data shown in these USCG exam diagrams. In that article there is also a video showing the steps.

The article also shows how to make annual tables for any station. It takes just 4 pages per year, per station. We do not need the historic books that covered all of North America. We need just the stations covering the tidal waters we navigate.

Our book Inland and Coastal Navigation covers the use of the NOAA website, and our Navigation WorkBook 18465Tr  has practice exercises. In practice a new challenge arises in finding the nearest station you care about (illustrated in the links above), or you can use a program like qtVlm or OpenCPN that used tested harmonic data from NOAA and they show where all the stations are.

Here is a video summary of this article, which includes a demo of our recommended approach to the NOAA data.

* * * Thanks to Seattle Maritime Academy instructor Robert Reeder for alerting us to these USCG exam updates.

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.