Depths, Contours, Soundings, and Groundings in ENC Navigation

Electronic navigational charts (ENC) provide a vast array of safety features not available using raster navigational charts (RNC), the echart equivalents of paper charts. These notes explain how charted depth information can be used in an electronic charting system (ECS), and in particular how qtVlm is well designed to study and practice these features using its built in NMEA simulator for power or sailing vessels. It is a good representative of any high-featured ECS. We use it in our forthcoming course on electronic chart navigation.

The subject is tied to how ENC present depth contours and soundings. Indeed, the proper use of depth contours is a key to good work with ENC. To be more precise, we are not really talking about properties of the ENC themselves, but rather, what ECS such as qtVlm can do with the specified format of the ENC sounding data in compliance with recommendations of the IHO and IMO. 

These two organizations call for the definition and use of three specific depth contours, and all modern ECS use these conventions. The user defines in the program preferences the values to be used for each of these contours, keeping in mind that they can be changed at any time to match the location and navigational goals of the vessel, as well as the scale of charts available. The three are Shallow water contour, Safety contour, and Deep water contour.

Shallow water contour

The shallow water contour is defined as marking that depth where the vessel will certainly go aground. It would typically be the draft of the vessel, perhaps modified by the range of the local tide. The working assumption is if you cross that contour you go aground, but there are important differences between the one requested, the one displayed, and the one that triggers alarms, as discussed below.

Safety contour

This is the most important of the three. It is intended to mark the boundary of guaranteed safe water for the vessel, meaning if the vessel remains outside of the safety contour they have no worries about grounding. This contour will be highlighted on the chart with a thicker, darker contour line, and its depth value is used to identify any isolated obstruction outside of the safety contour with the prominent isolated danger symbol (magenta circle with a transparent bold X inside) if its sounding is less than the safety contour depth you requested.

The safety contour will also be marked by two different water colors on either side, which will be true regardless of whether the mariner chooses a 4-color water pattern or a 2-color water pattern. The IHO and IMO also require that the COG predictor line (or anti-grounding cone) also trigger an alarm if the predicted vessel position crosses the safety contour.

Generally the safety contour would be chosen to be the draft of the vessel plus the user's choice of a safety margin below the keel, plus a correction for negative tides in the region. Ships and other large vessels also add a correction for maximum squat they might expect.

Safety depth

The goal of most routing would be to stay outside of the safety contour, but in some cases this must be crossed, and when doing so, special caution is required. One ENC display feature that assists with this is a fourth navigator input to the navigation program called the Safety depth.  The value of the safety depth is used to determine the color of the soundings.  If a sounding is equal to or less than the Safety depth, it should be printed in black text; if the sounding is deeper than the Safety depth it is printed in a gray color. Thus when sailing inside of the safety contour it can be easier to tell the deeper parts from the shallower. The parts deeper than the safety contour have gray soundings; the parts shallower, have black soundings.

The safety depth value is also used to change the background color of the important generic hazard symbols from transparent, when their soundings are deeper than the safety depth, to blue, when their soundings are equal to or less than the safety depth. See Role of the Safety Depth in ENC Display.

Deep water contour

This contour choice does not directly affect safe navigation and can be used as best suits the navigator. In a 4-color system it defines the outermost color boundary. We see several formulaic guides in online articles for choosing this, but they just generate a number with no specific meaning. We have suggested two practical applications, namely mark the 100-fathom contour on a coastal voyage to avoid the sometimes confused seas and currents along the edge of the continental shelf, or set it to your preferred anchoring depth to help look for anchorages ahead. Or maybe just choose the next deepest contour beyond the active safety contour to make a progressive color pattern to the water depth.

This contour is not important, other than it should be deeper than the safety contour, which in turn should be deeper than the shallow water contour. Generally the safety depth would be equal to the safety contour value or between that and the shallow water go-aground contour.

Setting ENC depth contours: The good, the bad, and the ugly

The good part is the contours we can choose, their clear presentation, their intentions, and their interaction with the ECS are as described above, which in principle provide valuable navigation aids. We have no such options using other chart forms.

The bad part is, selecting the contours we want to use is not as easy as we might guess. In all ECS, we are given the opportunity to type in the digital values we want for each of these contours (the requested values), but rarely will we actually get what we want drawn out on the chart (the displayed values). The problem is the only contours that can be displayed are ones that are already coded into the specific ENC we are using—and the only contours on the ENC are those that are on the RNC (paper chart) it was based upon. Thus if we want and ask for a safety contour of 20 ft on a US chart, we won't get it, because no US chart has such a contour on it.  

When this happens, ECS are instructed (by IHO S-52) to find the next deepest contour included in the chart at hand, and display that one. Requesting a 20-ft safety contour, will likely lead to a 30-ft safety contour, or on some US ENC there is also a 24-ft contour (more rarely) and it would take that one. Table 1 shows the contours available on ENC.

It is beneficial to know the gist of this table, determined largely by fathom charts, but even knowing the possible contours is not the end of the story. First, not all US charts have all these contours. You might have two charts next to each other with different contours. Samples (not next to each other) are shown below using a nice display feature of qtVlm that tells us all the contours in a specific chart in view, with the active safety contour underlined.

Beyond knowing what the options are, which is easily solved with this display, there is still a nuance in asking for it that can be traced to NOAA's policy of truncating the metric conversion of depths when creating the ENC based on the existing paper chart. The details are given in our book Introduction to Electronic Chart Navigation. 

In short, if you want a specific US contour and you have the ECS display set to feet, you must ask for something a foot or so lower than you want. If you ask, for example, for 18 ft, it will miss that one completely because it is in there as 17.7 ft. Ask for 16 to be sure to get 18, and so on.

Knowing how this works, one safe method is to look at the contours on the chart in use (viewed as feet  or meters) and cursor pick the contour you want as a safety contour to read its value, which in this case might well be reported back as 18 ft (or 5.4 m), but then ask for 16 ft, then look at the chart to confirm that the one you want is now bold with different colors on either side. Set to meters display, asking for 5.4 m will likely get you the one you want.

This extra concern called for in contour selection will all go away in a couple years, depending on where you are sailing. NOAA has an ongoing program of Rescheming all ENC and one of the many benefits of that program are new and consistent metric contours, as shown below.  Some areas are already completed, but others are a couple years out, as shown below.

The same considerations on setting the Safety contour digitally also applies to the Shallow Water (go aground) contour and the Deep Water (use as you please) contours.

An example of entering requested contours and seeing the consequences. Notice that soundings equal to or deeper than the Safety depth are in gray; others in black.

Subtleties in the use of the Safety Contour

The IHO and IMO require that ship navigation with ENC must offer the user the option to set alerts and alarms that will go off when the vessel approaches or crosses the safety contour. Most ECS that have this capability have also adopted this as an optional set up for the mariner. Thus they can turn on a COG predictor or anti-grounding cone and it will change color or set off some other visual or audio alert when it crosses the safety contour. You can choose to look ahead for any time interval, minutes or hours. It will also trigger when crossing any isolated danger (rock, wreck, or obstruction) outside of the safety contour whose sounding is less than the depth of the safety contour, or if no sounding is given.

This is a fine concept, but referring back to the notion of a sometimes bad aspect of the use of a safety contour, consider the case when there is not a contour in the chart that is close to what you want, and the next one deeper is quite a bit deeper than you want. When that happens, your otherwise useful alarm is now a nuisance. It is going off, for example, when you are crossing a 60-ft contour, when you have no danger at all above 12 ft, which is what you ask for.

One solution is just give up the audio alarm feature till the chart contours are more favorable, but there are other solutions that help clarify the situation.  qtVlm, for example, has implemented the system of Default Safety Contours recommended by Professor Adam Weintrit in his book The Electronic Chart Display and Information System (ECDIS): An Operational Handbook. In that system, if you request a safety contour that is 10 m (33 ft) or less and the only available safety contour in the chart is more than 67% deeper than you requested, then that selected contour is considered a default safety contour and it is marked with a less prominent line (lighter gray) than used for a closer safety contour.  It is still clearly different from other charted contours, but also different from that of a safety contour that is closer to what you requested. If the requested safety contour is deeper than 10 m, the default contour coloring is used if it is more than 33% deeper than you requested.

This is a seamless, behind the scenes enhancement to the presentation of the safety contour that reminds users when the safety contour in effect is not close to what was requested, and yet all other interactions with it remain unchanged. This is especially valuable when there is a mismatch of contours on adjacent or overlapping charts, as shown below.

Here we see a default safety contour in lighter gray in the bottom chart compared to the traditional ENC safety contour in the top chart. In both cases, we asked for 19 ft, and the top one had a contour at 30 ft, which was close enough to what we wanted to treat it as a traditional safety contour, but when viewing the bottom chart, its closest contour was 60 ft, so it gets relegated to a default safety contour status, which stands out from the other depth contours on the chart and also from the closer traditional safety contour that was closer to what was requested.

We also see here the danger cone alerts crossing the safety contour (orange) and the effective shallow water contour (red)—see related notes below. The safety contours are the ones in effect on the active charts. The orange alerts marked with yellow asterisks (discussed below) are going off because of the mismatch in contours; the COG predictor is detecting the safety contour from the adjacent chart, which can be seen on the lower chart.

Notice that soundings equal to or deeper than the Safety depth are in gray; others in black, which is a big aid when sailing within the safety contour.

Isolated danger symbol and the safety contour

Another nuance of the official safety contour procedures relates to the display of the isolated danger symbol for rocks, wrecks, and obstructions that are always underwater and lying outside of the displayed safety contour. These object symbols will change to the isolated danger symbol (prominent pink Philipps-head) whenever their known sounding is less than the depth contour that you requested, not the one that is actually displayed. Thus there is some virtue to requesting the safety contour that you really want (even though a deeper one might get displayed) and also setting the safety depth to this same depth. That way all the soundings will be black in waters shallower than that and any underwater hazard shallower than that will be danger symbols.

The ugly bits

What I had in mind as not a very pretty part of using the safety contour is the fact just mentioned that in the present state of US ENC, meaning before the rescheming is completed, we have cases where adjacent or overlapping ENC do not have matching depth contours. The above image is a good example. Different scales do not matter, but when the inherent contours differ it means the safety contour in effect can change, depending on the chart in view. The ECS is always choosing the next deeper one from what you requested if it does not find what you want, but that answer can change as you sail from one chart to the next, or you change the zoom level to change the active chart. Although this happens on other charts as well, the example shown here is an unusual exaggeration of this issue.

This problem is mitigated quite a bit with the use of the Default Safety Contour system, which alerts us to the presence of more than one safety contour, but it can still lead to unusual danger cone displays (the two with yellow asterisks shown above), because the ECS knows all the contours present on all of the loaded charts, whether or not it shows on the chart in view. It has to know that to trigger the alerts. Indeed, a great value of the ENC is you can load the ENC, and then navigate on an RNC and the alerts and alarms computed from the ENC will still be in effect. In this case it looks like the alarms are coming from the RNC, but in fact they come from the ENC below these that you cannot see.

Thus if you see your danger cone go orange in what looks like the middle of Nowhere, you can know that you are crossing the safety contour as defined on a chart you cannot see. In short, there is no safety issue here, we are just finding one of those places where the depth contours on overlapping charts are not the same. There are not many places this can happen, and in a year or so there should be none.

Danger cone alarms are also triggered by isolated dangers shallower than the active safety contour depth, so this behavior also changes in cases like this one.

Depth simulation in qtVlm

For navigation training and practice, the NMEA simulator included in qtVlm is an invaluable aid. There are several videos on the use of this tool, but for now I want to just summarize a new feature directly related to the topic at hand. That is, it will now simulate depth readings as your vessel moves across the chart. It can do this because no matter where you are on the chart you are between two depth contours (d1 and d2), and these define an ENC object called Depth area (DEPARE), which has two attributes, d1 and d2.  qtVlm simulates what your depth sounder would read by reporting back a depth of (d1+d2)/2 at all times, and you can display this in the depth meter (sounder) found in the instruments selection.

This is clearly a rough approximation, which appears as a bar graph in the histogram, with steps occurring as you cross a contour, but nevertheless this simulation can be used as a way to study depth sounding navigation as shown below.

If you had just the depth sounder trace shown above with no GPS working, you could find your position on this chart by matching the trace to your known heading and speed. This can be practiced with the simulator.

The simulator reads the depth below the location of the GPS on your simulated vessel that you specify in the boat dimensions tab (± 10 meters). These dimensions are entered in the same way they are specified if you were broadcasting your AIS location. Likewise when we get a ship's AIS signal, this is the way we learn where its GPS is located.

This shows the set up for a real size vessel icon with the GPS assigned to the starboard quarter, marked by a small red dot. The yellow dot forward, is one third of the LOA, which marks the rotation point of the vessel when turning in the simulator. The red ring with radius 10 m is effectively the simulated depth sounder profile on the sea bed. 

Going aground in the simulator

The simulator will go aground and stop the simulation whenever you cross the effective shallow water contour.  Recall from above, that if the shallow contour you request is not there, then the next deepest will be chosen for the water color change.  Also, if the exact contour you requested is not there, the shallow alarm does not go off at that displayed shallow contour and you do not go aground at that contour, but rather at the next shallower contour. In other words, if there are 5, 10 and 15m contours, and you ask for shallow at 7, then the 10 will be displayed, but you do not go aground or get the obstruction alarm until you cross the 5. 

You can test this behavior with the Rule tool. Just turn it on and extend it over the various contours or objects to see the color changes and notices. You will go aground whenever you detect a red obstacle warning.

If there are no shallow contours where you approach land, you go aground at the green foreshore, and with no foreshore, you go aground on the tan land.  With no ENC charts loaded, you go around at the boundary of the base map. There is a setting in Preferences that affects this choice, which is best set to the ENC chart borders. 

The grounding applies not just during simulation, but it is also taken into consideration when qtVlm  computes optimum routes, power or sail. Needless to say, it will not propose a route that takes you aground!  In this regard, the details can matter. If you do not have real dimensions assigned to your boat, then the grounding occurs when the vessel's GPS location crosses the shallow water contour or coastlines as noted above, but if you assign dimensions to your boat, then it is the rectangular profile of the boat that triggers the grounding. If your boat has, say, a beam of (C+D) = 4 meters, and your GPS is on the centerline (C=D), then you could drift abeam and go aground 2 m before the GPS gets there.

When running the COG predictor or danger cone, you will get an orange warning when the predicted position (which depends on the look-ahead time you have selected) crosses the safety contour, and you will get a red warning if it crosses the shallow-water contour.  If you have the audible alarms turned on with the danger cone activated, then these warnings trigger a graphic sign alarm and a sound. 

These warnings and alarms will also get triggered by crossing any isolated danger symbol or indeed any obstruction in any location whose sounding is less than the active shallow-water contour. The danger cone is the best way to afford this safety and to practice its function. The COG predictor line alone is less likely to hit the point source of an obstruction. 

You can investigate which alarms go off and when using the ruler tool, because it acts like a portable COG predictor and will go orange crossing the safety contour and red crossing the shallow water contour or any obstruction shallower than the shallow water contour.  You can sweep the Ruler tool line across an obstruction to see that alert. Orange alerts are announced "danger detected"; red alerts are announced "obstacle detected." Both can be used to trigger an extra visual and audio alarm, but the latter must be turned on manually first.

You can get realistic practice on grounding with dimensions assigned to your vessel, because then you will indeed go aground when that vessel outline passes over an obstruction whose sounding is less than the active shallow water contour. However, with the danger cone activated, you should not have this happen because it will warn you. That is why we are practicing this! Turn on a notable tidal current (Grib configuration / Corrections)  to see how this works when you are not going the way you are headed. 

To have the alerts, groundings, and routing restrictions work properly in the program, we must have the coastline detection set correctly, which is done on the ENC setup page.

If this is shut off, then the coastlines are defined by the borders in the base map, which are not nearly as accurate, and we would sail over or route over other shoalings as well. Likewise, assigning these to an ENC without it being loaded has the same effect.

Atmospheric Pressure: Look Close to See its Microscopic Pulse

We had an inquiry today about whether or not our Marine Barograph app would read and record pressures accurate to a few tenths of a mb on a per second basis and plot then showing sub mb precision. The answer is yes it will, but the question caught my attention. I have known for a long time that pressure oscillates on a small scale at all times, but this never effects our weather work, which looks for solid trends of over a few tenths of a mb—in a sense for just this reason, namely we do not want small oscillations interfering with our reading, no matter where they come from.

When we look at a plot of the pressure on a fine scale we see these oscillations, but when you look at just one plot the tendency is to consider this electronic noise. You might think you are just looking at the limits of the sensor and its circuits.  But that is actually not the case at all. These oscillations are real.

Below we see three old cell phones with our app loaded into them, converting them in a sense to about a $800 electronic barograph. Any iPhone model 6 or newer includes a pressure sensor—you may have one sitting around in a drawer.

The dip in the top unit on the left side of the graph of about 0.2 mb marks the time when, upon receiving the inquiry about the units, I picked this one up and raised it up to the ceiling, a distance of about 5 ft. This was to check how the scale responded to such a change.

The middle one did not have its sensor offset turned on so the value is off some, but this does not affect the topic at hand.  

Data are stored every second for 30 min, and then stored in different time bins for plotting longer periods, which is described in the help file.  You can export any of these time bins, which I did for each one covering the past 30 min.  We can see the plot of these in the phones themselves, and indeed pinch zoom in to see all details, but to make the point at hand, I exported them and then plotted them in Excel, as shown below.

Here we see all three plots; I shifted the gray one up to compare them, to compensate for its lack of calibration offset being turned on.  This whole note was pretty spontaneous!

The important thing to note here is these are three completely independent sensors, being run by three completely independent little computers, which are powered by batteries. I cannot imagine any common electrical properties of any of them—although the batteries were all on chargers at the time that does in  a sense link them.

In short, these oscillations in the pressure appear to be real variations in the physical pressure of this room. I do not know what causes it. Back when i was working in a laboratory, I would likely have stopped to figure out why, but now we just note this and move back to what we are now working on—a new course in electronic chart navigation.

Hallways in condo buildings are often kept at positive pressure to keep cooking smells indoors. Navy warships and first responder vessels keep their wheelhouse in relatively high positive pressure to protect from toxic gases, so these things could be studied with our app. Barometers in these ships have to be in sealed housings that have tubing leading outside to read atmospheric pressure.

There is some element of pattern present as very roughly outlined below, but this would take a serious mathematical analysis to understand.

This rough outline indicates that the oscillations have an amplitude of about 0.05 mb with a period of just over 1 minute. It is not clear what is causing this, but it seems it must be a real physical phenomena and not some artificial electronic anomaly—but that is not 100% clear. We have to rule out any link via the battery charger and investigate any way a wireless signal might induce this effect.

We will take these outside tomorrow for 15 minutes or so to see if that  makes a difference. They will also be on batteries without chargers for that and I will put each one in a metal can (Faraday cage.)  I don't think it will matter. I think we are seeing the pulse of the atmosphere here!

And I want to stress that this is no reflection on the actual accuracy of the pressure itself. With a good offset inserted, the reading is likely accurate to within a few tenths of a mb, which we know from our own calibration tank and comparison with other instruments, but we can't make comparisons any closer than that. But even though the accuracy on the full pressure readouts are not in this 0.01 mb range,  the oscillations themselves are likely real. That is the point of this note.

When we first started working on electronic barometers we also reported a very interesting physical affect of a small short pressure bump under a squall, which is indeed atmospheric science, but we have not learned more on this since then, although you see in that article a reference to an even earlier event we reported—we were not lucky enough to have our new app in these early days.  Maybe now that our app is available, more folks will study such events.

In short, a simple tool like our $15 marine barograph app is a powerful way to look into details of the atmosphere, besides its great value in helping shape a fast, safe course across the ocean... or predict when a front will cross our local lake. 

Let's finish with a bit more perspective on this. Here is the dial of a high precision Fisher aneroid barometer, which has the pressure scale marked off in 0.5 mb intervals.

_______

Note added Nov 26.  We have now done the outdoor test and the oscillations noted above are likely artificial, but we did indeed see what are more likely real sub-mb oscillations. See Sub-millibar Pressure Patterns, Part 2.

ENC Object: Tidal Stream, Flood and Ebb (TS_FEB)

 We have two ways to learn about tidal currents in a typical navigation program, such as qtVlm and OpenCPN that we use in our online courses, and numerous other commercial versions. 

First, we have the overlay of current and tide predictions that the navigation program itself computes for us, based on the harmonic constants we have loaded.  This source of tide and current data is our main workhorse for navigation, but it has nothing at all to do with the ENC charts themselves. It is a program function, unique to each program in the details of how it presents the results. These current predictions work perfectly well looking at ENC or at an RNC, but we can also shut off the charts completely and just use the  base map to obtain currents over the areas covered by the harmonics loaded.

Completely independent of those forecasts are the tidal current summaries that are included in the ENC and indeed in many cases on the related RNC as well, and it is these ENC current data that are the subject at hand.

The ENC object is called Tidal stream, flood and ebb (TS_FEB), the symbol is a current arrow, usually in pairs, one showing the flood direction, the other showing the ebb direction. On the RNC these are labeled with an F and and E, but not on the ENC—but  it is easy to tell because: "the Flood has the Feathers."  You can cursor pick these objects, but we do not learn more than shown, namely the average direction of the flood and ebb along with the average maximum speeds.

 

A cursor pick gives the report below.

There are a couple nuances to the use of this parameter. One is the S-57 standard for this current velocity is the average of the maximum spring currents, whereas the NOAA standard is to use the full average of all maximum currents. Thus we must keep in mind when reading these averages that during a full moon or new moon we can expect the peak max flow to be about 20% larger than we read on this ENC object. In principle this is not the case when viewing this object on the charts of other nations—with Canada being an exception for adjacent waters, because of our historic sharing of this data.

The second, more important point to keep in mind is these predictions on US and adjacent Canadian charts are at present not the latest values available—so the distinction between full average and spring averages is not really significant. It appears there is some internal NOAA communications catch up called for. One division makes the charts and codes the data we read here, but it is another division that makes the tide and current predictions that the chart makers depend upon. 

All of the related NOAA divisions are undergoing major changes these days—not to mention that they are also mostly working from home—so it is not too much a surprise that we experience a transition here, and indeed this is not a serious issue. But somethings can and likely will change soon with the ENC. The blue link in the ENC cursor pick report shown above, for example, brings up the text message shown, which refers to an official NOAA book that no longer exists. The last issue of that book was 2020. The new reference should be something like "For exact predictions see www.tidesandcurrents.noaa.gov" — or for completeness, there may in fact be 2021 books with a similar title that are no longer approved by NOAA that include the old 2020 corrections.

One of the reasons given by NOAA for discontinuing the annual printed current tables is the increasing use of digital sources of these predictions based on the harmonic constants that can be used to compute the predictions. And, indeed, NOAA makes these constants available to the public. The navigation program we use, qtVlm, is just one example of how convenient it is to get current data at any time, now or into the future. We also illustrate how you can confirm that the harmonics you have are valid.

Historically, the TS_FEB objects were placed on the RNC and ENC at each of the locations of the reference stations (now called harmonic stations)  and  subordinate stations. But over the years, NOAA has discontinued some of these stations, but some TS_FEB objects are still there, even through there are are no longer any official predictions for those locations.

It seems that the US ENC chart makers are aware of these issues and the role of the TS_FEB object is likely in a state of flux. For example, if we look at all US charts in the Eastern Strait of Juan de Fuca that are north of the ENC cell boundary at 48º 20' N, there were at one time about 100 of these objects. And indeed if you look at the latest Canadian charts that cover this region they are all still there, using the original US tidal data. But the US has removed all of these objects from the ENC in that region, in part, I suspect, because the data are no longer the latest values.

To be more specific, NOAA has current predictions at multiple depths, and we cannot just say "take the one closest to the surface," because at some stations there are multiple forecasts within 30 ft of the surface. In short, we have to look now to see what might be best. 

Also there is the much more practical point that many of the currents shown at the TS_FEB locations are not purely reversing currents as might be implied by just two arrows. Newer NOAA data shows that many of the stations are more rotary than thought some years ago. A pure reversing current has just two directions, with speed diminishing to zero as it changes from one to the other, whereas as a pure rotary current does not change speed, but just rotates from flood to ebb direction. Real currents are a  combination of both, showing an elliptical pattern in the plot of the current vector—typically in between pure rotary (ellipse pulled out into a circle) or pure reversing (ellipse squashed flat into a line). 

From our text Inland and Coastal Navigation. As a rule, coastal currents are more rotary than inland, but there are notable exceptions.

In short, these TS_FEB data in the region of our training charts (Eastern Strait of Juan de Fuca) can serve as a quick guide to current flow at peak strength, but the values shown may not reflect the latest knowledge. A couple examples are below.

Notes:

1. From cursor pick of the ENC symbol

2. Data from the last edition of these books. These data will become increasingly less dependable as NOAA updates their stations with new measurements. 

3. We get the average directions from a stations daily report; and then we figure the average speeds by downloading the full year of data and averaging the year's ebb and flood speeds in a spreadsheet. (We do not know where the average speeds are given, now that Table 2 is no longer supported.)

4. This is one of the objects removed from US charts, but still appearing on Canadian charts.

5. This is a random check of one station on the East Coast, using the Atlantic 2020 Table 2 .

Bruce Stark, Navigation Historian, Dies at 95

Bruce Stark’s classic treatment of the lunar distance method in 1995 sparked renewed interest in celestial navigation worldwide, quite beyond that one specialized technique. His work will be remembered as a key step in the history of marine navigation. 

He died peacefully in a Eugene OR area hospital following carotid artery surgery.  His wife Janice had close contact with him during this time, which was an unusual blessing in these COVID days that often prevent patient visits.

Bruce Stark became interested in celestial navigation as a sailor and outdoorsman in the late 70s, when he began the painstaking journey to becoming a leading expert on the navigation of Lewis and Clark, and especially the esoteric sub topic of celestial navigation devoted to finding longitude from sextant measurements of the distance between the moon and other celestial bodies, called the Lunar Distance Method. His innovative book on a new solution to this problem took him 20 years to perfect, but it received instant praise by experts in several countries.

Navigators had been following his reports on the development of his method along with extended notes on the navigation of Lewis and Clark in the Navigation Foundation Newsletters (NN), where they are still available to the public at www.navigationfoundation.com. Admiral Thomas Davies, founder of the organization, was the first to note the value of the work followed by in-depth reviews from several international experts.

Robert Eno from Iqaluit, Canada (NN, Issue 65, Fall 1999, page 7) in an extensive study of the method included these observations:  

"Stark has taken all of the tedious and complex calculations formerly required for lunars and reduced the whole process to a simple exercise of addition and subtraction." 

"Had he been born 200 years ago, he might have been remembered with the same veneration as Raper and Chauvenet whose names are now synonymous with lunars. "

"Now that Stark has made the process so simple, there is no excuse for the serious navigator not to try his hand at lunars."

The main point being made is, the Stark Tables provide an all manual solution. We have many ways to solve lunars these days with computer computations, but his method is all paper, no batteries required.

Later, Professor Jan Kalivoda from Charles University in Prague did another extensive review (NN, Issue 92, Summer 2006, page 12) that included these notes:

"I was astonished by their ingenuity. They don't repeat old solutions mechanically, but are significantly better than renowned works of the past, although they don't misuse the modern technical possibilities and go the fully traditional way of tabular and paper solution. It had to be an intellectual adventure to compose them and it is a delight to study them. "

"After Bruce Stark had published his tables, every sailing navigator (fondling the GPS in his pocket) can revert to the sea history in his practice very easily, if he chooses. He can be sure that with these Tables, the history of Lunar Distances is consummated now and the long line of rigorous methods for clearing them ends successfully—and for the first time, after all of these years."

The reason the Stark Tables kindled such renewed interest in cel nav and sextant work—which is ongoing—is the fact that the special sextant measurements required are difficult, taking exceptional sextant skill. But at the time of his book, there were not quick computer solutions to the lunar analysis, so the analysis of the sights was also very difficult. With the advent of his book, navigators had a way to hone their skills in sextant sights and then a way to see if they got it right. And on top of that, there is no heaving deck and ocean horizon needed for this practice. You can do this from a chair in your backyard.

We frankly do not  have much call for this method in practical navigation, but learning it makes you a better navigator, much as taking a few lessons on aerobatics makes you a better private pilot. On the other hand, a Long Term Nautical Almanac, including a set of Pre-computed Lunar Distances, along with the Stark Tables constitutes a Doomsday Navigation Kit, with which you can find your accurate location on earth without any semblance of technology or even civilization at all, period. There is not really any other way to do that.

Bruce had the devotion to detail needed for his work and a very reserved personality. It was Starpath Publications  who boldly added "Stark Tables" to his original title much later when we joined in to help with its publication and distribution. 

So it might be understood how pleased we were when he agreed to present a seminar here at Starpath on lunar distance and related Lewis and Clark navigation. This was in April of 2001, organized in conjunction with John Lewis, Puget Sound Maritime Historical Society, along with a representative of the National Park Service.  Bruce was nervous about an event like this that he had not done before, but it all went very well, and we became friends ever since, ultimately leading to our publication of his book. He went on to participate in other National Park Service events about Lewis and Clark. 

We were doubly fortunate and extremely grateful for his later donation of much of his navigation research library to Starpath, which is indexed online at www.starpath.com/library. We have made remote use of the documents open to the public and a several researchers have taken advantage of that. 

Bruce was active in navigational matters up to several years ago. One of his later interests was readdressing how much cel nav computations could be carried out with a simple slide rule, which he and I both carried around on our belts in early college days.

What always struck me was our shared strong opinion that dead reckoning remains the basis of good navigation. In many posts he would note that quality dead reckoning was as important as cel nav itself, and he ask for and motivated others to write more about it in the Newsletter, which was just one more important contribution to navigation practice.