Friday, October 29, 2021

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" — 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 48ยบ 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.


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 .

Monday, October 11, 2021

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 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 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.