Wednesday, May 8, 2024

Landmark Labels on ENC

We are one of the first in line to lament the poor coverage of terrestrial charting in electronic navigational charts (ENC) compared to the paper chart coverage we are used to.  And for good reason: we do most piloting relative to landmarks and much of the land mass on ENC is conspicuously blank—which can appear even more moonscape vacant depending how we have the display set up, as shown below.

Unlike viewing raster navigational charts (electronic copies of the paper charts), ENC let the user control many aspects of the display. Above we see an example of choosing to show "Important text only," which is a (misleading) official ENC display option.

If we compare that to what we see on the equivalent paper chart, we see what we are missing in that view.

It is not just the names that can be hidden, but NOAA ENC have very few elevation contours which can often help with piloting. 

Another reason we care about charted place names is a matter of basic safety and prudent seamanship. We teach that it is good policy to always keep in mind a verbal description of where you are, and maybe even note it in the log book that way, ie "Just passing west of Willow Island." Knowing this at all times we are prepared to describe our position over the radio in an emergency—which is much faster than finding, if you can, a read out of the Lat and Lon and reading that with its potential error.  Furthermore it makes the cruise more enduring if you learn these names as you go by.  The say-it-out-loud method is is also how we teach students to learn the stars in cel nav.

Thus these charted place names are valuable to navigation. But things are not so bad as they might appear.  They can be bad, as shown above, but they do not have to be. Below we turn on the text labels to see what we really do have in ENC.

The charted place names are actually all there on the ENC, they are just not as prominent as they are in the paper charts, which have the freedom to use large font sizes for some, and indeed print on a curve.

ENC have strict international rules of font size and orientation, although in some cases they do let labels (and associated symbols) move on the chart so that critical ones remain in view as you change the screen.  A folded paper chart on the chart table may be just hiding a note that a dashed line is marking a restricted Navy firing zone. On the equivalent ENC if you panned that notice off the screen it would suddenly reappear in a new position in view. 

In other words, the use of labels on an ENC is just one more aspect of the new chart reading skills we need to develop for ENC. We have to look at the charts in a new way. One thing that helps with this is the rule that  ENC chart symbols and labels stay the same size regardless of the display scale (zoom level). Thus crucial matters may become more apparent as we zoom into the region of interest.

The Future of ENC

As for other deficits of the terrestrial coverage of existing NOAA ENC, we can be confident that this will improve. First of all, a few nations do a better job with the elevation contours already, and the US certainly has extensive GIS data for all aspects of US mapping. 

To show that big agencies like NOAA should be able to solve this problem fairly soon, we can show how to do this ourselves already.  Beyond its outstanding ENC display presentation, the popular navigation and weather app qtVlm also offers the option to overlay on the chart GIS data as shape files  (.shp), a standard format for GIS data. 

In the sample below, I followed the instructions we have online  to add the roads to Lopez Island and water bodies and elevation contours to Blakely Island, and then (within qtVlm) limit the contours to the 100 ft intervals shown on the paper chart.

Once these are installed, we can get a tool tip presentation of the road names, heights of elevation contours, and related data for water bodies. In fact we learn there are more lakes on Blakely Island than the paper chart showed.

In other words, there is good reason to expect that the terrestrial coverage of future ENC will be even more valuable than that of the paper charts they are replacing. 

We are likely to see this take place first in the printed versions of the ENC called
NOAA Custom Chart (NCC). These are intended to be the (non-official) paper backups of the official ENC viewed on a computer screen or chart plotter.  NCC are user-created online from the NOAA NCC app that produces a PDF chart of the desired region, scale, and paper size, based on the ENC content for that region.  Then it is up to the user to get the chart printed at the chosen paper size.

It is during this NCC production that NOAA could offer the GIS overlay options such as elevation contours, roads, building, water bodies, etc to be added to the PDF they are creating... essentially just as qtVlm offers users the option as shown above. Thus we could end up with a new-generation of paper charts that are indeed superior to what we are now accustomed to.

Seeing this new data in the actual ENC themselves is likely further down the line. Even though a few other nations already have better contours, roads, and buildings, NOAA is likely pretty tied up with their massive process of rescheming all the ENC, which is a major ongoing improvement to the watery parts of the ENC. Not to mention that all nations are in the long process of preparing for the next generation of ENC, where the present IHO S-57 standard will be replaced by the new S-100 standard, which inherently includes a lot of new GIS content. These proposed changes are discussed in our text Introduction to Electronic Chart Navigation.

Monday, May 6, 2024

The World Sees Atmospheric Pressure at Work

This week is the 30th anniversary of the opening of the EuroTunnel (Chunnel) between England and France. The BBC commemorated the event with a story about the first underground meeting of the tunnels being dug from both sides that took place on Dec 1, 1990, four years before the actual opening of tunnel to traffic in 1994. They met roughly mid channel, with TV cameras at hand.

The fellow on the British side with orange t-shirt is Graham Fagg who in 2010 gave a description of the event, which can be heard on the BBC Witness program. In that recording from (3:59) to (4:28) we learn that when the hole was opened up big enough to walk through there came a sudden wind from the British side to the French side that was strong enough to blow his helmet off.  That wind is the subject at hand.

This wind is quite literally what we call in marine weather a channeled wind! It means the pressure on the UK side was higher than that on the French side and the area between the two sides was confined by a narrow channel. We just have a case here of a very narrow channel, not just steep hills on two sides.

Our goal is to estimate what that wind speed was, which is an exercise in resources—meaning, can we find the actual pressures at both ends at that time, and then can we make some semi-reasonable estimates of the wind speed.

Below shows the Chunnel viewed on Meltemus charts of UK in qtVlm, with overlaid ECMWF reanalyzed surface analysis for the approximated break-though time (11 to 12 UTC, Dec 1, 1990) when the wind was noted. (The New York Times had a good article about the event, but gave the wrong time of day due to a time zone error! — no link here as they no longer let non subscribers read their articles.)

The red line is the route of the Chunnel. The isobars are shown at 0.1mb spacing.  The inserts are meteogram plots of how the pressure varied throughout the day at both ends. The pressure gradient across the channel did not change from 11z to 12z, at (1036.0 - 1035.5)/26.9 = 0.5mb/26.9 nmi.

The ambient surface wind at this time was about 10 kts across the channel, but we must use wild approximations to estimate the wind in the tunnel.

We can for example just use the basic formula for wind responding to isobars that leads to the wind we see on the surface. We derive a simple formula for that in Table 2.4-1 of Modern Marine Weather:

 U = 40 kts/ [D x sin(Lat)]

Where D is the pressure gradient expressed in a special way. Namely it is the distance between 4-mb isobars expressed in degrees of Lat. On a map, we put dividers across adjacent isobars, then move that to the Lat scale. If the distance between the two isobars on either side of the point we care about is 180 nmi, then D = 3.0. 

So we have to convert our tunnel gradient to that format starting with: 0.5 mb = 26.9 nmi. 

0.5 mb x (4/0.5) = 4 mb = 26.9 nmi x (4/0.5) = 215.2 nmi = 3.58 Lat degrees (at 60 nmi per degree).

U = 40 kts / [3.58 x sin (51)] = 14.4 kts

Then for surface winds we have a surface friction reduction of 0.8 or so that leaves us with 11.5 kts, which essentially agrees with the observed surface winds—which should not be a surprise as that is the basic procedure used by the models, with a few subtle corrections.

The above is based on the physics of wind flow, but still a large stretch to project that thinking into the tunnel. It is at least a plausibility argument for the rough magnitude of  the wind.

In our textbook in Sec 6.2 on Wind Crossing Isobars (page 146) we give another way to approximate wind flow in channels that is purely empirical, meaning not computed, just observed. It is a rule we compiled based on how the local NWS forecasted wind speed (in the old days) in the Strait of Juan de Fuca and in the Puget Sound based on the pressure differences at each end of these channels. Our composite guideline is this:

Channel wind (kts) = 800 x Pressure gradient (mb/nmi),

which we can easily apply to what we know:

Channel wind = 800 x (0.5/26.9) = 14.9 kts.

So again, we see the order of magnitude of the wind speed we might expect in the tunnel.  And again, we cannot consider this rigorous science; wind flow in restrictions is very complex. We have just confirmed that indeed the wind was going the direction observed, and also about the right speed. We use this same approach to forecast or anticipate wind changes in our own waters based on pressure changes. It is part of our Local Weather web page.

There is also a physiological element of confirmation. The force of the wind is proportional to the wind speed squared. The force of 14 kts of wind is twice that of 10 kts of wind. At 17 kts it is three times stronger than 10 kts. In other words, there is a dramatic difference in what we experience in 10 kts vs even just 12 kts.

We know from our own experience that we could be in a wind of 10 or 11 kts that could blow our hat off if it hit at the right angle. And it would be noted, but not a focus point for any newscaster's story. But if this wind were much more, we know that it would be a focus of the conversation, which it wasn't. Note too that the wind came not at the moment when the flags were exchanged, but later when they had the hole opened up enough to walk through.

In other words, without any math or science considerations, we might guess the wind was about 8 to 10 kts, because less than that would not blow his helmet off, and much more than that would have clothes rippling in the wind and newscasters talking about it, which they did not.

Such visual effects of the wind is not unlike our view of whitecaps. At 10 kts there are some, if we look carefully; at 15 kts they are easier to see; but at 20 kts they are the dominant factor noted when looking at the water. 


Pressure remains a concern in all such tunnel travel due to the piston effect that can create high pressures in front of the train stressing gear and making travelers uncomfortable. The Chunnel has build in pressure escape valves all along the tunnel to prevent this.

Chunnel prices seem to be like Amtrak, which depends on availably, and season.