Sunday, December 10, 2017

Why the Book "Hawaii by Sextant" is Unique

There never has been in the past, nor will there likely be in the future,
        such a thoroughly documented study of a voyage
        relying purely on celestial navigation to cross an ocean.




The future part is easy. It is near impossible to find an ocean going vessel without a GPS on board, or in someone’s cellphone. From a legal point of view, it would likely be considered negligent to make such a voyage without GPS.

One might argue that a voyage could be or was navigated by cel nav without looking at the GPS, but even that does not really count. Knowing you have a backup solution changes the mentality of the navigator and biases the navigation. Not to mention that you are less likely to stand on deck with sextant in hand for hours waiting for the sun to peek out for a few seconds to get a sight. With a GPS in a box somewhere, you can more likely gamble that you will eventually get a sight and not have to work so hard at the moment… nor would you be forced to study limited data for hours to figure the most likely position.

Navigators can certainly document good cel nav practice underway in the ocean with detailed information, and such studies are indeed valuable contributions, but that is different from relying on it as the sole source of navigation, regardless of conditions. This book shows what it was like to navigate by cel nav with nothing else to go by but compass and log. It raises the questions you would have to face, and proposes solutions to analyzing difficult data.

Furthermore, suppose such a voyage were carried out and good records maintained. Then we have to fold in the probability that someone would devote the enormous amount of time and energy required to organize and present the information in a usable manner for students. We venture that this is highly unlikely…. maybe a few days sail, but not across an ocean.  A look into the past treatment of this challenge only reinforces this factor.

Why no such study exists from the past is a more interesting point–especially since we describe our book as being “in the spirit of early Bowditch editions.”  Bowditch’s American Practical Navigator (1802 up until 1900 or so) and also Norie’s Epitome of Practical Navigation (roughly same period) are two classic 19th century texts on navigation.  And sure enough each of these do include very detailed practice voyages with celestial sight data and logbooks. It is curious that they all involve voyages to or from Maderia, Spain, which must have some historic significance, but not an issue now.

The key point is that even though these records list the vessel names, voyage dates, the captain’s names, and the log keeper’s names, they are all fiction when it comes to the celestial navigation. In short, they made up the data to demonstrate what they wanted to teach… which is what all navigation teachers do to this day at some point.

These Bowditch and Norie Journals may have been based on actual voyages made at some time in the past, but the data presented is blatantly artificial. (We leave it as an exercise to confirm this observation. The books are online.)

We agree with these masters of navigation that this is the best way to teach the process. And certainly we do not even approach the skill and seamanship they represent, nor can we hope to emulate the high standards they set in navigation. The only point we make is data in Hawaii by Sextant is real; the comparable data in these classic texts was manufactured… and we know as well as anyone why they might do what they did.  After an ocean passage under sail, it is sometimes difficult to put the pieces back together to present a coherent picture of the full navigation, day to day–not to mention that the many details required to reproduce the results are tedious. Few would consider them worth preserving. As we point out in the text, and you see in our own records, when the going gets tough, the boat gets more attention than the logbook.

In more modern times there have also been a couple books published that present ocean exercises in celestial navigation, but these too have been based on manufactured data.  We thus maintain that Hawaii by Sextant is a unique contribution to the library of navigation textbooks.

It is obviously not at all unique to sail across an ocean by cel nav alone. Thousands of mariners over the years have done so. Bowditch did many times. It is not clear that Norie actually navigated. He was more famous as an author and book publisher at the time. He was the founder of what is now called Imray Nautical Publications in UK. That fact also is not surprising. Many early classic texts on navigation were from scholars who did not have practical experience underway. Bowditch, Lecky, and Thoms were notable exceptions.

PS. We have been told that there are several highly experienced navigators who do still have all the records of their early voyages done by cel nav alone.  We look forward to their publications if they choose to do so.  The more data we have of this type the better our learning will be. If these are from larger vessels (we understand they were commercial ships), then that too will add another perspective.


Wednesday, November 22, 2017

MSLP vs. MSLP

Someone has to care about the details of marine navigation and weather, or things slip by, which might one day show up and cause confusion, and we don't want confusion. Indeed, a hallmark of good navigation and seamanship is clarity in communications. Today, we ran across a gold nugget of doublespeak: MSLP.

We have published a book called the Mariners Pressure Atlas, which contains pressure statistics that are difficult to find, despite their great value for weather tactics in tropical storm prone waters of the world. The book contains global plots of the mean values of the sea level pressure, called mean sea level pressure  (MSLP) patterns (isobars) along with the standard deviations (SD) of these pressures on a month to month basis. The SD are a measure of the variation of the pressure we can expect purely from a statistical spread around the mean value. Sample sections are below.


These are the mean sea level pressures (MSLP) in this part of the world in July. Below are the SD values.


For example, in the tropics with a MSLP of 1012 mb and an SD of 2.0 mb, we know that an observed pressure of 1010 is one SD below the mean and a pressure of 1008 is 2 SD below the mean. When we observe an average pressure of 1007 mb, we are 2.5 SD below the mean. That takes on special meaning when we look at the probabilities.


In other words, the probability of normal pressure fluctuations being down 2.5 SD is 0.6%. A pressure that low is almost certainly (99.4%) not normal fluctuation—that is the approach of a tropical storm! The wind will for sure not warn of that at this point, and maybe the clouds on the horizon might not either, but there is indeed a tropical storm headed your way.

This powerful storm warning technique was well known in the late 1700s, early 1800s when ships carried accurate mercury barometers, but unfortunately with the advent of aneroids, by1860 or so this knowledge slipped away because they were not accurate enough then to do this job.  Eventually even the textbooks stopped talking about absolute pressures and just started preaching up or down, fast or slow, which is useless for this type of long range storm forecasting.  Now we have accurate barometers (including accurate aneroids), which is why we rejuvenated this classic method... mentioned in Bowditch, but sadly without a link to the crucial MSLP and SD data.

There are spot values of mean sea level pressures in Appendix B of the Coast Pilots.



But that is not the point at hand. What we are dealing with in the above is the mean value of the sea level pressure, which we periodically see abbreviated MSLP in weather and navigation documents.

Now, however, look at these weather maps from Australia and Canada.  The UK Met Office also uses the MSLP notation to describe a surface analysis map.



Now we have an all new meaning of MSLP.  This cannot be the mean sea level pressure we discussed above;  these are the actual values of the pressure at sea level at the valid map time. What is going on here is they are not calling the reference plane "sea level," which we often see, i.e., sea level pressure (SLP), but instead they are calling the reference datum "mean sea level."  MSLP in this context is the same as SLP.

Here is an example of aviation weather (METARs) using SLP; it is also used in some numerical model outputs.


Here is another example that shows the fluidity of the terminology.  The ECMWF defines

"MSLP is the surface pressure reduced to sea level." 

So they know that "sea level" is the same as "mean sea level," but they choose to help us make our point!

It is not unreasonable to tack on the "M"; the sea level does change with the tides (to a good approximation mean sea level, MSL, is halfway between MLW and MHW), not to mention that it varies with the pressure above it (called the reverse barometer effect). In fact, MSL is an even more complex concept, but in ways that do not at all effect our use of it as a pressure reference. For present context, this just reminds us to think through the terms we use.  We might note that on nautical charts building, towers, lights, bridge clearances are referenced to MHW, but spot elevations on the land and elevation contours are actually referenced to MSL.

We thus have in common navigation conversation both:

     MSLP = M - SLP,   being the mean value of the sea level pressure, and

     MSLP = MSL - P,   being the pressure at mean sea level.

If you found this abbreviation in some context of your work, and then went to a navigation or weather glossary to look it up, chances are probably only 50% that the glossary will come back with the appropriate answer for your inquiry. Put another way, you will not find an official glossary that has both definitions; they will have one, or the other.

... which I thought we should document, so no one gets the impression we are sitting around the office all day working on trivial matters.

PS. Just ran across this at the Navy site (FNMOC):

Sea Level Pressure (MSLP): The model-estimated pressure reduced to sea level. Units are in millibars; contour intervals are 4 mb. 

Maybe the "M" stands for "model-estimated"?