Tuesday, November 1, 2016

Rock Talk — Is it all awash, or not?

We are preparing new training materials on the use of electronic navigational charts (ENCs), which brings up an issue navigators face when moving from paper charts to ENCs—namely, rock symbols and how these differ between the two formats.

The most common rock symbol seen on a paper chart is a simple asterisk. This is a rock that shows above the surface when the tide is 0 or less, but it is covered by water when the tide is at or above mean high water (MHW).

Figure 1a. Rock symbols.

If there is nothing printed near the asterisk on the chart, then that is all we know about it. On the other hand, if there is an underlined number in parenthesis near it, ie (6), then that 6 is the drying height of the rock, meaning when the tide is 0.0 the top of the rock is 6.0 ft above the surface. At a tide level of 5 ft, the rock is just 1 ft above the surface, and any tide height greater than 6 ft covers the rock. The number is in parenthesis so it is not confused with nearby soundings.

When we start to work with ENC rock definitions, however, we run across a bit of a stumbling block in the definition of this isolated asterisk rock when there is no drying height charted.

For a historical perspective, we refer to the definitive American reference on chart symbols, the booklet called NOAA Chart No. 1, and specifically to the 8th edition, issued Nov, 1984. We see the rock in question as entry O(a) in Figure 1, which is distinguished from the same symbol with a drying height charted, which is rock O2.

Figure 1. Page from 8th edition of Chart No. 1, 1984

The plain asterisk rock, (Oa), is defined as  "Rock awash (height unknown)."  

Most symbols and labels used in the 8th edition of Chart No. 1 were based on those of an IHO (International Hydrographic Organization) resolution from 1952.  But note that the label of the "rock awash," (Oa) is in italics, in parenthesis, and not part of the normal sequence of rock labels in Section O. This is explained in that edition to mean that this is a symbol that does not have a counterpart in the 1952 IHO list of symbols. In short, this is a unique NOAA symbol, not an internationally adopted symbol, but there is more to this story.

When we move to the 9th edition of Chart No. 1 (April, 1990) we see two things. First, rocks are no longer in a Section O—Dangers, but are now in Section K—Rocks, Wrecks, Obstructions, and we see a new subsection of Section K called Supplementary National Symbols. The rock awash symbol is now called symbol "a" in this new list, which we might call Ka.

The change in section labels (ie O goes to K), and the motivation to separate out unique US symbols into a list called "Supplementary National Symbols" is likely due to the appearance of the first edition of the the IHO paper-chart symbol standards called INT 1 in 1987—between the 8th and 9th editions of US Chart No.1.

After the appearance of the IHO's  INT 1 standard, we find that the cataloging of common paper-chart rock symbols has remained essentially unchanged in the US from those of the 9th edition. A sample of the latest edition (12th, dated April, 2013) is shown below. 

Figure 2. Selection of paper chart rock symbols from NOAA Chart No. 1, 12th ed, 2013. (In this latest edition, "Aquaculture" was added to the K section.)

Notice that rock Ka is still a supplement and still called "rock awash." In contrast, if we look at the corresponding page of INT 1 (also called Chart No. 1) from Canada, UK, or elsewhere, we see: 

Figure 3. Selection of Canadian Chart No. 1, which follows INT 1. 

Notice that the US Rock Ka is part of the INT 1 K11 group, and it is in fact presented that way by all other nations.  Check the US definition of K11 and this rock is not included—that is, an isolated asterisk without any drying height specified.

You may fairly ask at this point—if not earlier!—why we care about such details?  The answer is this. Once we move to ENC usage, most rock information is included in the text descriptions of the rocks. The symbols are greatly simplified and we have to "cursor pick" (mouse or trackball click) the rock to read what kind it is. So a precise description of the rock is crucial.  We are also trying to figure out why this US Ka rock is left as a US Supplementary Symbol and not just moved into the K11 group.

It seems we are being called back to the definition of "rock awash." USA Chart No. 1 has always pointed out that if you need more help understanding the terms, refer to Bowditch, American Practical Navigator. Below is the latest Bowditch definition of "rock awash."
Figure 4. Bowditch Glossary (2002)

"Rock awash" with this definition (first part) is what we have called the symbol of a simple isolated asterisk rock on US charts for many years. (The second part applying to the Great Lakes where there is essentially no tides, is not relevant to the present discussion.) 

But we have to admit that the rest of the world does not use this terminology, and that fact comes more to the front when we start using ENC. The isolated asterisk rock K11 in INT 1 is defined as a "Rock which covers and uncovers, height unknown." This is frankly better and more precise terminology. 

The word "awash", outside of a US navigation context, means what we think it means: the top of the thing is above the water to some extent, with water washing up against it or just over it. Thus the US rock symbol Ka called "rock awash" really means "a rock that will be awash at some tide level, but we do not know what that level is." In short, it is a rock which covers and uncovers, height unknown, which is the definition of INT 1 rock K11.

It is not clear why NOAA maintains this definition of the Ka rock, unless it is tied to the tradition we see in the still active Bowditch definition and in navigation school training manuals (like our own). It seems that the rock Ka could be moved to K11, just as in the INT 1, and redefined as a rock which covers and uncovers, with height known or unknown above chart datum.  (We raise this point, but we are aware of the perspective: NOAA and its forerunners were making charts and explaining symbols long before most nations were, so they have history on their side.)

The word "awash" could then only appear in the definition of K12 (the plus sign with 4 dots) which is defined by both the US and INT 1 as "a rock awash at the chart datum," and which is, indeed, the IHO definition of "rock awash" as presented in S-32 the official IHO Glossary.
Figure 5. IHO Glossary, S-32, 5th ed, 1994. This is not the same as used in Bowditch (Figure 4), so we propose that the next edition of Bowditch include this alternative meaning.

It seems that change would simplify US paper chart symbols, and lead to an easier transition into the use of ENC. I know of at least one navigation school, who will no longer refer to the isolated asterisk as a "rock awash."

In a follow up note (Rock Talk 2 — RNC to ENC), we look at how rock symbols are presented in ENC. But before leaving I want to stress that the latest edition of NOAA Chart No.1 is a major milestone in such publications in that starting with the 12th edition, NOAA Chart No. 1 includes for each paper chart symbol the corresponding symbols used in electronic navigational charts, specifically those following the IHO S-52 standard used in ECDIS (electronic chart and display systems).

NOAA Chart No 1 is now more than ever a unique and especially valuable publication for all navigators, worldwide. There is a free pdf version online. To appreciate how lucky we are, the British Admiralty publishes this same data in two books NP 5011 ($35) and NP 5012 ($34).


A special thanks to Brian Voss, Librarian of the NOAA Library in Seattle. He has shown us many times the crucial value of real libraries and real librarians in this Google age. 

No comments: