Tuesday, November 1, 2016

Rock Talk 2: RNC to ENC

In a recent note (Rock Talk—Is it all awash, or not?) we discussed minor conflicts in paper-chart rock symbol terminology for a rock that covers and uncovers, height unknown (US supplemental symbol Ka and INT 1 symbol K11). One of the virtues of moving onto electronic navigational charts (ENC) is we no longer rely on the design of the symbol to convey specific information, and instead we get this from a direct query of the ENC database. This database uses an internationally accepted terminology that overrides national nuances.

Below is a summary of common paper-chart rock symbols with a graphic presentation of their relative soundings.

Figure 1. Common rock symbols used on paper charts and RNCs.

ENC charts, however, have simplified the actual symbols, not just for rocks, but also for other objects traditionally shown on paper charts. Part of this difference is schematically shown in Figure 2. Going forward, we can abbreviate "paper charts" with RNC (Raster Navigational Charts), because that form of an electronic chart is just an image of the actual paper chart.  The figure caption also refers to INT 1, which is the International Hydrographic Organization (IHO) standard for paper chart symbols.

Figure 2. Rock and terrain symbols are greatly simplified on ENCs. Left is a hypothetical RNC (paper chart, INT 1); right is the corresponding ENC. The six or so INT 1 rock symbols used on paper charts are presented on ENCs as just two symbols, with detailed attributes found by cursor pick. Likewise the attractive but challenging INT 1 tree symbols are replaced with a generic tree and cursor pick. Using ENCs, navigation schools will have to forgo tricky test questions distinguishing INT 1 coral from rocks, as these two are a single generic symbol on ENCs. Click the symbol to find out which. (We leave it as an exercise to decide which is coral and which is rocks in the RNC.)

The use and meaning of rock symbols on ENCs are not the same as they are on paper charts. We have gone from seeing some 6 or 7 "rock symbols" that tell us much about the rock from the symbol alone and knowledge of the tide height, to seeing just two different rock symbols on an ENC, some of which may not show at all, depending on how we have the optional soundings set—which brings up the crucial ENC topic of the "isolated danger" symbol, which is a powerful new feature of ENCs; the subject of a later note. 

When using ENCs, we must learn to get crucial information from clicking the symbol (called a cursor pick) and reading about it in another window. The rock symbols alone no longer convey detailed information.

This change in chart reading practice required when using ENCs can be a challenge, depending on individual experience. Having used paper charts for 30 years, my initial attitude toward these simplifications of the symbols, and rock symbology in particular, was negative, and I was not timid in complaining about it. However, the more I have used ENCs and studied the goals of the IHO in their "new" system (it is actually some 10 years old at this point!), I have changed my opinion on this. 

Although we might miss our traditional symbols, there is much virtue in not having to learn all the nuances of the traditional US rock symbols in the first place. In fact, many mariners who did not need to know these details to pass a navigation exam may not have been aware of all the information contained in the paper chart symbols. It is not even that transparent when searching Chart No. 1, without training on the use of that important publication.

Using ENCs, we only have to teach that there are two types of (common) rock symbols, an isolated asterisk or an isolated plus sign in a dotted circle. To know more about that rock, just click it. The procedure is easier to learn than memorizing multiple symbols, and probably a safer way to use charts. We must train ourselves to click every rock that is near our route. 

An asterisk is a rock that covers and uncovers as the tide changes between 0 and MHW (a K11 or K12 rock in INT 1), and a circled plus sign is a rock which is covered when the tide is 0 (K13). Note that we see a plus sign with or without a ring of dots on an RNC, but it aways has a ring of dots on an ENC. A ring of dots in all charting means a special hazard.

The way we learn more specifically what an asterisk rock means on an ENC is a cursor pick. Then we will be told the "value of its sounding."  An asterisk rock with the (9) beside it on an RNC will report a sounding of -9.0 ft in the ENC. In ENC reports, a negative sounding is a drying height.  

Figure 3A K11 rock shown on an RNC (left) and on an ENC (right).
Note drying heights are negative numbers on ENCs. Here the drying height is 0.8m = 2.6 ft.

A plus sign with 4 dots on an RNC (INT 1 symbol K12), will show up on an ENC also as a plain asterisk, and a cursor pick will report sounding of 0.0. Recall that all soundings are relative to 0 tide height, so when the tide is zero, this rock is right at the surface.

Figure 4. A K12 rock shown on an RNC (left) and on an ENC (right).
A sounding of 0.0 means this is a "rock awash" in the IHO definition, but not in the Bowditch definition, a point we discussed in 
Rock Talk—Is it all awash, or not?

Figure 5. Another example of a K11 (NOAA Ka) rock shown on an RNC (left) and an ENC (right).

Figure 6. Example of a K12 rock shown on an RNC (left) and an ENC (right).  Both are official Canadian HO products. Note the slight color pallet differences between US (Figure 4) and Canadian RNCs (Figures 5 and 6). ENCs meeting IHO standards all use the same color pallet, regardless of national origin, which is another small step forward. 

Note, too, that on paper charts, a ring of dots is an indicator of extra danger placed around a single rock, group of rocks, or just a rocky area. On ENC, the underwater rock symbol includes the ring of dots regardless of the sounding on the rock if known—there is no plain plus sign rock symbols on ENC.

Notice in both Figure 5 and 6, how much "cleaner" and more precise the chart looks on an ENC in these examples, compared to the largest scale RNC for the region. This often can be an aid to navigation, BUT we must be careful when over zooming, as shown in both of these pictures. Over zooming an ENC maintains the sharp lines, which gives the impression of a higher accuracy, which might not be justified by the actual survey data the chart is based upon. Both RNCs and ENCs are intended to be used at their native scale, although in practice we often over zoom them. The difference is the RNC gets fuzzy and pixelated, which is a warning of sorts, but the ENC does not.

Here is a summary of the comparison of ENC and RNC rock symbols:

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