There were several questions raised in that note that have taken more research. In short, that note just described the situation without providing any reasons for it. Since then I have made progress on understanding these ENC properties, thanks primarily to good support from NOAA's Office of Coast Survey (OCS), who do indeed care about the best use and understanding of their products.
There are two basic factors that lead to the unusual properties we end up with on the presentation of the ENC—I am not discussing the content of the charts at all here, just the issues of size, shape, boundaries, and scales.
First, the US ENC are made from the available RNC. That is, to make a vector chart of a region, step one is bring out the largest scale RNCs of the region, and then digitize these products in a standard ENC format. Digitizing means converting a curved shoreline or depth contour into a sequence of straight line segments and storing a data base of these points, which is then included in the ENC files we load. Then an echart program can draw the chart on the fly each time we change a display scale or move to a neighboring or overlapping ENC. It is a complex process to make these, but much of the data are already digitized and all new survey data comes in digitized as well.
Then at the location of each aid to navigation (ATON) the data describing it are entered into another data set included in each ENC. (Though probably not so well known, the header file components of RNC kap files include the precise Lat-Lon of each prominent aid as a georeference point of the RNC, which insures these are in the precisely right place to match the Light List and latest Local Notice to Mariners; recall RNC and ENC are updated as needed every week or so.) It is this step of the ENC production that could add much more ATON information than possible on an RNC, and often this is the case—but not always. In the case of rock symbols, for example, the ENC descriptions sometimes include less information that we can get from the rich set of RNC rock symbols described in Chart No. 1.
That is the way the ENC are produced, but they cannot be published in this direct one-to-one relationship between a new ENC and the long established RNC, which brings us to the second factor affecting the ENC boundaries we see.
The reason they cannot be the same is the International Hydrographic Office (IHO) has standards on these ENC echarts that all participating nations must follow, and the US is indeed a participating member [see References]. This situation has pros and cons. The main con is, the US does not have complete freedom to produce ENC in any scheme they might deem most logical based on our RNC system. The main pro is we end up with charts that match an international standard that better serves the thousands of foreign vessels that visit the US each year, and indeed then makes foreign ENC familiar to US mariners visiting their waters.
The primary IHO restriction that affects the boundaries and scales of US ENC is no ENC can overlap an adjacent one within the same scale band. That brings us to a key factor in understanding the boundaries of ENC, namely the concept of scale band or usage band. The IHO defines 6 usage bands, and recommends the scale ranges that each of these bands should cover. The table below summarizes these as best I can tell. These definitions are useful for selecting charts in catalogs and for understanding the names, and in turn the ENC boundaries we see in navigation programs.
The US values do not coincide with the IHO values because the US had been making these ENC before the IHO made their recommendations. The British Admiralty ENC is marked with a footnote [*] because although they do use the IHO values for their own ENC, they do not use the name Scale Band, but more often prefer Usage Band. I think the reason for that is they have a vast worldwide chart distribution network (they distribute the charts made by other nations), and different nations have different definitions of the actual scales in each usage band as shown. The Canadian one is notably different.
The scale band is a key to understanding the names of all ENC. In the case of US products, the first digit after “US” is the scale band. Thus for the chart we discussed earlier, US5WA18M, the US means US product, and the 5 means this chart has a scale band 5 (i.e. between 1:5,000 and 1:50,000). The WA means the chart is either wholly within the state of WA or when covering overlapping states, the majority of the chart is in the state of WA.
Scale bands 3 to 6 are generally associated with a specific state. Bands 1 and 2 are often labeled: EC (for East Coast), GC (for Gulf Coast), and WC (for West Coast), but there are some overlaps in these descriptors with the state descriptors.
We can think of the “M” as a reminder that the native units of these charts are metric (although we can display them in other units if we choose), or we can think of it as a filler since all ENC worldwide must be 8 characters long—I would have to agree that dealing with an extra M is better than dealing with leading zeros.
The fact that the actual sequence of numbers (i.e. the 18) do not always follow a logical sequence along a waterway is just a matter of history in the production sequence, much as the chart numbers of the RNC are not in any particularly logical order. If anything, the ENC are more often in sequence than the RNC counterparts.
Thus we have this sequence of charts coming in from the ocean toward San Francisco:
US2WC12M (1: 1,200,000)
US2WC06M (1: 811,980)
The first two (scale bands 1 and 2) can overlap as they are different scale bands, but none of the three band-2 charts can overlap. Likewise the last two here (band 5) cannot overlap and will change charts at a specific line. The NOAA Online Interactive Catalog is the best way to study this, short of having the built in convenient presentation of Rose Point navigation software, mentioned earlier.
In learning to use this system we might also keep in mind that the US ENC scale bands are not the same as historically been used in the US for paper charts. The US Chart Scale Classifications are defined in Chapter 1 of any US Coast Pilot and Bowditch as:
The IHO paper chart categories are also shown, in part to show these are broad categories with no definitive boundaries. In my own experience, I tend to think of 1:40,000 to 1:80,000 as "medium," being between "large" (less than 40k) and "small" (more than 80k).
It is not clear if this difference in scale range definitions between our traditional paper charts and modern ENC has any significance. It certainly provides navigation schools and USCG exams nice ways to make trick questions, but probably not much more than that. The standard US paper chart classifications never had, to my mind, much influence on practical navigation, but the new ENC scale band system does indeed have a meaning, i.e. it determines the third character in the name of any ENC, worldwide. It could be that the upcoming new edition of Bowditch will address this matter.
So now we can go back to our example from the earlier article to see why the boundaries of the ENC equivalent to RNC 18474 (ie US5WA18M) are what they are—again, the NOAA Online Interactive Catalog is the best way to follow along. When NOAA sets out to cover the ENC charting of the region covered by 18474 (scale 1:40,000), they start out by digitizing the largest scale RNC charts that cover that region. In this case, the north half of this chart is covered by RNC 18449 at a scale 1:25,000. (The south half of 18474 does not have any larger scale over the bulk of those waters.) These are both scale band 5, so to meet IHO standards this chart must be broken up. They cannot overlap, so that breaks up the region into two parts 5WA14 and 5WA18. Then we notice that RNC 18449 also overlaps RNC 18450, which is scale 1:10,000, which is also scale band 5, so it cannot overlap and has to be separated out. Thus we are building up the irregular rectangular shape of the ENC.
The same thing takes place to the south with overlapping 18448 and 18453, both also in scale band 5. Note that the Port Orchard and Gig Harbor charts, which are also in scale band 5, are not cut out because they lie wholly within their respective ENC cells. They do not “overlap,” meaning part in and part out. Individual ENC charts are often referred to as "cells."
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We are left then with just two points raised in the first article, namely the confusion that might arise from the ENCs internal description of the corresponding RNC given for each ENC. These will always correspond to the name of the RNC that has the largest overlap with the ENC that is of the same scale. There will not often be a close correspondence, and in fact this formula for the internal description of the ENC can often be notably wrong, as in the case of US4WA.
OCS is aware of the confusion this might cause in some cases, and they are considering solutions. It is a big project, however, to name these more precisely. In principle, an algorithm could be written to make these names more realistic. For now the key point for mariners is to simply understand this limitation when reading the ENC descriptions that their echart programs reproduce. They are not making this up, this is what the ENC says it is.
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The other issue is the area of missing US ENC in waterway between Victoria and Race Passage, covered by US RNC 18465 or Canadian RNC 3440. This waterway is all Canadian waters, which must have contributed to the results at hand. But we still have the unique situation that an American vessel could sail from an American ENC of Haro Strait’s Canadian waters toward Race Passage and onto another US ENC, but have to cover up to 23 nmi without any US ENC.
|Sample of the NOAA Onliine Interactive Chart Viewer|
It is not unprecedented that US ENC cover Canadian waters, but in this case we are forced to use the Canadian ENC, because of another IHO requirement that no two Nations can make an ENC of the same waters within the same scale band. This means the US had to cut out a section (now the missing part) of the US ENC equivalent of 18465 (called US4WA34M) and mariners must now use Canadian ENC called CA470075. This was a direct conflict of overlapping scale bands (note the 4 in each chart name) from two nations, both of which had previous ENC of this region. This particular cell is now covered exclusively by Canada as a direct result of negotiations between the US and Canada about how they were going to meet the IHO standards along all of their long and complex common borders. In short, this is a compromise in these negotiations.
This situation stands out because many commercial vessels, as well as some racing yachts, prefer to use the same source of ENC, which is not possible in this unique case. We need the Canadian ENC of these waters, CA470075.
Individual Canadian ENC were not available in the past, but as of a month or so, they are in principle for sale. This one has a price of CND$25. It can be seen on Canadian Hydrographic Service Interactive Chart Index, which is good way to view all of their charts, print and electronic… but save that link. It is hard to find.
|Sample of Canadian Online Interactive Chart Viewer|
I would like to make a special thanks to Steve Soherr of the Customer Affairs Branch at NOAA's Office of Coast Survey. He has helped me and many others with charting and related questions many times over the years. The above notes, however, are my interpretation of the resources, and any misunderstanding or misstatements are my fault alone.
References on IHO requirements and US-Canadian ENC negotiations
The key acronym at IHO for the topic at hand is WEND (worldwide electronic navigational chart database).
WEND Principles (from IHO)
Facts about Electronic Charts (from the IHO)
S-57 Interactive Object and Attribute Catalog (www.s-57.com) Takes some patience, but it is all there!
The US-Canada agreement on ENC
The specific case of Pacific NW waters.
As noted earlier, the ENC format is still a bit in flux, but not to worry. It will soon be replaced with a new standard and much of this might change. See S-101 Product Specification for the Electronic Navigational Chart