Official US charts with chart numbers started with the founding of the US Coast Survey in 1807 that evolved through several forms into the modern NOAA. Historic charts can be tracked down at historicalcharts.noaa.gov.
The first Boston Harbor chart, for example, was called No. 337 in 1836. It was then replaced with No. 246 in early 1900s, and then in 1974 the system many of us grew up with was established.
And then this chart was called 13270, a number that lasted until its final edition, when all paper charts were discontinued at the end of 2024.
That numbering system grouped charts using the first two digits as follows:
19xxx = Hawaii charts
18xxx = Pacific Coast and Salish Sea Charts17xxx = SE Alaska16xxx = Alaska14xxx = Great Lakes
13xxx = Atlantic, Long Island to Maine
12xxx = Atlantic, Cape Hatteras to N end of Long Island11xxx = Gulf Coast and Atlantic up to Cape Hatteras
To reminisce on this and learn all those chart numbers, see How to Get a Copy of a Discontinued NOAA Chart.
With the demise of the the NOAA paper charts—which brought with it the end of all raster navigational charts (RNC)—the only official charts left were the electronic navigational charts (ENC). The "sunsetting" of the paper charts should not have been a surprise; it was announced in 2019 and they began the process almost immediately.
Many mariners first confronted ENC this year, but they are not at all new. They were defined by the IMO and IHO in 1990, approved for use in 2000, and they have been mandated to be used by all SOLAS vessels since 2018.
NOAA's first official ENC were published in 2003. The original NOAA ENC (now called legacy), some of which are still in use, especially on the West Coast, have wildly irregular shapes, and multiple scales (over 100!), that come about from the strict IHO rules on how they must match up and not overlap, yet stemming from existing paper chart data. Below is a sample from the Eastern Strait of Juan de Fuca.
The shapes and scales of the legacy ENC are chaotic, but the file names (chart numbers) are in fact fairly systematic.
The chart number starts with the originator's country code (US) followed by the scale band of the chart, which is the compilation chart-scale range for legacy ENC, followed by the state abbreviation (USPS conventions), followed by two digits which is the chart ID, and that is followed by an "M." No one seems to know why this is an M. We do know the full name according to IHO must be 8 characters, so they needed something. Some have suggested it means metric, but we do not need to be told that because all ENC, worldwide, must be metric, plus others have said there is no evidence this means metric. The "no one," "some," and "others" referred to are NOAA personnel.
So, for example, chart number US5WA16M means:
US = United States made the chart5 = scale band 5, which in legacy terms means 1:5,000 to 1:51,639WA = Washington state chart16 = chart IDM = filler to make 8 characters
Note that ENC retain actual chart names similar to the paper chart they were based upon. US5WA16M, for example, has the name "Approaches to Admiralty Inlet, Dungeness to Oak Bay."
Here are the scales in use
There is a detailed explanation of the rescheming program on line at Rescheming and Improving Electronic Navigational Charts.
The rescheming began slowly in late 2019, early 2020, which introduced a new numbering system, but it was not described on a NOAA website till mid 2024. I have since discovered that it was indeed presented on an IHO website in 2017.
The new chart numbering system was not noticed much because the rescheming was slow getting started, and we had most of the time just a few reschemed charts amid mostly legacy versions—as we have now in WA state. But the reschemeing is now nearly done, and the new numbering has since been added to the latest rescheming overview.
There are still 8 characters to the name of every ENC worldwide, and they still start with the national IHO Producers Code (US, CA, FR, GB...), followed by the scale band, but the meaning of the rest of the characters is new.
The regional code replaces the simple US state abbreviation, and the cell location within a matrix replaces the previous (chart number + M).
In principle, and in some cases, this is a simple system. For example look at scale band 3 charts in LA.
The above are scale band 3 charts for LA with neighboring charts from TX, MS, AL, and FL. Green ones are available now. Red ones are planned.
Many states, such as LA, have only one regional code, which is the state abbreviation + 1, LA1 in this case. So all charts in the state use regional code LA1. Other states like CA, WA, NY have multiple regional codes, i.e., WA1, WA2, WA3, etc.
We see that the SW corner is cell location (A, A); they then increase to the north and east, ending when it reaches a neighboring regional code.
It would be nice to stop at this point saying that is the new system, but that would would leave out some details that might confuse us when encountered.
First, we note that for scale band 5, the regional code based on state ID can be replaced in the vicinity of some specific ports with its United Nations Location Code (UN/LOCODE). Also when multiple state codes are used, it is not always clear what the logic is for their specifications. See for example, this plot of scale band 5 charts of NY.
But looking at how they are named, we see the influence of the new rules.
NY has more than 1 region code, and on scale band 5 they use NY1, NY2, and NY9. In California, scale band 5 uses CA1, CA2, CA3, and CA4, which appears to be latitude and scale band dependent. In NY, the codes seem to be longitude dependent, but not in sequence.
Also in NY we see the use of three UN/LOCODES: NYC (New Your City), HEP (Hempstead), and PTJ (Port Jefferson). The location code abbreviations are likely understood by local mariners, but likely less so by visitors. Zoom in on the charts, and it should be clear what they refer to.
Just about every city in the world has a UN code, which mariners might be familiar with on some level, because the ones that are ports are used in Type-A AIS voyage data for the reported destination. That AIS output is manually entered, and mariners can put what they want, but the intention is to use the UN codes, which is common. Looking at the practical side, however, many ships forget to change this once they leave a port. So often the destination broadcasted is actually where they just left from!
So the summary is, most ENC chart names still include the state, but it will be followed by a number whose whose meaning may not be obvious and may depend scale band.
The exceptions are the scale band 5 charts that instead of the state regional codes, will (on just a few charts) use UN codes for what NOAA refers to as "Principal ports (based on cargo, fisheries, and tourism)." NY uses three of these, one of which is certainly more "principle" than the other two. CA charts in production propose to use four of these: OAK (Oakland); NTD (Port Hueneme); LGB (Long Beach); and SAN (San Diego). One might not have guessed that Port Hueneme (why-NEE-mee) made the list, but we probably learn more about this Port because of this.
The UN/LOCODES are also used in principle for scale band 6 ENC, but the NOAA does not yet have any of these—and the couple we used to have are now discontinued.
The last point to address is the cell location ID at the end of the file name. It is based on a matrix, unique to the state and to the scale band, whose SW most cell is called AA. In some cases, as above for LA scale band 3, the pattern is clear. In other chart regions it is not clear, and what we see in the SW corner is a matrix location that is not easy to decipher.
Below is a sample of where the DE and NJ charts meet.
The NJ charts look fine with AA in the SW corner, but the DE charts start with DE in the SW corner—which is just a coincidence that is trying to fool us! It has nothing to do with the state abbreviation. This is the D row of the E column, with the location of the non existent AE chart indicated here in red. This chart has not been created yet, but it might be once the legacy chart US5DE10M gets reschemed.
And to see why this is AE instead of AA, we have to look to the NW. Delaware has charts running up the Delaware Bay, as shown below.
Again, the sequence of DE charts (blue) runs into a legacy chart (USDE13M) that has not been reschemed. Once that is done, we see where there might be a chart LA next to the existing LB, which would mark the A column that we can extend south to see the matrix location of a chart AA—if it were to exist. A key to understanding this cell location system is to realize that there does not have to be an actual chart for every location in the grid. Notice that FF is missing in the above graphic.
So it seems that NOAA starts by defining an area that is going to be reschemed at, say, scale band 5. Then they lay a rectangular grid of scale band 5 charts over that area, which are labeled starting with AA at the SW corner. Then wherever they make a chart under that grid it gets that grid label for its location specification. In some cases AA location will be a real chart, but in many others it will not.
Hopefully this helps understand the intended matrix system.
So we have a new chart naming system that is better than the paper chart "eighteen-thousand means West Coast," and better than the legacy ENC system, which added a state designator, but the rest the name conveyed no information, and could be very misleading.
In US5DE13M, shown above, for example, the same chart covers three different locations, separated by as much as 20 nmi, and adjacent legacy chart numbers had no relationship to each other. We knew that system had to go, and now it is nearly done.
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