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Transmittals, exchange sets and loose cells

S-57 data is transmitted from place to place is a variety of formats. This can be a tad confusing. It confuses me at times and it confuses my dog. This article is an attempt to cast some illumination on the subject.

To begin with we need to be fairly precise about terms. Specifically we are not simply dealing with S-57 but the ENC (Electronic Navigational Chart) product specification. S-57 describes a fairly generic mechanism for exchanging geographic data. ENC is a specialization of S-57; a product. This describes a sub-set of all that might be possible with the broader S-57 and ties it down to some specifics. Eventually we get to a list of allowed entities, how they relate to each other and how they get updated.

The world is divided into cells. Cells are regular and rectangular. They are not the same as charts. A chart typically is designed for a particular purpose and is the right shape and coverage for that purpose. Cells on the other hand are designed to give a complete tiling so often several cells are required to give the same coverage as a particular chart. Cells come in six different flavors called Navigational Purpose which roughly correspond to scales.

1

Overview

covers the most area

2

General

 

3

Coastal

 

4

Approach

 

5

Harbour

 

6

Berthing

most detailed sort of cell

I use the word ‘roughly’ because there is no real consistency between producing agencies as to how the differences between Navigational Purpose are managed. In general the more detailed Navigational Purposes use a smaller compilation scale and so correspond to larger scale charts.

For each sort of cell there are two types of file that we are initially interested in: a base cell file and an update file. The base file is the first type of file that is ever issued for a cell and contains the all the basic chart data. Update files are produced later and contain incremental updates. That is they contain only the information needed to update a cell from its current version to the next version. So it is essential that updates are applied in order. This is what the Update Number is for.image

The name of a cell file is made up like this:

CCPXXXXX.EEE

| | | |

| | | |----- EEE = update number

| | |-------------- XXXXX = individual cell code

| |------------------- P = navigational purpose

|----------------------- CC = producer code

So, for cell EC09M which is Chesapeake Bay from a compilation scale of 1:200,000 (navigational purpose Coastal) produced by NOAA :-

· US3EC09M. 000 is the base cell.

· US3EC09M. 001 is the first update to the base cell.

· US3EC09M. 002 is the second update

· And so on…

Unfortunately this reasonably simple scheme is not the end of the story. Occasionally the producer of the cell will roll all the updates to date into a re-issued base cell. In this case the cell will be published as a base cell (.000) but the update number will not be zero. So the publication sequence might look like this:

File Name

Update Number

US3EC09M. 000

0

base cell

US3EC09M. 001

1

1st update to the base cell

US3EC09M. 002

2

2nd update to the base cell

US3EC09M. 000

2

re-issued base cell which include updates #1 and #2

US3EC09M. 003

3

1st update to the re-issued base cell and also the 3rd update to the original base cell

US3EC09M. 004

4

2nd update to the re-issued base cell and also the 4th update to the original base cell

So how do you tell the real update number from a .000 cell? Well fortunately it is usually possible to read this information from the cell. So encoding the update number in the file extension is interesting but stops short of actually being useful because you may well have to read the update number from the file anyhow.

We are still not done though.

Occasionally the cell producer will create a whole new edition of the cell. This is comparable to a new edition of a paper chart. In this case a new base cell is created and the update number also gets reset because subsequent updates will apply to the new edition base cell and not the old one. In fact it would be and error to attempt to apply an update for a new edition to a cell of the previous edition. This means we also need to track edition number which is not part of the filename at all but can be read from the cell. Here is the complete publication sequence:

File Name

Edition Number

Update Number

US3EC09M. 000

1

0

1st edition base cell

US3EC09M. 001

1

1

1st update to the base cell

US3EC09M. 002

1

2

2nd update to the base cell

US3EC09M. 000

1

2

re-issued base cell which include updates #1 and #2

US3EC09M. 003

1

3

1st update to the re-issued base cell and also the 3rd update to the original base cell

US3EC09M. 004

1

4

2nd update to the re-issued base cell and also the 4th update to the original base cell

US3EC09M. 000

2

0

2nd edition base cell

US3EC09M. 001

2

1

1st update to the 2nd edition base cell

US3EC09M. 002

2

2

2nd update to the 2nd edition base cell

Confused yet? There is more to come but this is probably the worst of it. To recap:

· A file name ending .000 is a base cell containing a complete set of chart data which can be displayed. We refer to these files as loose cells and it is the way that several agencies distribute their chart data. You need code to actually read the edition number and update number from the cell.

· A file name ending .001 to .999 is an update to a base cell. This does not contain information that can be displayed directly; the update always needs to be applied to a base cell first. The correct base cell will have the same file name but with a .000 extension. Unfortunately several of these may have been issued and the update will only apply to one of them.

In Nuno we display the combined edition and update number as a single decimal number. So for example 2.003 is edition 2 update 3 and 11.589 is edition 11 update 589. This is quite a convenient representation because it allows rapid comparison of both edition and update numbers in an easy and familiar way:- 3.004 is more up to date than 2.845.

Ok that’s loose cells pretty much dealt with but this is not how S57 data is supposed to be distributed. Next time we’ll look at Transmittals and Exchange Sets.

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