Charts are square, countries aren’t, it’s a problem. There may be a vision of a utopian world where country boundaries are straight and align with a geodetic grid but the reality, as usual, is much messier. The tricky bit is that each national hydrographic office will create ENC cells with bits missing. The political ramifications of drawing a map of someone else's country can be severe. Politicians just don’t like this sort of thing and Google have managed to upset people to a remarkable degree by misplacing the odd line. So the only sensible thing to do is to create chart up to the national boundary and then stop. Of course by safe I mean politically safe. For the mariner it is a pain in the transom akin to sailing off the edge of the world.
This means we end up with multiple ENC cells of the same area at the same compilation scale but from different providers. The reason you might care about this is that you could easily end up with duplicate or similar features from both cells overlaid on each other. Depending on how your chart display software renders this it will probably look a mess, could easily be confusing and might even be dangerous.
Fortunately the good people at the International Hydrographic Office have already thought of a solution to this. Unfortunately it requires a degree of international cooperation and we (I am speaking for the whole human race now) are not very good at that. None the less several countries have been giving it a go following the principles of the Worldwide Electronic Navigational Database or WEND. This is a great idea and would be even better if it worked. I am not saying it won’t work but there is a way to go yet. One small step in the right direction has just been announced by NOAA and the Canadian Hydrographic Service. They have conceded that the boundary between the US and Canada is not straight so they have agreed on using a wrinkly one instead.
ENC cells are often cataloged and managed by the coordinates of the cell corners. This is fine most of the time and implies that the cell is square. However within the cell is a coverage object which really defines the shape of the area covered by the cell data. This can be a polygon of as much complexity as needed. So if two countries cooperate they can arrange the coverage boundaries along the international border, each country then charts its own bit and everyone is happy. The UK and France are a good example of where this works well. What is happening up on the Canadian border seems to be a little different though. They are still dividing up who does what but, see the above diagram, they are not following the border. Instead they are making sure the the cells will fit together properly but Canada is charting some of the US and visa versa. Why are they doing it this way? I have no idea. Please tell me if you can shed any light on this. Pragmatically the reason does not matter too much. The end effect will (should) be a set of ENC cells which join up nicely.