Resolution

[Gudern]

Xpian you are right - since the very beginning I have been talking about PPI and not DPI.
I even explicitely said in the first post that I assumed no impression to avoid this confusion.

Redrobes and Waldronate thanks, you are on topic because what you wrote seems to have answered my original question.
I'll sum it up and please correct me if I misunderstood something.

- There is a physical support for the map (a screen) which is approximately a square 30 cm by 30 cm - it may be bigger or smaller but I take this example which corresponds to a diagonal of 42 cm.
- People generally watch it at a distance of around 40 cm. From Redrobes tutorial and Waldronate's remark follows that the eye resolves then approximately 200 PPI (or 80 ppcm)
- So the size of the map in pixels directly and uniquely follows from the 2 numbers above. It is 2400 x 2 400 pixels. If it is less, the eye will see individual pixels and the map will be pixelated. If it is more, it is useless because the eye won't resolve the additional pixels so that a bigger size is an overkill (and useless work)
- If I want to represent a continent 3 000 km x 3 000 km, each pixel represents 1,25 km in reality
- Follows that I have to draw the map so that everything that is bigger than 1,25 km must be shown because its size on the map will be AT LEAST 1 pixel.

Of course all that assumes that I do a map where the scales are real and not a map where there is no relationship between the real size of represented objects and their representation on the map. And this is indeed my idea.
So is it right that the answer on my question is : "In your case take 2 400 x 2 400 pixels. If your screen is bigger or smaller than 30x30 cm, just apply a proportionality" ?