Resolution
A bitmapped image is a rectangle of pixels and the resolution of the image is the number of pixels in each direction. For example a video monitor screen might have a resolution of 1280x1024 pixels. Higher resolution images have more pixels and can therefore describe a similar image in more detail than a low resolution image. This term is pretty much universally recognized.
An image size has different meaning depending on who you ask. Often it is synonymous with resolutuion however - just the number of pixels. When the image is mapped onto something physical or something representing a physical device then it might have real world size. For example a monitor screen has a width and height of viewing area. The image could be printed to a sheet of paper having known dimensions. Whenever (and only when) an image has a known resolution mapped to a known physical size, then it can be said to have a pixels per inch value. As soon as anything has a pixels per inch, dots per inch, or lines per inch or similar kind of metric then it also has a maximum spatial frequency. All of these names are used to describe whether an image will look good but usually miss out on an important but implied 3rd value which is how far away from the image it will be viewed. For a monitor or printed sheet of paper it is assumed that it will be of a short, perhaps half meter, distance. When viewing a poster or bill board the distance will be much larger and therefore the image might look just as good at a much lower dots per inch. So here is some technical proof and guidelines about what constitutes a good value.
The most important surface on which an image must fall is the retina of an eye and the lens / pupil of which has a diameter of about 4mm in medium to low light conditions. As the light dims the pupil opens up and when bright it closes down. The maximum resolution that anything could possibly detect given superhuman retina still depends on the pupil size so in theory during low light conditions the eye is potentially sharper even if less sensitive. Its all a bit moot but allows for a calculation of absolute maximum angular spatial frequency and thus pixels per inch at different distances.
Angular resolution can be determined from the Rayleigh Criterion for mid green (500nm) light in a 4mm pupil as:
sin(theta) = 1.22 * 500x10^-9 / 4x10^-3
theta = 0.153mrads
So that means that at half a meter away, Mr Supereyes can see lines spaced at about 0.077 mm apart which is 330 pixels per inch.
A billboard by the side of a road 10m away could not be resolved by Mr Supereyes beyond 1.5mm which is 17 pixels per inch.
So if you print a character sheet or page of book to be read up close then 600dpi is about the maximum that you will need. A picture to hold up and share 300dpi. A battle mat viewed at about a meter away 150dpi and a poster on the wall maybe 100dpi. For comparison, a top spec 17" laptop screen is 1920x1200 which is approximately 130dpi. Any digital camera purchased for printing full images onto A4 will be a bit pointless after 10 megapixels.
Its also worth mentioning that if your going to print any of the maps onto A4 which is 8.3 x 11.7 then at 300dpi that means images much larger than 2500x3500 are a bit wasted - but you can cater for large format printers if you fancy.
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