There is an emerging consensus that maps need to be digitized at a minimum of about 300 dpi in 24 bit color if all the important details are to be captured. Will occupy several hundred megabytes. The most difficult part of this process is digitizing the map. Digitizing from film produces the least satisfactory results, but it is also the easiest and least expensive method for those that have small numbers of maps to digitize. The drawbacks in working with a film intermediary are that there are unavoidable losses in resolution and color fidelity in comparison with other methods.
Scanners produce better results, but are limited by the size of the maps they can handle.
There are a few scanners that can work with large maps. The Library of Congress obtained its scanner as a donation. Cost and some technical considerations make it doubtful whether many libraries will want to imitate the Library of Congress in this respect, although they have obtained excellent results using this scanner. In my opinion, high-end digital cameras are presently the best option for those contemplating creating large numbers of research quality map images. The total cost of a digital camera back and associated equipment is somewhat lower than that of a Tangent scanner.