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I passed along some of the debate about resolution and map scanning
to Columbia Head of Preservation, who worked on the oversized image
project--which included scanning geology maps. She sent me this
this message and was willing to have me pass it along to the list.
/SusanKlimley
 
I got your printout from MAP-L about scanning at 75dpi for display.
 
There are several issues here.  First, it depends if someone is using
the scanned image just for browsing/identification or to be read
online.  If it is just for browsing/identification, with assumption
that actual work with the map will be done with the original or other
paper copy, then a display version at 75 dpi will probably work fine.
If the idea is for someone to be able to actually use the map online,
then the first step is to find out the size of the smallest letter or
number on the map and apply the formula that Cornell has come up with
to determine what dpi is needed to make that smallest letter legible.
For a 1mm letter, as we know, you need approximately 200 dpi with
24-bit color.  The pixel depth (black and white, gray scale, or color)
makes a difference.  A black and white scan of 1mm needs about 600 dpi
to be legible, and grayscale is somewhere inbetween.  Maps with larger
letters will need lower dpi, obviously.
 
The other big question is whether it makes sense to scan just for
browsable images.  Today's equipment may only be able to handle 75 dpi
comfortably, but every year that changes, and a 75 dpi scan that is
deemed acceptable for browsing now will be considered much too poor a
few years from now.
 
It has been estimated that at least two-thirds (if not much more) of
the cost of scanning goes for the labor of selecting the items to be
scanned, getting them ready, manipulating them physically during the
process, and then putting them away again.  The difference between a
low-resolution scan and a high-resolution scan is less costly than
re-scanning at a later date because you have to go through the whole
selection and physical handling routine all over again.  Given than a
low resolution scan has limited use (only for browsing, not for
printing or reading at the screen) and that even for browsing it will
be considered poor quality in a few years, it makes much more sense to
make a high resolution scan that gives a fully legible image (based on
the formulae mentioned above).
 
The hig resolution "master" image can then be used to derive lower
resolution versions for easy browsing use, or other purposes.  As
equipment capabilities improve, re-scanning does not become necessary,
because you can simply derive better images from the master file.