----------------------------Original message---------------------------- [log in to unmask] 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.