Hi Greg,
One potential problem with offsite maps is the difficulty in delivering them onsite for use without damaging them. We've managed to keep our map collection on site for that reason. also, the inefficiency of a patron requesting an offsite sheet, then it comes
in, only for him to need the next sheet west, for example, and have to wait another day (more at weekends) to get it.
***
I strongly second the suggestions to index all series material sent offsite.
If you also manage to scan the material before moving it offsite, you can then hyperlink the indexes to allow patrons to point-and-click on the indexes to see the scanned maps.
e.g.
catalogue search results for L509 AMS series, showing index thumbnail attached to the series record
[click on that record's title to get:]
catalogue record itself, with attached index
[click on index thumbnail to get:]
zoomable index as seen in online viewer
[click on 8th/bottom blue button in viewer to get:]
interactive version of index
[click on any pink square to get]
thumbnail image[s] for that grid square
[click on any thumbnail to get]
desired scanned image
[click on arrowheads in viewer window to get:]
next sheet in the direction of the arrow (e.g. the sheet to the west)
Brendan Whyte
National Library of Australia