Just to add a footnote to Ned Heite's comments on map overlaying which might
be of  interest.
A few years ago a retired Ordance Survey (the british government department
wh make maps) surveyor was doing some interesting work on an early 17th
century crown map in the Forest of Dean, Western England. He found that the
map was based on accurate measurements but that it was inaccurate because it
didn't take into account gradient. However, it was possible to compensate for
the effect of slope using modern maps and thus replot very accurately the C17
map measurement by measurement. I am unsure if this work was ever published.
 
Paul Courtney, Leicester, UK