Having used the great Sanborn maps for years I have always wondered how
they were compiled.  It is clear that agents came out to the communities
(larger cities had their own offices?) and first spent a great deal of time
making the base map (using existing surveys for the base maps?) and then
every 5 years or so doing a thorough field check of each and every
structure (they even have such details as new outhouses, bay windows and
porticos!) and modifying the map accordingly.
        I know on some of the later maps, 1920s and up to the 1950s, that they did
paste-overs on top of earlier maps where there were changes and the
recorders were always careful to note the date of changes (I found that
holding a bright flashlight behind will help illuminate the various layers
and changes (oh for digging to be so easy!).  Where these paste-overs
Sanborn original field maps (many are in the hands of municipalities) from
which updated maps were compiled and published?
        Perhaps most importantly, are their notes and draft maps for these field
maps which could be used by researchers (hoping that there are notes such
as "this structure burned since last survey" or "building under
construction at time of survey"?)
 
        Dan W.