tic marks on blueprints?
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Most often the Tic marks are there for reduction/enlargement reference. The plan may have been set up for microfilming. All too often the microfilm process is not exact. Your original at 1:1 is shrunk to 1/1000 of original size, and then via lens it gets enlarged and printed at 1:.998 and thus the machine needs adjustment, or you can only use the plan as reference. The cited dimensions hold in compared to measured distances. Thus a 48" long original plan may now measure 47.9" long. Off by 1/8" overall.
Wm.
Wm.