It's from the Latin for "hemp" because it was often made of hemp; add a letter & it means to take a survey
[DOC] [TLE] Canvased - definition of canvased by The Free Dictionary [PAR] A heavy, coarse, closely woven fabric of cotton, hemp, or flax, traditionally used 
for tents ... and from Medieval Latin canavsium, both ultimately from Latin 
cannabis, hemp; see cannabis.]