"Blotter Plan" circa 1816

I had a question from a friend and former colleague Allan Day who is now retired but operates a survey research firm for Ontario surveyors looking for historical information, both Provincial and Federal. http://home.cogeco.ca/~surveyresearch/

He has asked me what a "Blotter Plan" is as he has not run across the terminology.

I think it is a copy of an original drawing that was made on onion skin type paper at the same time as the original but I don't know for sure the process if I'm correct.

I do recall seeing old 'description books' that were composed of onion skin duplicates of the original inked descriptions.

But it's been a while.

Any assistance available ?

Thank you.

Cheers,

Derek