In a military context, especially by the Finnish Non-Linear Artillery of The Continuation War, the Fire Correction Circle or (Fire) Correction Converter Dial is a fully mechanical auxiliary device which was constructed of plywood and transparent plastic and which used to be used to calculate targeting values for non-linear artillery and mortars. By using a Correction Circle the artillery observer is able to report the necessary fire corrections to the unit firing without knowing exactly where the unit is firing from.
References
Fire Correction Circle Wikipedia(Text) CC BY-SA