In drying, the gelatine contracts, and, necessarily, causes a deformation of the tissue, which curls up at the edges and loses its planimetry.    From Wordnik.com.  [Photographic Reproduction Processes] Reference

Find the surface of each area by planimetry and multiply this value with the runoff coefficient of that particular area (average reduction factor at the end of the design period) to find the surface of the reduced tributary area.    From Wordnik.com.  [1. Intake structures] Reference