Abstract: A system for determining kelly-to-riser position on floating drilling platforms is presented. The system removes (e.g. filters) uncompensated heave from surface sensor measurements so that bit depth can be accurately calculated. During operation, surface sensors on the drilling platform measure heave compensator action and kelly movements. Each heave sensor reading is then added to a kelly sensor reading in order to obtain a value termed kelly-to-riser. Each kelly-to-riser value is fed (inputted) into the system of the present invention and a filtered reading is returned for use in calculating bit depth and rate of penetration (ROP). The filtering system is termed a linear extrapolative filter (LEF). The LEF generates filtered readings by performing a linear regression on measurements taken over its pre-specified filter interval. It then predicts the current kelly-to-riser position from the resultant trend line.