A two-factor data table in a stream sensor study estimates values between measured conditions. Rows pH (pH units) 6, 7; columns distance downstream (km) 20, 40; contaminant level (mg/L) matrix: at row 6: 42, 51; at row 7: 42.6, 51.8. Using linear interpolation in both directions, estimate contaminant level at 6.5 pH units and 35 km.