A chemical mechanical polishing apparatus and method can use an in-situ monitoring system. A measurement of a position of a carrier head and a sinusoidal first function can be used to define a second function that associates measurements from the series with positions on the substrate. For each measurement in a series from the in-situ monitoring system, the second function can be used to determine a position on the substrate where the measurement was taken. In addition, a measurement of the position of the carrier head, a time when the measurement of the substrate property is made, and a phase correction representing lag resulting from a processing delay in generating the measurement of the position of the carrier head can be used in determining a position on the substrate where a measurement of a substrate property was taken.