Methods and networks are disclosed for optimizing PMD measurements on an installed span of fiber. For a method of the invention, a light system transmits light over the installed fiber. A PMD measurement system measures PMD on the fiber over a time period based on the light. A temperature measurement system measures ambient temperatures of the fiber over the time period. The PMD measurements and the ambient temperature measurements are then processed to determine an optimal temperature range for performing future PMD measurements on the fiber. The optimal temperature range indicates the best temperatures to perform measurements, the best time of day to perform measurements, the best time of year to perform measurements (such as summer or winter), etc.