1. A calibration process for calibrating a first loop feature value estimation method using a first locally measurable loop characteristic and a first set of parameters, comprising: taking measurements of said first locally measurable loop characteristic for a first plurality of loops; obtaining a reference data set representing reference estimates of said loop feature value, said reference estimates being obtained by performing a calibrated second loop feature value estimation method using a second locally measurable loop characteristic and a second set of parameters, the first loop feature value estimation method and the second loop feature value estimation method being different methods; and determining calibrated parameters so as to minimize a deviation between said reference estimates and estimates that can be obtained by applying said first loop feature value estimation method to said measurements using said calibrated parameters as said first set of parameters.