Section: New Results

Calibration of TRANUS Adjustment Parameters

One of the most difficult steps in calibrating the parameters of the TRANUS land use model, concerns the estimation of its adjustment parameters (so-called shadow prices), that allow to “absorb” imperfections of the model or the data. The main difficulties are the non-linearity of the underlying equations and the fact that some of these equations give rise to loops between intermediate system variables: modifications of some of these variables entail modifications of others and vice-versa. In other words, the concerned part of TRANUS is a dynamic system. Currently, users of TRANUS perform the calibration by semi-automatic (at best) trial-and-error.

We have started investigating more systematic solutions to this. A first step has been to explicitly pose the estimation problem in the form of an optimization problem, with clearly stated cost function and constraints. Next, we have found ways of splitting the problem into separable subproblems, concerning the estimation of adjustment parameters for different economic sectors. In particular, the housing/land sectors can be calibrated independently of the others. A simple gradient descent was shown to be sufficient, both theoretically and experimentally, to achieve this calibration. We are currently investigating strategies to estimate the adjustment parameters of the remaining sectors.