The [ISO 1101] standard specifies the form errors with geometrical tolerances using the zone concept.To complete this concept, we present a generic method which adapts to any geometry and allows to describe any kind of errors. Thus,we can dissociate the part errors according to reference categories: position, orientation,form, waviness and roughnesses. Starting from a cloud of poinds representing the error measurement, the "modal" method decompose, like Fourier series,this error in a sum of sorted errors according to the ircomplexity degree (a number of "wavinesses"). In addition, we propose to show, on a simple example, that according to error complexity to be characterized, an interpolation by the modal method allows to optimize the measuring strategy.