The modern permanent magnet undulators are usually equipped with motors that have integrated feedback electronics. These are essentially rotary encoders that indicate the position of the motor axis. In addition, undulators are also equipped with linear encoders that provide the absolute value of the gap between the magnetic structures or the position of the magnetic girders relative to the undulator frame. The operating conditions of undulators should take into account the risks of failure of electronic equipment under the influence of radiation. In case of encoder failure, the motor or encoder must be replaced. To avoid the need to return the undulator to the magnetic measurement laboratory, reference measurements are required to restore the position of the magnetic structure after replacement. In this article, reference measurement procedures for planar and helical APPLE-X undulators used at the European XFEL are presented.