I. Introduction
At Desy the Petra II machine is in the process of reconstruction into the third generation synchrotron light source PETRA III [1]. One eighth of the machine circumference is completely redesigned to produce proper synchrotron radiation for the external user beam lines. The beam guiding elements and diagnostic parts are mounted on 34 girders. On each girder three magnets are located. The position accuracy of the quadrupole magnetic axis with respect to the beam trajectory must be within 100 ( on average). The quadrupole magnets (Fig. 1) are of the “eight” type [2], [3] with lengths of 0.44 m and 0.72m and apertures of 70 mm in diameter. The integral field nonlinearity in the good field region is better than . The magnets are described in detail in [4].