The PSF of a segmented mirror telescope is hardly affected by the segments alignment and it can cancel the performances of the Adaptive Optics Systems. The piston and tilt of each segment must be uniformly adjusted in relation to the rest of the segments. Furthermore, the direct detection of the alignment error with natural stars would be desirable in order to monitor the errors during the astronomical observation. We have studied the lost information of the piston error by the presence of atmospheric turbulence in the measurements of curvature and present a new algorithm to obtain the local piston using the curvature sensor. A phase wrapping effect is shown as responsible for the loss of curvature information to get the piston error map well enough; this happens not only in the presence of atmospheric turbulence, but also without it.