A high resolution hydrogen line survey of Messier 31. I: Observations and data reduction

New observations with the Westerbork Synthesis Radio Telescope are presented of M31 in the 21 cm line of neutral atomic hydrogen. The observations and calibration are described, and the various stages of data reduction are covered, emphasizing the nonstandard reduction methods employed. Maps are presented in various formats, showing the data from different aspects in order to give a complete view of the whole data set. Radio photographs of the channel maps are shown at full resolution, and sets of position-velocity maps are presented at lower resolution. A radiophoto of the total H I surface brightness is also reproduced. The most striking features of the survey, such as the many regions which seem devoid of H I, are briefly discussed. Multiple peaks which occur at many positions of the observed line profile are probably caused by a warp