The NDT[3] was presented as an alternative to ICP-based scan matching algorithms. The problems of ICP-based algorithms derived of the establishment of correspondences do not appear in the NDT approach. However, the NDT requires accurate and dense sets of readings to work with. Consequently, the NDT is not well suited to perform scan matching with sensors that produce sparse sets of noisy readings, such as ultrasonic range finders. In this paper, an extension of the NDT to deal with sparse sets of noisy readings, the sNDT, is presented. The necessary processes to overcome the sparsity of sonar readings, as well as those involved in the scan filtering, are described in the paper. Experimental results compare the new approach to other well known scan matching methods.