Preliminary hard and soft seafloor substrate map derived from an unsupervised classification of multibeam backscatter and bathymety derivatives at Tinian Islands and Tatsumi Bank, CNMI. The dataset was derived using Reson 8101 backscatter data, bathymetric variance and bathymetric rugosity. The sonar frequency is 240 kHz for the Reson 8101 backscatter data, which were resampled to a 5 m grid cell size prior to the classification. Limited seafloor photographs for groundtruthing are available for Tinian and therefore no supervised classification was performed and we are unable to visually or quantitatively evaluate the accuracy of the unsupervised classification seafloor substrate map in a robust manner. However, in locations such French Frigate Shoals, NWHI and Tutuila, American Samoa, ground truth data are available and the unsupervised classification method is a good predictor of substrate type in similar depth ranges and seafloor environments.