Data Types and Preprocessing 2.1. Multibeam Data [4] All multibeam sonar data, in appropriate data formats, were processed using the Caris HIPS/SIPS data processing software. Bathymetric data were corrected to mean sea level (MSL) using tide information acquired during surveying or a tide model derived from known harmonic constituents using the XTide software (http://www.flaterco.com/xtide). A despiking filter was applied to all multibeam data sets to remove the majority of ‘‘erroneous’’ data. The filter calcu- lates the slopes between every ping and its neighbor’s port and starboard. If both slopes exceed a defined value (a value of 5 was typically used) and are of opposite sign then the ping is rejected. Most data sets were also visually inspected to further remove spikes and to check the results of the despiking filter. Overlapping multibeam data sets were then used to correct roll biases and refraction errors. A number of the deepwater multibeam data sets had refraction errors (typically in depths >2000 m) because of inaccurate sound velocity profiles applied during data acquisition. Refraction artifacts were minimized using the refraction editor in the Caris HIPS/SIPS software. Other multibeam data sets that were not compatible with the Caris software were edited using the MBSystem software suite [Caress and Chayes, 1995]. Presently, multibeam data cover 11.6% of the marine grid area (Figure 2a), the bulk of these data sets are located in the Gulf of Papua and were acquired as part of the MARGINS source-to-sink research program. The multi- beam data sets used to construct the bathymetry grid are summarized in Table 1.