Large-area seafloor imaging will bring significant benefits to various fields such as academics, resource survey, marine development, security, and search-and-rescue. The authors have proposed a navigation method of an autonomous underwater vehicle for seafloor imaging, and verified its performance through mapping tubeworm colonies with the area of 3,000 square meters using the AUV Tri-Dog 1 at Tagiri vent field, Kagoshima bay in Japan (Maki et al., 2008, 2009). This paper proposes a post-processing method to build a natural photo mosaic from a number of pictures taken by an underwater platform. The method firstly removes lens distortion, invariances of color and lighting from each image, and then ortho-rectification is performed based on camera pose and seafloor estimated by navigation data. The image alignment is based on both navigation data and visual characteristics, implemented as an expansion of the image based method (Pizarro et al., 2003). Using the two types of information realizes an image alignment that is consistent both globally and locally, as well as making the method applicable to data sets with little visual keys. The method was evaluated using a data set obtained by the AUV Tri-Dog 1 at the vent field in Sep. 2009. A seamless, uniformly illuminated photo mosaic covering the area of around 500 square meters was created from 391 pictures, which covers unique features of the field such as bacteria mats and tubeworm colonies.
Field evaluation of sounding accuracy in the multi-narrow beam echosounder, SeaBeam 2112, on R/V Kairei was carried out in the seafloor with a depth of deeper than 9,000 m. Bathymetric data were gathered while the ship holds on station. Variations of pitch, roll, and heave indicate that the ship was so stable that the motion of the ship except for heading did not affect for soundings. The depth uncertainty in each beam was estimated by forming the ratio of the standard deviation of the soundings in each angular bin to their mean. The soundings in observation range from 8,500 m to 10,900 m. The uncertainty in most beams from beam angle -24° to 27° is below the quality assurance (< 0.5%) demanded by the manufacturer. That of outer beams from the beam angle -24° and 27° is larger than 0.5% because of beam pointing errors by motion of the ship and topographic relief. The uncertainty of all beams at a constant heading was lower than 0.5%. The depth uncertainty near the vertical beam was lower than 0.1%. Soundings by SeaBeam 2112 on R/V Kairei in deeper than 10,900 m of water depth satisfy Order 1b of Standards for Hydrographic Surveys (S-44) by the International Hydrographic Organization.