Underwater photogrammetry application for coral reef mapping and monitoring

Underwater mapping techniques such as underwater photogrammetry is one of the latest trends in survey techniques in mapping coral reefs. The underwater photogrammetry technique uses a non-metric camera in 3D reconstruction of coral reefs that will produce photo maps up to in 3D. This article explains how the process of data acquisition to the process of producing coral reef photo maps. The study area used surface terrain of coral reefs with dimensions of 25 m long and 2 m wide at depths of less than 5 m. Acquisition of photos using a type of waterproof sport-fisheye camera with automatic data processing to rendering using the Structure from Motion (SfM) algorithm. This platform was chosen because it is easy to obtain, quite inexpensive and practical in its operation. In addition, it has a good quality of lowlight, affordable, and have large FoV. For maximum image quality, the camera calibration is done with the average error obtained from camera calibration is equal to 1.10 pixels. Geodetic measurements are also carried out on the control points in the modelling process. Data was processed using 822 photos for photo orientation with overlap rate of > 75%. The result is a 1: 100 scale photo map with a Ground Sample Distance (GSD) value of 0.395 m. The 3D models of coral reefs are well-formed although the resulting image still has some weaknesses, for example, the appearance of colours that depend on the level of water clarity and the camera filters used.