Two-Dimensional Video Mosaics for Mapping and Monitoring Coral Reef Resources.

Gracias, Gleason, Lirman, Gintert, Doolittle, Reid, Negahdaripour

Coral reef communities have experienced significant declines in abundance and distribution over the recent past prompting the innovation of technological tools to assess and monitor coral reef condition. Diver-based video surveys have frequently been used to measure ecological indicators of reef condition such as coral cover and species diversity. The video in such surveys is typically collected along a transect, and individual frames are sometimes stitched together to form panoramic strips. Recent advances in video mosaicing technology now allow the accurate registration of images acquired over multiple parallel strips thereby extending the coverage of the resulting mosaic to two spatial dimensions.

Mosaics constructed over two spatial dimensions (2D) have several advantages over conventional strip mosaics constructed along a single spatial dimension. For example, coral size distributions can be measured from a 2D mosaic, even though they are typically impossible to acquire from strip mosaics because only the smallest corals are completely imaged along a single transect. The spatial extent of features such as vessel grounding scars, which are often too small to map from the air and too large to efficiently map by a diver, can be measured from a 2D mosaic. Two-dimensional imagery from repeated surveys can be accurately georeferenced to assist change detection algorithms, unlike transects which are exceedingly difficult to precisely duplicate at multiple times. Therefore, 2D mosaics can also support the analysis of spatial distribution of benthic communities.

Two-dimensional video mosaics could be widely adopted as a component of coral reef monitoring strategies. The flexible mosaicing algorithm adapted for this work allows the technique to be used in a variety of applications from low cost surveys with hand held video to mapping deep reefs with ROV-based video. Mosaics of plots on the order of 20 meters on a side can be constructed in very high detail (millimeter resolution) with today's desktop computers. Two-dimensional video mosaics can fill an information gap for managers by providing measurements on the plot scale that complement measurements made over the regional scale from satellites and buoy networks as well as measurements made on the colony scale by divers.