Application Of Photogrammetry On Underwater Survey Of Coral Reefs

Coral reefs are one of the important ecosystems in the ocean,with high biodiversity and primary productivity,and have great economic,social and ecological values.At present,coral reef ecological survey methods mainly include Reef Check,quadrate photo and transect video.However,these methods can only sample some of the two-dimensional information of the coral reefs.Photogrammetry is an extremely effective technique,possessing potentials for investigating the whole two-dimensional or even the three-dimensional information.As the refraction of the water column,the optical path from the water bottom to the camera becomes broken lines instead of straight lines.The previous researches showed that the camera model of underwater photogrammetry can keep the original collinearity equation form by equivalently amplifying the principal distance.However,it is unclear whether the collinearity equation form will lead to some model errors or some larger systematic errors.Furthermore,it is unclear about the question how to ensure that the images meet the photogrammetric overlap in large-area(more than 200 m~2)of underwater photography.For this reason,the study proceeded the application of photogrammetry on underwater survey of coral reefs.So in this paper,model errors produced when the collinearity equation based on equivalently amplifying the principal distance is used for underwater photogrammetry were discussed in indoor flume experiment.Then a diver underwater shooting scheme was designed for the coral reefs experiment in Weizhou Island,and the underwater photogrammetric systematic errors in a large area(more than 200 m~2)were analysed.Finally,the Live Coral Cover(LCC)interpretation analysis experiment was carried out to illustrate the application of photogrammetry on underwater survey of coral reef.As a result,the following conclusions are obtained:1)underwater photogrammetry based on the collinearity equations and self-calibration bundle adjustment does not produce significant model errors.2)The diver underwater shooting scheme can ensure that the images meet the overlap requirements of photogrammetric processing.3)Large-area(more than 200 m~2)underwater photogrammetry(a Free-Net Bundle Adjustment)will produce a nonlinear deformation(systematic errors),and the value of the deformation is larger than that of in air.4)The underwater photogrammetry based the Free-Net Bundle Adjustment in a large area of coral reefs is mainly suitable for applications that do not very focused on the geometric accuracy,e.g.the LCC and its uncertainty expressed as the length/area ratio,and the Digital Orthophoto Maps of about 2000m~2 coral reef area in Weizhou Island which was obtained from 38950 underwater images processing for display purpose.Compared with traditional interpretation methods,photogrammetry has some advantages in estimating LCC:1)it can estimate LCC in the form of area ratio which is closer to the definition of LCC.2)The uncertainty of LCC estimation can be estimated through the interpretation of multiple virtual paths.3)In principle,it is expected to get rid of the dependence on the scale of reference ruler for the interpretation of LCC in transect video The results extend the photogrammetric application to the field of coral reefs,and are of great importance and are instructive for applications of photogrammetry on underwater survey of coral reefs,which is beneficial to improve the technical level of coral reef ecological survey.