Low-cost Bathymetry Modelling of NE Brazil Protected Reef Areas: Focus at APA Costa dos Corais
Channel: Encontro Recifal Brasileiro
Date: 2020-07-01

SEOANE, J.C.S.; SANTOS, E.E.S.; CAON, J.A.; PEREIRA, P.H.C.

Digital Elevation Models (DEM) are essential for a better comprehension of different terrains, as they represent the original geomorphology, and through processing, such as color classification of data, shaded relief, slope, direction of faces and curvature, allows for a range of transdisciplinary interpretations of great value to the management of conservation areas. South APA Costa dos Corais, Alagoas, is the chosen area for the study, a fully protected Conservation Unit (CU) where existing bathymetric data does not attend management’s needs for precision, as data are scattered and little detailed. Remote sensing through echosounding is an adequate tool for mapping, causing no direct interference to the sensitive environment. A previous scout survey identified a reef, at about 20m depth, chosen for detailing (15 km2). Tide-corrected data points in a ~200 x 5m point grid were surveyed using a commercial echosounder attached to a GPS. Modeling of the continuous bathymetric surface DEM used a Multiquadratic Radial Basis Function (RBF) interpolator and generated a 15m pixel DEM, with submetrical RMS=0.46m. Two other CU in Bahia had similar results of submetric errors using RBF interpolators for tighter grids. PMM Recife de Fora was the first to be surveyed using a commercial echosounder in June 2007 (25 km2 @ ~50x10m, RMS=0.33m, 10m pixel), while PMM da Coroa Alta was surveyed between June 2014 and April 2015 (61.5 km2 @ ~100x2m, RMS=0.26m, 14m pixel). The ‘Topo to Raster’ algorithm for elevation point to grid conversion yields similar results in all cases. The large range of bathymetric detail is of huge importance in marine protected areas, as they are the basis to the knowledge of ecosystems. We present a low cost methodology, which has potential to become widely applied, and compare three case studies of survey design for different Conservation Units, to aid management in marine protected areas.