A new video survey method of microtopographic laser scanning (MiLS) to measure small‐scale seafloor bottom roughness

Abstract

A novel video survey method measures small-scale seafloor bottom roughness in fragile and deep-sea habitats called microtopographic laser scanning (MiLS). Using a controlled submersible platform, an attached downward-facing video camera with a single optical laser can return imagery to detail the bottom profile at a resolution of ~1-2 cm. The method compares the position of the underlying substratum and laser dot between successive video frames to determine distance traveled in the forward direction and substratum height. The video imagery is processed using photogrammetry to calculate small-scale topography (horizontal and vertical axes). MiLS is adaptable for most aquatic habitats as it can be executed using any platform that can move forward with a constant slope over the desired transect. Traditional techniques of measuring small-scale roughness are largely restricted to easily accessible habitats and often yield measurements that are relative and not comparable among different habitats and studies. Quantifying roughness in ways that permit comparisons is critical to understanding effects of bottom roughness and would benefit many fields of aquatic science. With its versatility, ability to access remote locations and output of quantified measurements, MiLS has the potential to fill this need. It is also likely this method will be useful in subaerial habitats such as wetlands. Here, we describe the MiLS equipment, theory, and method in detail, and then demonstrate its application in a lab trial and in a field study in a deep-sea (≤450 m depth) sponge and coral habitat where its high resolution, accuracy, and precision is made evident.

Roughness, resistance, and dispersion: Relationships in small streams

“…This method allowed for measuring for the first time long continuous transects of riverbed roughness in difficult-to-access reaches of small streams at low flow depths. Other recently developed methods, including photogrammetry [Bird et al, 2010;Lane et al, 1994;Westaway et al, 2003], LiDAR (Light Detection and Ranging) [Brasington et al, 2012;Kinzel et al, 2007], laser scanning [Du Preez and Tunnicliffe, 2012;Gonz alez et al, 2007] and structure from motion [Fonstad et al, 2013, Woodget et al, 2015, provide three-dimensional topographic measurements of the riverbed at various scales. The accuracy and resolution of these observations from above the water surface depend on the water depth and surface roughness, light conditions and accessibility of the stream [Fonstad et al, 2013;Smith and Vericat, 2014].…”

A new arc–chord ratio (ACR) rugosity index for quantifying three-dimensional landscape structural complexity

“…Similar to Friedman et al (2012), the ACR method orthogonally projects the surface onto a plane of best fit (POBF) rather than a horizontal plane-decoupling rugosity from slope at the scale of the surface (Du Preez and Tunnicliffe 2012;Friedman et al 2012). The innovation of the ACR method lies in the analysis used to generate the POBF: identify and isolate the boundary data (step 3), and use a linear polynomial interpolation of the boundary data to generate the POBF (step 4).…”

“…Two studies recently proposed similar methods for measuring the rugosity of a two-dimensional profile independent of slope (Du Preez and Tunnicliffe 2012;Friedman et al 2012). Instead of projecting the surface onto a horizontal plane, the methods use a plane of best fit (POBF; Fig.…”

Triangulation hand‐held laser‐scanning (TriHaLaS) for micro‐ and meso‐habitat surveys in streams

“…a total 0.8% uncertainty of the brick length (2 mm), one can conclude that the error of the CLT‐measurement is equal to or less than the HHP positioning error. This corresponds to previously reported accuracies of less than 1 mm (Du Preez & Tunnicliffe, ; Noss & Lorke, ). The minimum spatial resolution between points of the raw data was between 0.04 and 6 mm in the case of the Kaltenbach riverbed topography and between 0.9 and 12.5 mm in the case of water surface measurement.…”

“…Hence, measurements are mainly limited to exposed bars at low flow conditions (Alho et al, 2009;Bertin & Friedrich, 2016) and selected sites like gravel riverbeds below shallow flows (Chandler et al, 2002). Underwater stereo-photogrammetry (Briggs et al, 2002), micro-topographic laser scanning (Du Preez & Tunnicliffe, 2012), and mechanical levelling techniques (Furbish, 1987;Clifford et al, 1992;Nikora et al, 1998) provide very high resolution and accuracy (< 1 mm), but only for small spatial scales (< 1 m).…”