Abstract
Abstract Echograms are used to visualize fisheries acoustic data, but choice of colour map has a significant effect on appearance. Quantitative echograms should use colour maps, which are colourful (have a perceived variety and intensity of colours), sequential (have monotonic lightness), and perceptually uniform (have consistency of perceived colour contrast over their range). We measure whether colour maps are colourful (M^(3)>0), sequential (rs=±1), and perceptually uniform (ρ = 1) using an approximately perceptually uniform colour space (CIELAB). Whilst all the fisheries acoustic colour maps tested are colourful, none is sequential or perceptually uniform. The widely used EK500 colour map is extremely colourful (M^(3)=186), not sequential (rs=0.06), and has highly uneven perceptual contrast over its range (ρ=0.26). Of the fisheries acoustic colour maps tested, the Large Scale Survey System default colour map is least colourful (M^(3)=79), but comes closest to being sequential (rs=−0.94), and perceptually uniform (ρ=0.95). Modern colour maps have been specifically designed for colour contrast consistency, accessibility for viewers with red-green colour-blindness, and legibility when printed in monochrome, and may be better suited to the presentation and interpretation of quantitative fisheries acoustic echograms.
Highlights
Echosounders are routinely used in marine science to survey the underwater environment
The EK500 (34%) and “Rainbow” (16%) colour maps were the most frequently used for echograms, followed by Large Scale Survey System (LSSS) (8%), “Greyscale” (7%), and “Other” (14%)
The choice of colour map has a significant effect on the appearance of an image and the detail revealed (Campbell and Robson, 1968)
Summary
Echosounders are routinely used in marine science to survey the underwater environment. Signals are typically recorded as power, in Watts, and converted to target strength (TS), or volume backscattering strength (Sv), in decibels, to study the distribution, abundance, and behaviour of animals (Simmonds and MacLennan, 2005). Acoustic data are recorded as a matrix of signals X(i, j) , where i is the range index and j is the along-track distance index. The range of X to be visualized (determined by the scale bar) is divided into k equal bins, and pixels are mapped . The available radiometric resolution of an echogram reduces as k reduces, and an echogram often has lower dynamic resolution than the original acoustic data. Changing the colours in an echogram affects the visual appearance of its content in the same way that changing the colours in a photograph would change the appearance of its subject
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