Table 3 Data abstraction with the broken-stick model

The broken-stick model (BSM) [26, 28] is a piecewise linear approximation method, used here to represent time-depth dive and hydrographc profiles with a smaller number of points than their full resolution. This has been used as the abstraction algorithm on-board SRDLs for dive data and hydrographic data on CLS-Argos tags from 2007 onwards.

The number of iterations of the algorithm that are carried out on board the tag can be adjusted. The principle of the algorithm is to use line segments to approximate a shape, adding points to the piece-wise linear representation where it differs most from the original shape.

In the case of dive profiles, the differences between the BSM dive profile and the original profile recorded by the tag are measured vertically, i.e. metres. At each iteration of the algorithm, the point in the original profile with the biggest vertical distance from the abstracted profile is identified and added to the abstracted profile. This process is repeated until the desired number of points is reached [23]. The depth points chosen by the BSM are stored at the tag’s internal resolution of 10 cm and the time resolution (4 sec or multiples) is known implicitly since it is linked to dive duration.

For CTD upcasts, which consist of fixed points as well as points chosen by the BSM, the first iteration of the algorithm is taken as the piecewise linear profile made up of the deepest point, the surface point and the intermediate fixed depth points.