Review of: Multisensor validation of tidewater glacier flow fields derived from SAR intensity tracking, by: Rohner et al. for The Cryosphere (tc-2018-278)

Abstract

Rohner et al. present a detailed validation study of ice velocity measurements derived from intensity feature tracking using Sentinel-1A and RADARSAT-2 SAR images. The study area is a tide-water glacier in Western Greenland (Eqip Sermia) which showed substantial fluctuations in recent years. The derived velocity maps are compared with field data, derived from 3 GPS stations, a Terrestrial Radar Interferometer (TRI) and repeated UAV surveys, in an effort to demonstrate the accuracy and limits of the method and data sets. Statistics are provided of the outcomes and are visualised in histograms, scatter plots and difference maps. The authors report a good agreement with the in-situ data with minor differences near the calving front ascribed to the characteristics of the different techniques. The ice velocity maps are also compared with Greenland-wide C1 * Pg. 9 Ln 8: "which corresponds to an area of about 25×25 m." Considering the chosen template size used for the tracking it actually corresponds to a much larger C3 TCD Interactive commentPrinter-friendly version Discussion paper area. * Pg. 16 Ln 10: ". Left uncorrected, these introduce biases in the estimated magnitude of surface velocities (Nagler et al., 2015). ". Did the authors apply any such corrections? If not, what is the estimated bias introduced by this? * Section 4.2: A large part of the discussion concerns an intercomparison of an annually averaged version of the '5m' maps with products from MEaSUREs and CCI. The authors find substantial differences near the calving front and margins but also further upstream. These differences are reported as an underestimation of ice flow in the operational products and according to the authors this implies an underestimation of ice fluxes and an overestimation of Greenland mass loss when used for mass balance assessments. These claims are, however, unsubstantiated by the current work for a number of reasons, aside from the issue regarding resolution described before.