Surface topography and snow depth measured during the ALERT2018 campaign (MAP Last Ice) at station ALERT2018_09

Measurements of surface topography have been performed using a Terrestrial Laser Scanner (VZ-400i, RIEGL, Horn, Austria) on First-Year-Ice during the ALERT2018 campaign (Multidisciplinary Arctic Program (MAP) - Last Ice) off Alert, Nunavut, Canada in the Lincoln Sea in May 2018. The scans were performed across an approx. 100 times 100 m wide patch. The laser scanner was mounted on a tripod approx. 2 m above the surface and had a wavelength of 1550 nm. The scan pattern “panorama 20” with an angular resolution of 0.02 degree and a scan time of 180 s was used. At 20 m this corresponds to a 0.7 cm and at 50 m to a 1.7 cm horizontal resolution. A laser pulse repetition rate of 1200 kHz with a maximum measurement range of 250 m was used. Due to the snow conditions and the laser wavelength, a maximum range of only about 100 m was achieved. The roll and pitch accuracy of the laser scanner ranged from 0.009 to 0.014 degree which translates into a 1.57 cm to 2.44 cm vertical error per 100 m distance. For further interpretation we used the mean of 2 cm for the laser scanner accuracy. The individual scans were each registered to a master scan position using three space-fixed cylindrical retro-reflectors (10 cm diameter) in the RiSCAN Pro software. The standard deviation of the reflectors calculated during the registration was between 0.5 cm and as much as 2.2 cm when the laser scanner was not entirely stable during windy conditions. Snow fall, moving targets, and the area outside the area of interest were manually removed in the RiSCAN Pro software. Surveys conducted during larger snow fall events were not further analysed. After registration, a full point cloud was created with mean horizontal resolutions of 5 cm. The snow depth was calculated from the surface topography using the length of the rod where the reflector sitting on level ice was mounted, the distance between rod end and reflector centre, and the z-coordinate of the reflector as measured by the laser scanner