When using the instrument altimeter to estimate ice thickness, a correction for atmospheric pressure variations is required. The pressure sensor measures absolute pressure. However, the data are presented relative to a nominal atmospheric pressure defined when the pressure offset was set. In other words, a constant mean atmospheric pressure has already been subtracted from the absolute measurements. The data must therefore be corrected for fluctuations in atmospheric pressure relative to this assigned mean value. These fluctuations are associated with the passage of high- and low-pressure systems. They occur on time scales from days to weeks and, if uncorrected, can correspond to variations of up to ±0.5 m in water level. A time series of barometric pressure is required to apply this correction. Because atmospheric pressure variations are associated with large-scale synoptic weather systems, the barometric measurement does not need to be made at the exact instrument location. Atmospheric pressure measurements obtained within a few tens of kilometers of the deployment site typically result in residual errors of less than ±0.05 m after correction [Waves in the Summer, Ice in the Winter (2010)].
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