How do you compute sea ice freeboard using GLAH13 data?

"Freeboard" is a measure of the height of sea ice above the ocean surface. The geoid height field ("d_gdHt") cannot be effectively used as a proxy for sea level in order to estimate freeboard, because current models of the Earth's geoid are too coarse to account for the local variations in geoid height that ICESat/GLAS actually detects.

A suggested method is to use a high-pass filter to remove the geoid and ocean dynamics effects, essentially filtering out the low-frequency, long-period variability (from geoid and ocean dynamics) and leaving the high-frequency, short-scale variability from sea ice. Some biases still exist, so an offset may be required to make the lowest elevation sea level, while assuming at some point there will be open water. This method is not suggested over a long section of a transect, but it works reasonably well within a given region.

Kwok et al. used synthetic aperture radar (SAR) data to find open water and confirm sea level at specific locations, and estimated snow cover.

Kwok, R., H.J. Zwally, and D. Yi. 2004. ICESat observations of Arctic sea ice: A first look. Geophysical Research Letters 31, L16401, doi:10.1029/2004GL020309.