Significance of GUND - Geoid undulation ?
I am looking at some AD charts that show values for Elevation and GUND at the thresholds. eg HKMO shows GUND -94.58Ft and elevation 199.6Ft at the threshold of Rwy 21
Looking up GUND tells me that it stands for Geoid undulation, but I am none the wiser for knowing that. Could it refer to deviation from the geoid used by GPS to determine altitude ? Does it have practical significance ?