A Derived Model of Alaskan Sea Surface Topography: a critical piece of the vertical datum transformation for ellipsoid referenced hydrography

With coasts that form three >1700 km arms which spiral outward from a point near Kodiak Island, Alaska provides a large-scale site for the study of the relationship between geodesy and sea level. The surface of a static, uniform ocean will follow an equipotential, orthometric surface. Long lived geostrophic currents or prevailing winds result in localized and large scale variations between its surface and the geoid datum. The topography of the sea surface (TSS) relative to the geoid is a critical component for conducting hydrographic survey operations. We modeled TSS in Alaska by examining locations which had both historical tidal datums and a GPS occupation of a nearby benchmark. From observations at over 100 such stations throughout Alaska, we observed that the sea surface forms a SW dipping slope which is ~1.4 m above NAVD88 (GEOID12B) near the SE corner of Alaska, and ~35 cm above NAVD88 near Adak Island ~2700 km to the east. Point observations of TSS are extended to all regions via a Laplacian interpolation. This TSS model, in conjunction with the Geoid12B model and a model of tidal amplitudes can be summed to derive a datum separation between the GPS reference ellipse and chart datum. Keywords— Alaska hydrography; topography of the sea surface; tidal datum; VDatum;