| Popis: |
Altimetry based ocean surface topography is crucial for deriving global/regional ocean surface current via geostrophy. To this end, many global/regional MSS (mean sea surface) or MDT (mean dynamic topography) models have been developed, primarily in a geodetic sense, sea surface topography is a direct representation of vertical separation between sea surface and a reference equipotential surface (i.e. geoid surface). Therefore, sea surface topography is another crucial input for vertical datum transformation such as transforming ellipsoid height to MHW (mean high water) tidal datum. NOAA's VDatum is a comprehensive suite of tools for performing vertical transformations among a variety of tidal, orthometric and ellipsoidal vertical datums (https://vdatum.noaa.gov), allowing to convert geospatial data from different horizontal/vertical references into a common system. Generating a topography of sea surface for datum transformation (or datum TSS) is different in many ways from developing the existing MDT models for deriving ocean current. The main objective is to better model a vertical separation of sea surface itself from the reference geoid surface, not only producing accurate sea surface slope/gradient, which is necessary for estimating ocean current, also placing an emphasis on nearshore estuary and riverine upstream areas where vertical datum transformations are frequently applied for coastal hazard planning purposes. Therefore we propose an approach to create a regional datum TSS for VDatum by integrating i) retracked altimetry SSH (sea surface height) data with custom geophysical corrections and vertical offset adjustment referenced to NOAA's water level gauges; ii) GNSS campaigned water level gauge input to merge with altimetry SSH; iii) airborne gravity based geoid model (xGeoid20B model from NOAA's National Geodetic Survey) that provides enhanced coastal gravity signal and helps reduce errors coming from nearshore SSH. |
