Vertical Datum Unification in the Philippines from Analysis of Tide Stations

Being an archipelagic country, the Philippines has yet to adopt a national vertical datum. A datum is usually defined by averaging sea-level observations at one or more fundamental tide gauges. However, the mean value of the local datum at tide gauges does not coincide with geoid. This paper describes the results of determining the potential difference between datum zones by using a direct and indirect method as applied in the vertical datum unification of tidal stations in the Philippines. The results show that the potential differences computed from least square adjustment differ if data from gravity surveys and geoid modeling (e.g. NRMDP data) from those data acquired from GPS and levelling (NAMRIA). However, discrepancies were in the in the ellipsoidal heights used. These differences therefore had to be reconciled. Addressing this concern is important since the same data can be used to correct a future geoid model.