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USING AN OFFICIAL UNDULATION MODEL FOR ORTHOMETRIC HEIGHT ACQUISITION BY GNSS

Abstract Since the beginning of 2006 the vertical control system in Israel has been ellipsoidal. The Survey of Israel constructs and maintains the ellipsoidal vertical control system based on permanent Global Navigation Satellite System (GNSS) stations. Nowadays, the vertical control system's objective of providing a framework upon which topographers and engineers can base and adjust their heights is achieved more easily by GNSS measurements. Despite this, the vertical orthometric control system is still in use since many height data users prefer the orthometric system. However, the characteristics of the orthometric control system are localized. The Geoid Undulation Model is one of the foundations of the geodetic infrastructure. It is used to connect ellipsoidal and orthometric heights. Intensive utilization of GNSS for geodetic, surveying and engineering applications necessitates the rapid development of an undulation model. Today, the efforts to develop a geoid undulation model with an accuracy level of one centimetre over an entire country demand multiple resources. This paper suggests a method for developing an orthometric control system with a reasonable accuracy level on a nationwide basis by means of GNSS measurements, ellipsoidal vertical control and an official geoid undulation model. The best available geoid undulation model can be used as the official geoid undulation model, regardless of its accuracy level. Research was conducted in Israel in order to test the feasibility of the idea of using an official geoid undulation model. Two kinds of official models were tested, the world wide geopotential model GPM98B and a local countrywide model computed by the Survey of Israel. We compared the orthometric height differences obtained by GPS measurements using the undulation model with the known orthometric differences. This paper presents the results of these experiments. It verifies the capability of the suggested technique to define seamless orthometric vertical control, adequate for most geodetic and surveying purposes, via a low-cost and fast procedure.