Official GNSS-derived Vertical Orthometric Height Control Network
Since the beginning of 2006, the vertical control system in Israel has been ellipsoidal. The Survey of Israel (SOI) will not continue with nationwide precise leveling. Instead, data from permanent Global Navigation Satellite Systems (GNSS) stations are used to develop and maintain Israel's ellipsoidal vertical control system. Currently, the objective is to provide a framework on which topographers and engineers can base and adjust their heights, and this is achieved more easily by GNSS measurements. Despite this, many users of height data prefer the orthometric system, even though its control is localized. A 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 development of an undulation model. Efforts to develop such a model with an accuracy level of 1 centimeter over an entire country demand multiple resources. This paper suggests a method for developing a national, orthometric control system with a reasonable accuracy, using GNSS measurements, ellipsoidal vertical control, and an official geoid undulation model. Research was conducted in Israel in order to test the feasibility of using an official geoid undulation model. Two kinds of official models were tested: the worldwide geopotential model (GPM98B) and a local, countrywide model computed by the Survey of Israel. We compared orthometric height differences obtained by GPS measurements with the known orthometric differences. This paper presents the results of those experiments. The technique proposed in this paper was found to define seamless orthometric vertical control adequate for most geodetic and surveying purposes, at low cost and fast. Of special interest in the paper is the comparison between the proposed idea and the National Geodetic Survey (NGS) attitude toward GPS-derived orthometric heights. The advantage of using the official geoid undulation model for the acquisition of orthometric heights is demonstrated.
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