MODELING OF ORTTHOMETRIC HEIGHT FROM MULTI-NETWORKS OF GNSS/PRECIS LEVELING IN CANADA

Abstract As a result of wide spread in Canada use of satellite based positioning techniques, especially Global Positioning System (GPS), a greater attention has been focused on precise determination of geoid models with an aim to replace the classical leveling with Global Navigation Satellite System (GNSS) measurements. In this study, geometric technique of deriving orthometric height from GPS survey along a profile and the use of EGM 96 geoid model for deriving orthometric height from GPS data (using GNSS solution software) are discussed. The main focus of the research is to critically examine the potentials of these methods with a view to establishing the optimum technique as an alternative to classical differential levelling. From the results obtained, the standard errors are 1.453m and 1.450m for EGM 96 model and the geometrical approach respectively. From the graphical representation of the residuals from the two methods, it was observed that the two curves suddenly became sinusoidal from station 9 (corresponding to SB08 in the tables). This similarity pattern of the residuals makes it difficult to draw a conclusive judgment between the two methods examined; however, from the standard errors, it could be inferred that the geometrical technique gave a better result over EGM 96 model. Key Words: Geometrical Interpolation, EGM 96 Model, Orthometric Heights, Ellipsoidal Heights Introduction The classical Vertical control is composed of several hierarchical networks which follow the principle of "working from whole to parts". The primary Vertical Control Network contains loops of first order precise leveling of some hundreds of kilometers in length. The accuracy of precise leveling (high precision leveling) should be at the millimeter-level per kilometer (Bomford, 1980). The other subnets in the network are densifications of the primary one, according to the needs. It is worthy to note that it is reasonable to establish a third-order network in the densely populated area. However, the difficulties involved in precise leveling are well-known; for example, Eriksson et al, (2002) observed that even with the most advanced technology of motorized leveling, it took some 25 years to accomplish the first-order network in Sweden. Due to such difficulties, it is actually impossible to get heights for lower-order networks with absolute accuracy (relative to the higher-order) better than 5-10 cm (Steinberg and EvenTzur, 2005). Because there was no alternative to precise leveling, as a tool, to achieve the objectives of primary leveling networks prior to the GPS era, it was naturally referred to as vertical control. However, current researches have shown that GNSS measurements are quite more effective for monitoring vertical tectonic changes over a wide area. Consequently, the ideas of Vertical Ellipsoidal Control, or 3-D Geodetic Control, based