North American height datums and their offsets: The effect of GOCE omission errors and systematic levelling effects

Abstract

One of the main scientific objectives of the Gravity field and steady state Ocean Circulation Explorer (GOCE) gravity field satellite mission is its contribution to the global unification of height systems. In this study, we compute the offsets of three height datums in North America (NAVD88, CGVD28 and Nov07) against a common equipotential surface. NAVD88 and CGVD28 are the official vertical datums for the USA and Canada, respectively. Nov07 is the latest unofficial adjustment of the first-order levelling network in Canada. This datum is only used for the validation of geoid models. The offset for each datum is determined from a combination of ellipsoidal, orthometric and geoid heights. The ellipsoidal heights on benchmarks come from the GNSS networks of Canada and the USA while geoid heights are computed from currently the best GOCE-based geopotential model in North America, i.e., go_cons_gcf_2_tim_r3. The orthometric heights of the GNSS stations are available from the adjustments of the vertical control networks of both countries. Among the various factors that contribute to the uncertainty of the computed datum offset, we investigate the effect of the omission error of the GOCE geoid by means of the EGM2008 model. In Canada, where GNSS/levelling stations are irregularly distributed over the landmass, the effect of the GOCE omission error on the computed offsets reaches one decimetre. Due to the much more densely distributed GNSS/levelling stations in the USA, the effect of the GOCE omission error on the offset of NAVD88 is 3 cm. Therefore, the effect of the omission error of GOCE-based geopotential models should be taken into account in the height datum unification on the North American continent if we aim at the one centimetre accuracy.

The GBVP approach for vertical datum unification: recent results in North America

“…The difference of the MSL at the two coasts is about 60 cm and is explained by the difference in MDT (see, e.g., Hayden et al 2012). Many regional distortions are also found in CGVD28 contributing to approximately a one-meter distortion from west to east (Amjadiparvar et al 2013a). A large northwest-southeast tilt in the US levelling data (Wang et al 2012), the reason for which is still unknown (Marc Véronneau, personal communication), is also quite prominent.…”

“…CGVD28 was constrained to the MSL at five TG stations in the Pacific and Atlantic Oceans and in the St. Lawrence River. A comprehensive review of the levelling-based datums in North America can be found in the work by Amjadiparvar et al (2013a). Although no technical paper on CGVD2013 has been published yet, general information about the new Canadian vertical datum can be found in Lamothe et al (2013) and Véronneau et al (2014a, b).…”

“…These two factors proved to be very important in the estimation of the North American datum offsets. It was shown that improving the density and spatial distribution of the GNSSlevelling stations decreased the effect of the GOCE omission error on the estimated mean datum offsets in North America (Amjadiparvar et al 2013a;Sideris et al 2014).…”

North American height datums and their offsets: Evaluation of the GOCE-based global geopotential models in Canada and the USA

“…Corrections for various systematic errors were applied to the levelling measurements in Canada and the USA: refraction, rod calibration, magnetic field and astronomical (tidal) corrections. A stepwise adjustment procedure was applied to reduce the accumulation of systematic errors in the levelling network in Canada [1].…”

“…The effect of the deflection of the vertical on the computed geoid height is at the submillimeter [8]; therefore, it can be neglected in equation (1). All these errors affect the quality of the GNSS/levelling geoid heights as an external data set for the evaluation of the GGMs.…”