The effect of noise on geoid heights in Stokes-Helmert method
20/09/2016 | 17:00 | Session 3: Local/regional geoid determination methods and models
Author(s): Yosra Afrasteh, Abdolreza Safari, Ismael Foroughi, Michael Sheng and Robert William Kingdon
Yosra Afrasteh, Abdolreza Safari, Ismael Foroughi, Michael Sheng and Robert William Kingdon
Noises are an inevitable part of gravity observations and they can change the accuracy of height datum if they are not treated suitably in geoid determination. In order to provide data to solve the geodetic boundary value problem, surface gravity observations must be transferred harmonically down to geoid, which is called Downward Continuation. This step magnifies existing noise in Helmert gravity anomalies because of the inherent numerical instability of inverting the Poisson integration, despite later smoothing when evaluating the Stokes’s integral. The effect of noises in different steps of Stokes-Helmert geoid determination approach is intended in this research.
The territory of Iran, from 44 to 62° longitude and 24 to 40° latitude, was considered for the purpose of this study. The global gravity model EGM2008, up to degree/order 2160 was used to compute the free air gravity anomalies on a regular grid on topography. The SHGeo package, developed at the University of New Brunswick (UNB), was used to estimate the effect of noises when using the Stokes-Helmert method. After transferring the free air gravity anomalies to Helmert space, three different noises were added to the data and their effect was evaluated by comparing the results of ‘noisy’ and ‘clean’ data after each step.
Results revealed that if the downward continuation of 5*5 arc-min surface points is required, the standard deviation of differences between ‘noisy’ and ‘clean’ data on topography will increase by 15% with respect to the corresponding standard deviation on geoid. This increase will be almost 40% if the 1*1 arc-min points are required for downward continuation.