A gravimetric marine geoid for the Red Sea was computed using the remove-compute-restore method with RTM reduction and the 1D-FFT technique with Wong-Gore modification , using shipborne gravity , the DTU13 altimetric gravity and the GOCE DIR_R5 GGM up to degree/order 160 and the EGM2008 from degree/order 161 up to degree/order 2190. The question is how to evaluate this model taking into account the absence of costal GPS/Leveling data?
Dear Sir: I recommend to read the article attached, by Wong and Gore.
Also, look out the table (picture) from "Gravity, Geoid and Height Systems Proceedings of the IAG Symposium GGHS2012, October 9-12, 2012, Venice, Italy": "After removing the effects of the global model and residual terrain corrections, anomalies had a mean of 0.18 mGal and standard deviation of 4 mGal if you take the EGM2008 model. If you take EIGEN-6C model, the mean residual anomalies are about 0.17 and the standard deviation 6 mGal."
Best wishes, Mario E. Sigismondi
Dear Ahmed,
You have raised a widely neglected but fundamental flaw in the use of GPS for elevation measurements. GPS estimates of latitude and longitude are extremely accurate because the target locations are calibrated with reference to the nearest ground stations, whose exact 2D relative positions are known by conventional trigonometry or astronomy. Thus allowance is made by GPS for constantly changing atmospheric distortions of the electromagnetic spectrum, which can be taken into account in real time.
The elevations of local ground stations above the centre of the Earth's oblate spheroid is not known with similar accuracy. There are many reasons for this but GPS calculations assume that sea level closest to the local ground stations conforms to one of the conventional spheroids and that every ground station elevation is fixed above that datum.
There are several flaws in those assumptions, for example: Firstly, in the middle of the Indian Ocean the nearest ground stations may be a thousand kilometres distant, so atmospheric distortions are incalculable because of curvature of the earth's surface; Secondly it is assumed that the Earth's radius conforms to that spheroid everywhere and the solid crustal surface is not subject to tidal forces or expansion due to subterranean volcanic forces; Thirdly, it is assumed that there has been no change in elevations of ground stations above sea level or its associated spheroid since that elevation was most recently measured; Fourthly it is assumed that all GPS satellites in the cluster were orbiting at accurately known distances from the centre of the spheroid at the time the location elevation was specified.
The total of these inaccuracies can amount to at least tens of metres in many remote locations. This can be confirmed by checking GPS elevations while standing on a beach. For your Red Sea problem you need to check the locations of your local GPS ground stations carefully, particularly that their elevations above the GPS assumed geoid have not changed.
Hope this helps.
Malcolm.
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