Evaluation Of EGM2008 by Means Of GPS/Levelling In Uganda
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Date
2013-04
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Addis Ababa University
Abstract
The global gravity model EGM2008 is evaluated in various regions of the country to assess if it is
good enough for geodetic applications. The evaluation method involves comparison of geoid
heights computed from the model with those computed at irregularly distributed GPS/levelling
stations. For testing the model, a total of seven levelled benchmarks available in Uganda which
belong to the New Khartoum datum are used. The spatial positions of these benchmarks were
determined at mm accuracy, with respect to ITRF2008. The agreement between the EGM2008
geoid and the geoid undulation derived from GPS/levelling over the seven irregularly distributed
benchmark points has a standard deviation of 0.255m, with a mean of -0.859m. The datum offset
may be due the choice of Wo (potential of the geoid) and Uo (potential on the surface of the
ellipsoid); using GRS80 for the gravitational reference system and WGS84 for the geometrical
reference system; some possibly different tidal conventions; but, by using the same method of
analysis for Ethiopia and Uganda, these absolute offset effects are eliminated when comparing the
two so that the computed difference [0.118 m] in datum offset for the two states does tell us
something about the differences in levelling datums. The standard deviation of 26 cm suggests that
sparser, irregularly-distributed and inhomogenous gravity data for Uganda was used in the
development of EGM2008 not ruling out errors in levelling since there is barely any documentation
pertaining the accuracy of results obtained regarding the levelling network in Uganda.
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Keywords
EGM2008, GPS/Levelling In Uganda