Since 1990, positioning technology has undergone a dramatic improvement in terms of accuracy
and accessibility. Prior to 1990, national geodetic datums were considered to be static with fixed
coordinates assigned to the datum origin by convention. Datum coordinates were dynamic only as
a consequence of re-observation and re-adjustment, or localized disturbance. This strategy has
been suitable for terrestrial surveys within stable continental areas. The accessibility of Precise
Point Positioning “PPP” is now widespread with the provision of free online processing facilities
by various geodetic agencies. Such ready accessibility to a global datum has serious implication for
this technology with static geocentric datums. The Geocentric Datum of Egypt (ITRF1994 Epoch
1996 based on GPS Observation Campaign 1996), for example, is now offset by −42.0 to +32.0cm
from the ITRF2008 Epoch 2015 due to the inexorable tectonic movements of the Nubian plate
since the datum realization in 1996. Unless the local geocentric datums are kept in step with global terrestrial reference frames, discrepancies between the two will increase in magnitude over
time and will become discernable, even using PPP. The PPP users nowadays are unaware of this datum offset and incorrectly assume that a coordinated datum monument is in error if there is found
to be a disagreement greater than the level of accuracy of the positioning technique. In this study, to
avoid the discrepancies between the applied datum and On-Line GNSS Services, a semi-kinematic
datum and related deformation mode are proposed for Egypt. The result also confirmed that the
PMM model was giving good performance for the other parts out of Egypt. The evaluation study
shows the best performances for the Egyptian deformation model over the ITRF2008 PMM. |
