Global Positioning System Reference
In-Depth Information
Average
(cm)
Maximum
(cm)
RMS
(cm)
E
0.95
2.35
1.00
N
1.12
2.52
1.18
Table 1. Statistics of positioning differences between GPS RTK-Network with the total station
To check repeatability of results (internal accuracy), another independent survey was
carried out after two days and 4 hours to re-determine the coordinates of the same marked
points previously determined by GPS. The average and maximum values of the differences
between the results of the two surveys are given in Table 2. As the table demonstrates,
repeatability testing showed that the average value for differences in the total planimeteric
coordinate estimation was at the cm level, while for ellipsoidal height determination it was
1.56 cm. These differences can be attributed to changes in the quality of the measurements
used, which mainly resulted from differences in the number of the observed satellites and
their geometric distribution. These parameters have affected the quality of the network
computations of the measurement corrections and the quality of coordinate estimation at the
rover.
Average
(cm)
Maximum
(cm)
E
0.85
1.23
N
1.15
1.88
h
1.56
3.22
Table 2. Statistics of coordinate discrepancies between different observing sessions
Unlike traditional levelling, GPS derived heights are referenced to an ellipsoidal datum
(WGS 84) and do not depend on local gravity variations, whereas in most levelling works
and mapping orthometric heights are used. Orthometric heights reflect changes in
topography as well as local variations in gravity. They are referenced to the geoid, which is
an equi-potential level surface of the Earth that is closely associated with the mean sea level
on a global basis. To convert ellipsoidal heights from GPS (h
GPS
) into orthometric heights
(H), geoid heights are needed, such that:
H = h
GPS
- N
(23)
where (N) is the geoid height. Thus, with the use of one receiver and employing NRTK to
determine ellipsoidal heights, orthometric heights can be determined if a good geoid model
is available.
To assess accuracy of
orthometric height determination by using NRTK, the second test was
performed in Dubai, using the DVRS network. The Dubai gravimetric geoid model was
used, which was developed by integrating a comprehensive set of gravity measurements
with GPS, levelling and digital elevation data. The computed geoid fits GPS/levelling at the
3-4 cm level RMS (Forsberg
et al
. 2001). The test was performed on a network consisting of
41 benchmarks of the second order levelling network. Orthometric heights at these
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