Global Positioning System Reference
In-Depth Information
benchmarks were first estimated by combining ellipsoidal heights determined by using the
Dubai NRTK with the local gravimetric geoid model data and were next compared to
known orthometric heights of the benchmarks. The test area spanned approximately 22.7
km x 7.8 km in the Easting and Northing directions respectively, representing the area
acquiring the most demanding survey works in the Emirate of Dubai. The height difference
between the highest and lowest points in this test was approximately 34.5 meters. Each test
point was occupied for a period of a few seconds, representing an ordinary working
environment. The standard deviations of the ellipsoidal height determination for the
occupied points of the test network ranged between 1.05 cm and 5.47 cm (El-Mowafy
et al
.,
2005).
Figure 5 shows the differences in orthometric heights between using the “NRTK GPS +
geoid heights” and the known orthometric heights of the benchmarks. On average,
differences were within ±5 cm, with a maximum value of 7.04 cm. The statistical results of
the differences are presented in Table 3. The average value of the absolute differences was
2.4 cm with 3.05 cm standard deviation. The differences towards the north-east were greater
than those at the south-west region of the test. This can mainly be attributed to accuracy of
the geoid model used in the test area. Figure 6 illustrates the
surface plot of the height
difference
between the two methods. These results show that no significant systematic
errors were present. The achieved accuracy is considered precise enough for third order
levelling, which represents the majority of levelling works being carried out.
10.00
8.00
6.00
4.00
2.00
0.00
-2.00
-4.00
-6.00
-8.00
-10.00
0
10
20
30
40
Points
Fig. 5. Height differences between GPS Network RTK geoid heights and precise levelling
(cm)
Max.
difference
Average of absolute values
σ
2.40
7.04
3.05
Table 3. Statistics of height differences between the GPS network RTK + geoid and precise
levelling (cm)
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