Global Positioning System Reference
In-Depth Information
is 19.3 cm. When the TG03 is refined with LSA of orthometric height differences on the 58
baselines among the 22 reference and 9 test benchmarks, the accuracy of the refined TG03
model (version 1) is ±15 cm in terms of the RMSE of geoid heights at 9 test points. Hence the
improvement of the model is approximately 42%. The refined version of TG03 (version 2)
using CS fitting revealed ±19.2 of RMSE of geoid heights at the test points. The third version
of refined TG03 was computed using LSA of geoid height differences on the baselines with
estimated variance information from iterative MINQUE algorithm, and the internal
accuracy of the computed geoid height values having ±4.9 cm RMSE at 31 points were
obtained. As expected the 2
nd
order polynomial type local GNSS/levelling geoid model
revealed the worst results with ±46.6 cm RMSE of geoid heights at the test benchmarks. All
the results can be compared using the summary statistics at Table 5. For further reading on
the applied methods for TG03 local refinement in Çankırı area and the associated case
study, Erol et al. (2008) can be referred to.
refining method
min.
max.
mean
RMSE
TG03
-
-10.8
60.4
19.3
26.2
refined TG03_ver.1
LSA of ΔH on Baseline
-28.7
23.1
0.0
15.0
CS fit., 1
st
order
model
-25.4
43.3
0.0
17.6
refined TG03_ver.2
polynomial
test
-45.4
28.5
0.0
19.2
local geoid model
2nd order polynomial
-120.1
85.7
0.0
46.6
Table 5. Statistical comparison of TG03 and its refined versions in Çankırı (units in
centimetre) (Erol et al., 2008)
4. Summary of results and remarks
This chapter compares geoid models from various scales in Turkish territory and aims to
provide a road map to GNSS users in practice, with regards to how to choose, compute and
use of the geoid model as a tool for transformation of GNSS ellipsoidal heights to the
regional vertical datum. As the traditional levelling techniques for obtaining precise height
information are left aside, the improved accuracy of the geoid, as a modern technique for
vertical control called, known as GNSS(-geoid) levelling, can be contemplated as an
alternative for practical height applications. In the numerical evaluations, presented as part
of this chapter, the recently released global geoid models, which include the data by the
latest gravity field satellite missions, CHAMP, GRACE and GOCE, were tested against the
terrestrial data. The results from this indicate the absolute accuracies of the two ultra-high
resolution combined global geopotential models, EGM08 (
ℓ
max
= 2190) and EIGEN-6C
(
ℓ
max
= 1420) in Turkey were calculated around ±17 cm, which means that these global
models can directly be used for GNSS levelling in small scale map production and
applications that requires regional orthometric heights with decimetre accuracy. A
comparison on validation results of satellite only global models put EIGEN-6S and
GOCO02S forward that these models were calculated using GOCE and GRACE missions'
data until 240, 250 maximum degrees of expansion, with ±44.0 cm absolute accuracy at the
test points. Comparing these models, performance of the GGM03S (
ℓ
max
= 180), the GRACE
only model, stayed rough in representation of the local gravity field in the region. Therefore
in modelling the regional hybrid geoid, EIGEN-6S and GOCO02S may provide better
performances.
Search WWH ::
Custom Search