Home' Position : Position Jun Jul 2015 Contents Another challenge that arose with four test points along
Bells Line of Road was the inability to observe directly over a
benchmark due to limited sky view or multi-path issues. This
problem was overcome by placing an arbitrary (eccentric)
mark a short distance away at a location with more favourable
observing conditions. The AHD height of the benchmark was
then transferred to the arbitrary mark using reciprocal EDM
heighting. A spirit level was not used because this could not be
achieved by one person in the field.
Results: flat terrain
Each test point was occupied between three and six times with
NRTK GNSS using CORSnet-NSW and 3-minute observation
windows. The AHD heights obtained by applying the three quasi-
geoid models for each test point were then compared to the
official, published values.
We found thatAUSGeoid09 allows AHD height
determination with an accuracy of about ±0.03m (1 sigma)
in flat terrain. This accuracy agrees very well with previous
studies across NSW and Australia. As expected, using
AUSGeoid09 rather than its predecessor AUSGeoid98, resulted
in substantially better agreement with published AHD heights
(at the 700% level).
Comparing the results obtained with AUSGeoid09 against
those using AGQG2009 illustrated, as expected, the benefit that
the introduction of the geometric component of AUSGeoid09
has had on the determination of AHD heights with satellite
positioning technology (250% improvement). For all test points,
AUSGeoid09 provided heights that are about 30-50mm closer to
the published AHD values than those obtained using AGQG2009.
This improvement is consistent with the geometric 'sliver'
component of AUSGeoid09 generally amounting to about
-0.05m or less in this area. The evolution from AUSGeoid98 to
AGQG2009 and AUSGeoid09 has significantly improved the fit
between GNSS-derived and published AHD heights in this part
of the study area. So far, so good.
Results: mountainous terrain
The same procedure was applied to the remainder of test
points, located in mountainous terrain along Bells Line of
Road. In this part of the study area, AUSGeoid09 allows AHD
height determination with an accuracy of about ±0.06m (1
sigma), i.e. half the accuracy of flat terrain. Again, as expected,
AUSGeoid09 provided substantially better agreement with
published AHD heights than its predecessor AUSGeoid98
Interestingly, two test points showed much larger
discrepancies (at the 120mm level) to the published AHD
heights than all other test points in the study area. At point 18,
this disagreement may be attributed to mark subsidence, but
the limited data available precludes a definitive answer in both
cases. If these two test points were removed from the analysis,
the accuracy of AUSGeoid09-derived AHD heights improves to
±0.05m (1 sigma).
Now things get really interesting. Comparison of the results
obtained with AUSGeoid09 and AGQG2009 showed that, contrary
to the findings in flat terrain, the introduction of AUSGeoid09's
geometric component overall has not had a positive effect in
this part of the study area. Closer inspection revealed that for
elevations below 500m, the geometric component improved the fit
to published AHD heights by about 10-20mm.
However, for elevations above 500m, the geometric 'sliver'
component appears to degrade the fit by about 10-30mm. For
elevations above 1,000m, this negative effect is even larger. While
we recognise that the sample size is small, this does indicate
possible problems with the geometric component at high
elevations -- not surprisingly, as it is well known that suitable
datasets for the generation of the geometric component are
notoriously sparse in mountainous regions.
It also needs to be remembered that the gravimetric geoid
is weaker in mountainous regions because (1) gravity data are
limited, (2) existing gravity data are biased along the ridges (roads)
and creeks for ease of access, and (3) the terrain effect is less well
modelled in regions of large elevation changes in topography.
Investigating the effect of the geometric 'sliver' component
in more detail, it is interesting to note that the improvement of
fit to published AHD steadily decreases from east to west in the
study area, from about 55mm in the western outskirts of Sydney
to zero near Kurrajong. Heading further west through the Blue
Mountains, the geometric component increasingly degrades
(almost linearly with distance) the GNSS-based determination of
AHD heights in the study area, culminating in up to 80mm at the
highest elevation near Lithgow (see Figure 3.).
This can be explained by the decreasing density of datasets
available for the empirical determination of the geometric
component away from metropolitan areas. It is also interesting
that across the entire study area, featuring both flat and
mountainous terrain, AUSGeoid09-derived AHD heights are
always lower than the published AHD heights. This indicates
that there is room for improvement in regards to future versions
of the AUSGeoid model, provided additional datasets are
collected or included in this region.
In order to provide another visual perspective of these results,
cross sections were generated showing published AHD heights
and NRTK GNSS-derived AHD heights based on the three
quasi-geoid models investigated (see Figure 4.). The cross
sections run from left to right in a west-to-east direction and
have been scaled and exaggerated (separately for each part of
the study area) to allow visual inspection.
Across both terrain types, it is clearly evident that
AUSGeoid09 (green) provides a far better fit to published AHD
(dark blue) values than its predecessor AUSGeoid98 (light
blue). In flat terrain (eastern part of study area), AUSGeoid09-
derived heights are consistently closer to published AHD than
AGQG2009-derived heights (red), showing the benefit of the
geometric component. The shape of all quasi-geoid-derived
cross sections is very similar to the shape of AHD in this part
of the study area.
Figure 3. Difference in fit to published AHD heights between AUSGeoid09
and AGQG2009 derived AHD heights in the study area.
30 position June/July 2015
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