Using GNSS to establish

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Using GNSS to establish a Height Datum on a Survey
Richard Stanaway QUICKCLOSE
www.quickclose.com.au
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Overview of GNSS Heighting
Height systems and Geoid models
AHD and MSL
Measuring ellipsoid Heights
Working with height datum offsets
Victorian case-study - Murray Valley LiDar
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The Geoid
Universität Stuttgart
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Heighting Surfaces
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Differences between MSL and the geoid (Mean
Dynamic Topography)
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H h - N
N h - H
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Australian Height Datum
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GDA94 Ellipsoid Height
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Ausgeoid98
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local AHD offset (o) can computed by GNSS/GPS
measurements at local PMs.
o H(AHD) h(GDA94) N(Ausgeoid98)
H(AHD) h(GDA94) - N(Ausgeoid98) o
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Complications..
AHD20?? 0.1 to 0.2 m higher (sea level rise)
Regional and localised subsidence (groundwater
changes)
Levelling errors and mark disturbance
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Antenna Reference Point and Phase Centre
GNSS measurements made to phase centre (and not
antenna reference point)
trunnion axis of the GPS
Different antennas have different phase centre
offsets!!
Good advice to use phase centre heights rather
than ARP heights if different antennas are used
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Steps to get local AHD using GNSS/GPS
1. Determine phase centre offsets / models

1. Ensure correct geoid model is used (Ausgeoid98 or
   Ausgeoid09 in Victoria)

3. Take measurements at local high order (3rd
order or higher) PMs/PSMs to compute offset (o)
(The Ausgeoid surface should be parallel with AHD
over a localised area)

1. Apply the offset to Ausgeoid derived elevations
   in order to align them with local AHD

Some later GNSS systems have a local geoid
builder in their software (local AHD calibration).
Alternatively, a local geoid model can be created
and loaded into a GNSS controller or software
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LiDar Ground validation - example
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Ground Validation Points
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Local AHD Control
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Control Information
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Static Data Processing - Setup
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Network and Baselines
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Network and Baselines
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Baseline solutions
Results
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Thank You!
More information at
www.quickclose.com.au/publications.htm
richard.stanaway_at_quickclose.com.au