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Using GNSS to establisha Height Datum on a Survey
www.quickclose.com.auRichard Stanaway QUICKCLOSE
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
The Geoid
Universität Stuttgart
Heighting Surfaces
Differences between MSL and the geoid(Mean Dynamic Topography)
H = h - N N = h - H
Australian Height Datum
GDA94 Ellipsoid Height
Ausgeoid98
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
Complications…..
AHD20?? – 0.1 to 0.2 m higher (sea level rise)
Regional and localised subsidence (groundwater changes)
Levelling errors and mark disturbance
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
Steps to get “local” AHD using GNSS/GPS
1. Determine phase centre offsets / models2. 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)
4. 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
LiDar Ground validation - example
Ground Validation Points
Local AHD Control
Control Information
Static Data Processing - Setup
Network and Baselines
Network and Baselines
Results
Baselinesolutions
