Using GNSS to establish a Height Datum on a Survey

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Using GNSS to establish a Height Datum on a Survey
Richard 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 / models 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) 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

Baseline solutions Results