Using GNSS to establish a Height Datum on a Survey Richard Stanaway QUICKCLOSE.

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

Thank You!

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