Vertical Datum Issues in New Zealand Nic Donnelly

Vertical Datum Issues in New Zealand Nic Donnelly – Geodetic Data Analyst 5 March 2008

Global Context

New Zealand Height Systems • Unlike the US, New Zealand does not have a single national vertical datum • 13 major leveling datums (and many minor local datums) • Datum origins in terms of MSL at 12 different gauges Major Leveling Datums

Issues • Most vertical datums in NZ were defined over short time frames (1 -2 years) in non-ideal locations so they may not represent long term MSL • MSL changes over time (~50 years since datums defined) • Orthometric heights only on benchmarks beside major highways – sparse coverage • Nationwide leveling inefficient

Height Modernization FROM THIS TO THIS

NZGeoid 05 • NZGeoid 05 developed in conjunction with Curtin University of Technology, Perth, Australia • Geoid determined using the following datasets: – – – Terrestrial gravity observations Marine gravity observations Satellite altimetry Digital topography data Global geoids • Geoid verified with: – GPS derived heights – Leveling derived heights Marks with GPS and Precise Leveling Heights

NZGeoid 05 • A gravimetric geoid (as opposed to geometric) • Ranges from 0 to 40 m above GRS 80 ellipsoid • Provided on a 2 arc-minute grid (~3. 7 km) • Sub 10 cm nationwide accuracy at this resolution

NZGeoid 05 Accuracy

NZ Vertical Datum • Ellipsoidal heights authoritative • Geoid plus offset used to transform from ellipsoid to leveling datums • Offsets can also be used to transform heights between leveling datums (even if they don’t overlap)

Height Modernization Products • • • Ellipsoidal heights on about 20000 geodetic marks Online height conversions to obtain orthometric heights Downloadable conversion software Geoid grid files available Continued maintenance of existing benchmark infrastructure

Next Steps • Surveyor-General Vertical Datum standard • Continued education of the spatial community to encourage geoid model usage for appropriate purposes • Maintenance plan for geoid and existing benchmark network • Possible update when EGM 08 becomes available