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

2. Global Context

3. 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

4. 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

5. Height Modernization FROM THIS TO THIS

6. NZGeoid05 NZGeoid05 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

7. NZGeoid05 A gravimetric geoid (as opposed to geometric) Ranges from 0 to 40 m above GRS80 ellipsoid Provided on a 2 arc-minute grid (~3.7 km) Sub 10 cm nationwide accuracy at this resolution

8. NZGeoid05 Accuracy

9. 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)

10. 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

11. 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 EGM08 becomes available