1

Geoid and Geoid Change: Discussion Topics

Roger Haagmans, Boulder, 21October 2009

2

Geoid and geoid change: personal interest

Unification of height systems

Improved estimates of sea level change

My house

3

Geoid and geoid change

• Gravimetric geoid: definition equipotential surface

• What about change?

“GRACE”, future constellations and/or terrestrial gravity measure all changes that cause the equipotential to change in time: “all periods and all spatial scales”

– Change in time wrt “static mean” choice should integrate to zero otherwise the Earth is growing/shrinking

– For every epoch the mass redistributes regionally (e.g. land ice to sea) but the global integral is every time the same

4

Current status

“Static geoid”

• Took years to arrive at present situation (gravity and auxiliary data (topography/density))

• Produce gravimetric geoid with sufficient weight for the local data (local data should determine the local to regional scales rather than a global model!!!)

• Re-analysis of “higher” latitude satellite data residuals (GRACE + GOCE) for improving the geoid

• carefully analyse error characteristics: also in comparison to GNSS & levelling and Altimetry & MDT

5

Errors in gravimetric geoids vs gps/levelling or satellite altimetry/MDT

6

Errors in gravimetric geoids vs gps/levelling or satellite altimetry/MDT

7

MSL Satellite Altimetry vs tide gauge:harmonising geometry

• MSL altimetry to mean tide gauges over same period of time

• Airborne connection

• Future coastal altimetry: CryoSat “SAR altimeter” ?

hA GNSS hB GNSS

MSL altimetry

Globally consistent

W0

Choice of W0

W0

8

Global height system unification

• Global height unification based on satellite only models

(commission and omission error analysis necessary)

• Could include regional height reference point and global degree 1

• Height system correction for all terrestrial gravity and levelling in

same system

Stokes & GGM Stokes & GGM with MWG

9

Geoid change and surface change

What does it mean to isolate a specific effect of the geoid change e.g.

PGR & GIA?

- incomplete according “definition” but maybe ok for application

- accurate enough from satellite data or drowning in corrections

errors/filters and noise (do we understand the error characteristics)?

- accurate enough from ground data combination of geometry and

gravity?

- does equipotential change necessarily mean geometrical change

(at ocean expansion from temperature!) ?

10

Geoid change and surface change

- 2 effects on measurement:

+ geometric change in ΔH= Δh- ΔN over characteristic distance

(if distance << phenomenon expected change in ΔH <<)

+ set up of instrument different (spirit level) following different

local gravity vector as a consequence mass change (too

inaccurate ?)

- tide gauges follow change as well: change in height reference point

possible

- Re-analyse levelling heights and keep certain benchmarks for

validation purposes

11

Geoid change and surface change

USER REQUIREMENTS:

- Geoid definition: space-time validity with certain accuracy

- GNSS based Levelling: regional requirements probably ok?

- tide gauges/altimetry & height system unification: global requirements

- put setup for North-America in global context (gravity changes from

satellite missions relate to mass transport/re-distribution in the Earth

System - difficult and unnecessary to try to isolate specific phenomena in

first processing and data base – then tailor made product)

- planning tools (with simulator capability) for future networks (used to be

a strenght of geodesists)

- do we need to define specs for future satellite missions (towards a

continuous operational series ?