Integration of Future Geoid Models

Dan Roman and Yan M. Wang

NOAA/NGS

Silver Spring, MD 20910 USADecember 3-4, 2008

Overview

• Introduction

• History of geoid modeling at NGS

• Tentative date of publication of GEOID09

• Plan and specification of future of geoid models

• Interaction between the users community and NGS

What is a geoid

• An equilpotential surface which coincides with an idealized mean sea level (mean sea surface minus permanent ocean dynamic topography)

• The reference ellipsoid (e.g. GRS80, or WGS84) is an approximation of the geoid

• The geoid height is the vertical distance between geoid and reference ellipsoid

Why we need it

• Conversion surface between NAVD88 (orthometric) and GPS (ellipsoidal) heights

• Defines next national vertical datum• Computation of deflections of the vertical• Surface Gravity Interpolation• Mean ocean dynamic topography?• Time varying geoid (gravity)?

Why we need geoid (cont.)

• Why not GPS (ellipsoidal) height?- historic convention: height above sea level:HAULOVER PIER, N. MIAMI BEACH

h=-25.02 m H(NAVD88)=+2.972 m

NEWPORT, NARRAGANSETT BAY

h=-28.36 m H(NAVD88)=+1.612 m

……

Why we need geoid (cont.)

• Water flows usually from high to low orthometric height, not necessarily from high to low ellipsoidal height

• Useful to identify inundation area, plans for storm surge, environmental change monitoring

Geoid used in GPS campaign

• Simple relationship: H=h-N

h – GPS (ellipsoidal) height

H – orthometric (e.g. NAVD88) height

N – geoid (e.g. GEOID03) height

This formula is accurate to a fraction of mm

Geoid modeling History at NGS

1. Milbert, D. G., 1991b: GEOID90: A high-resolution geoid for the United States. Eos, 72(49), pp. 545-554.

2. Smith, D.A. and D.G. Milbert, 1999: The GEOID96 high resolution geoid height model for the United States, Journal of Geodesy, V. 73, N. 5, pp. 219-236.

3. Roman, D.R. and D.A. Smith, 2000, Recent investigations toward achieving a one centimeter geoid, presented at GGG2000 session 9 of the IAG Symposium in Banff, Alberta, Canada from July 31 - August 4, 2000.

4. Roman, D. R., Y. M. Wang, W. Henning, J. Hamilton, Assessment of the New National Geoid Height Model—GEOID03, Surveying and Land Information Science, Vol. 64, No. 3, 2004, pp. 153-162

Geoid computation theories

• Stokes theory

– Using gravity measurements, requiring global coverage

– For precise geoid, the Earth’s topography and its density need to be known

Geoid computation theories (cont.)

• Molodensky theory – Using gravity measurements, no need for

knowledge of the density of topography– It does not provide geoid, rather a similar but

non-equilpotential surface – the telluroid.– The distance between telluroid and the

Earth’s surface is called the height anomaly

Height anomaly and geoid height

• Under the first order approximation

– Conclusion: no matter which theory is used, the topography and its density are needed to be known for precise geoid computations

HHGg

N)2(

Few words about NAVD88

• NAVD88 is the official vertical datum of the United States

- It is based on leveling and gravity, very accurate locally

- No satellite data used

- Tied to Rimouski (zero point)

Geoid Height DifferencesNAVD88-GGEM02S

Discussions• The southeast to northwest trend

• The most significant difference occurs in the Pacific Northwest region

• 98cm bias between NAVD88 and GGM02S geoid surface

• Father Point/Rimouski (zero point of NAVD88) has a bias of 80 cm above the global geoid of GGM02S

Geoid in NGS 10 Yrs Plan

• A gravimetric geoid will serve as the next National Vertical Datum

- High accuracy of the geoid (2 cm) and high resolution (<= 1 arc minute)

- Fixing the southeast to northwest tilt in the NAVD88

- Combing satellite data (models) and surface data (gravity, GPS/leveling, …)

- Deciding and adopting a new W0 value for the US and establishing relationship with a global vertical datum

How to reach cm accurate geoid

• Theory and computation methods development

- Develop and refine theories to the highest accuracy

- Study the effect of the data spacing and data accuracy on geoid; provide guideline for data collection

- Test and evaluate numerical computation methods to increase computation accuracy and speed up the computation

How to reach cm accurate geoid (Cont.)

• Data collection and processing- Collect surface gravity data with accurate positioning,

absolute or relative - Airborne gravity – GRAV-D, data processing and

filtering, downward continuation- Compiling and cleansing digital elevation models to the

highest resolution- Acquire mass density of the topography- Re-compile and compute the potential number

differences from NGS leveling database

National and International cooperation

• NGS visiting scientist program• IAG working group “North American Geoid”

chaired by D. Roman • IAG study group “Towards cm-Accurate Geoid -

Theories, Computational Methods and Validation” chaired by Y. Wang

• NOAA internship• Cooperation between US, Canada, Mexico,

Virgin islands

National and International cooperation (cont.)

Data issues:• Obtain DEM data from any available sources• Collection of mass density data• Data exchange between US government

agencies• Data exchange between US, Canada, Mexico,

Virgin island counties• Support and cooperate with State on Height

Modernization Program (data collection and modeling)

GEOID09

• GEOID09 will become available around ACSM09

• Data format and access method are the same as the GEOID03

• Interpolation program intg can be downloaded at NGS web site

• Expect dm changes at some BMs

Future Geoid modeling plan

• One geoid model per year for internal and external evaluation and testing

- Models will be published at NGS web site- Data format, grid size and coverage areas will

be kept the same, except specially announced- Suggest (plan) a user discussion group hosted

by NGS- Create and publish FAQ for geoid & related

issues (simplify GEOID page?)

Summary

• Next national vertical datum will be a model based on satellite and surface data

• Beta version of the models will be published at NGS web site

• Expect a big change along the road

• Welcome any feedback from the user community

Point of contact

By phone:

Dr. Dan Roman: 1-301-713-3202 x 161

Dr. Yan-Ming Wang 1-301-713-3202 x127

By e-mail:

Dr. Dan Roman: Dan.Roman@noaa.gov

Dr. Yan Ming Wang Yan.Wang@noaa.gov