Geoid Modeling and GRAV-D: Gravity for the

Redefinition of the American Vertical Datum

Beaumont, TexasJune 8-9, 2009

Renee ShieldsHeight Modernization Manager

• Define the NSRS– “The NSRS must be more accurate than all activities which

build upon it, while still being practicably achievable.”

• Maintain the NSRS– “NGS must track all of the temporal changes to the defining

points of the NSRS in such a way as to always maintain the accuracy in the NSRS definition.”

• Provide Access to the NSRS– “NGS must develop and maintain guidelines for users to

access the NSRS at a variety of accuracies.”– “NGS will publish all coordinates of defining points of the

NSRS with an epoch tag and will furthermore publish velocities relative to that epoch-tagged set of coordinates”

http://geodesy.noaa.gov/INFO/ngs_tenyearplan.pdf

From the NGS 10-Year Plan

• The official national coordinate system of the U.S. federal government which includes:– Geodetic latitude, longitude and height– Scale, gravity, and orientation– How these values change with time

• Components include:– National and Cooperative CORS– Network of passive monuments– Official national shoreline – Precise orbits of GNSS satellites used to define NSRS– Models and tools to describe how all of these

quantities change over time.

Mission – The NSRS is…

Science of Height mod – 3 kinds of Heights

Orthometric Heights (NAVD88):

Traditionally leveling

Ellipsoid Heights (NAD83): GNSS

Geoid Heights (NAD83): gravity

and modeling

(NAVD88) H

H = Orthometric Height (leveling)

H = h - N

TOPOGRAPHIC SURFACE

h = Ellipsoidal Height (GPS)

N = Geoid Height (model)

h (NAD83)

Ellipsoid

N

GeoidGeoid Height (GEOID03/09)

Ellipsoid, Geoid, and Orthometric Heights

NAVD88 Heights - Conventional Leveling

BM

BM

100’

Backsight Foresight

ROD 2

INSTR

Backsight

ROD 1

Foresight

ROD 1

INSTR

5’6’105’

HI

99’

7.5’ 4.5’96’

enter GNSS…

Can we get accurate heights with GNSS?

Guidelines to obtain accurate heights through GNSS

N O A A T e c h n ic a l M e m o r a n d u m N O S N G S - 5 8

G U ID E L IN E S F O R E S T A B L IS H IN G G P S - D E R IV E D E L L IP S O ID H E IG H T S( S T A N D A R D S : 2 C M A N D 5 C M )V E R S IO N 4 .3

D a v id B . Z i lk o s k iJ o s e p h D . D 'O n o f r ioS t e p h e n J . F r a k e s

S i lv e r S p r in g , M D

N o v e m b e r 1 9 9 7

U .S . D E P A R T M E N T O F N a t io n a l O c e a n ic a n d N a t io n a l O c e a n N a t io n a l G e o d e t icC O M M E R C E A t m o s p h e r ic A d m in is t r a t io n S e r v ic e S u r v e y

Equipment requirements

Field Procedures/Data

Collection Parameters

Basic Control

Requirements

Processing/Analysis

Procedures

Equipment Requirements

• Dual-frequency, full-wavelength GPS receiver– Required - observations > 10 km– Preferred - ALL observations regardless of length

• Geodetic quality antennas with ground planes– Choke ring antennas; highly recommended– Successfully modeled L1/L2 offsets and phase

patterns– Use identical antenna types if possible– Corrections must be utilized by processing software

when mixing antenna types

“Fixed” HeightTripod

Equipment Requirements

Data Collection Parameters

• VDOP < 6 for 90% or longer of 30 minute session

• Session lengths and sampling rate

• Track satellites down to 10° elevation angle

• Repeat Baselines– Different days

– Different times of day

• Detect, remove, reduce effects due to multipath and similar satellite geometry between sessions

Comparison of 30 Minute Solutions - Precise Orbit; Hopfield (0); IONOFREE(30 Minute solutions computed on the hour and the half hour)

MOLA to RV22 10.8 Km

Day 264dh (m)

Hours Diff.

Day 265dh (m)

Day 264 minus

Day 265 (cm)

* diff >2 cm

Mean dh (m)

Mean dh minus "Truth" (cm)

* diff >2 cm

14:00-14:30 -10.281 27hrs 17:00-17:30 -10.279 -0.2 -10.280 -0.514:30-15:00 -10.278 27hrs 17:30-18:00 -10.270 -0.8 -10.274 0.215:00-15:30 -10.281 27hrs 18:00-18:30 -10.278 -0.3 -10.280 -0.415:30-16:00 -10.291 27hrs 18:30-19:00 -10.274 -1.7 -10.283 -0.716:00-16:30 -10.274 27hrs 19:00-19:30 -10.274 0.0 -10.274 0.216:30-17:00 -10.287 27hrs 19:30-20:00 -10.276 -1.1 -10.282 -0.617:00-17:30 -10.279 27hrs 20:00-20:30 -10.261 -1.8 -10.270 0.617:30-18:00 -10.270 27hrs 20:30-21:00 -10.251 -1.9 -10.261 1.518:00-18:30 -10.277 21hrs 15:00-15:30 -10.270 -0.7 -10.274 0.218:30-19:00 -10.271 21hrs 15:30-16:00 -10.276 0.5 -10.274 0.219:00-19:30 -10.277 21hrs 16:00-16:30 -10.278 0.1 -10.278 -0.219:30-20:00 -10.271 21hrs 16:30-17:00 -10.286 1.5 -10.279 -0.320:00-20:30 -10.259 18hrs 14:00-14:30 -10.278 1.9 -10.269 0.720:30-21:00 -10.254 18hrs 14:30-15:00 -10.295 4.1 * -10.275 0.1

"Truth"14:00-21:00 -10.275 14:00-21:00 -10.276 0.1 -10.276

Two Days/Same Time

Difference = 0.3 cm

“Truth” = -10.276Difference = 2.3 cm

Two Days/Different Times

-10.254-10.295 > -10.275

Difference = 4.1 cm

“Truth” = -10.276

Difference = 0.1 cm

-10.254 -10.251 > -10.253

REPEAT BASELINE DIFFERENCES BY DISTANCE

0

1

2

3

4

5

6

0 5000 10000 15000 20000 25000 30000 35000 40000

BASELINE LENGTH (M)

RE

PE

AT

DIF

FE

RE

NC

ES

(C

M)

0113-H83A3.1

1933-01213.1 0092-1933

3.1

6669-00924.7

6669-6666 4.9

172 BASELINES - 3% Above 3 cm

Station pairs with large repeat base line differences also result in large residuals. NGS guidelines for estimating GPS-derived ellipsoid heights require user to re-observe these base lines.

Four Basic Control Requirements

• Occupy stations with known NAVD 88 orthometric heights, evenly distributed throughout project

• Project areas < 20 km on a side, surround project with at least 4 NAVD 88 bench marks

• Project areas > 20 km on a side, keep distances between GPS-occupied NAVD 88 bench marks to less than 20 km

• Projects located in mountainous regions, occupy bench marks at base and summit of mountains, even if distance is less than 20 km

• Perform 3-D minimally constrained (free) adjustment

• Analyze adjustment results

• Compute differences between GPS-derived orthometric heights from free adjustment and published NAVD88 BMs

• Evaluate differences to determine which BMs have valid NAVD88 height values

• Perform constrained adjustment with results from previous step

Processing: Five Basic Procedures

Sample Project

• Area: East San Francisco Bay Project– Latitude 37° 50” N to 38° 10” N– Longitude 121° 45” W to 122° 25” W

• Receivers Available: 5

• Standards: 2 cm GPS-Derived Heights

Primary Base Stations

CORSHARNNAVD’88 BMNew Station

121°40’W122°35’W

37°50’N

38°20’N

LA

TIT

UD

E

LONGITUDE

Primary Base Station

MOLA

MARTLAKE

10CC

D191

29.6km25.8km

38.7

km

19.0km28.7

km

25.7

km 38.3km

31.6

km

GPS-Usable Stations

Spacing StationPrimary Base Station

8.2km

CORSHARNNAVD’88 BMNew Station

CORSHARNNAVD’88 BMNew StationSpacing Station

121°40’W122°20’W

37°55’N

38°16’N

LA

TIT

UD

E

LONGITUDE

Primary Base Station

Session A

Session BSession C

Session D

Session ESession F

Session G

CORSHARNNAVD’88 BMNew StationSpacing Station

121°40’W122°20’W

37°55’N

38°16’N

LA

TIT

UD

E

LONGITUDE

Primary Base Station

Session A

Session BSession C

Session D

Session ESession F

Session G

Observation Sessions

Texas Primary Control – CORS

TXAN – San Antonio RRP2, TX

TXBM – Beaumont, TX

TXME – Memphis, TX

Primary Control - Texas High

Accuracy Reference

Network (1993)

HARN/Control Stations(75 km) 5.5 hr 3 days different times

Primary Base(40 km) 5.5 hr

3 days different times

Secondary Base(15 km) 0.5 hr

2 days different times

Local Network Stations(7 to 10 km) 0.5 hr

2 days different times

GPS Ellipsoid Height Hierarchy

Height Modernization Project

= Existing NGS Level line= New HMP Level line

= HARN= Primary Base Network= Secondary Base Network= Local Base Network

HARN - Average 50km, max 75kmPrimary – 20-25km,

max 40km

Secondary - Average 12-15km, max 15km

Local – Average 6-8 km, max 10km

• 1 National Geodetic Survey, Retrieval Date = JUNE 6, 2009• AC6323 ***********************************************************************• AC6323 DESIGNATION - BEA5 B• AC6323 PID - AC6323• AC6323 STATE/COUNTY- TX/JEFFERSON• AC6323 USGS QUAD - VOTH (1993)• AC6323• AC6323 *CURRENT SURVEY CONTROL• AC6323 ___________________________________________________________________• AC6323* NAD 83(2007)- 30 09 15.32032(N) 094 10 50.43369(W) ADJUSTED • AC6323* NAVD 88 - 13.115 (meters) 43.03 (feet) ADJUSTED • AC6323 ___________________________________________________________________• AC6323 EPOCH DATE - 2002.00• AC6323 X - -402,394.366 (meters) COMP• AC6323 Y - -5,504,987.110 (meters) COMP• AC6323 Z - 3,185,163.813 (meters) COMP• AC6323 LAPLACE CORR- 0.04 (seconds) DEFLEC99• AC6323 ELLIP HEIGHT- -14.472 (meters) (02/10/07) ADJUSTED• AC6323 GEOID HEIGHT- -27.51 (meters) GEOID03• AC6323 DYNAMIC HT - 13.098 (meters) 42.97 (feet) COMP• AC6323• AC6323 ------- Accuracy Estimates (at 95% Confidence Level in cm) --------• AC6323 Type PID Designation North East Ellip• AC6323 -------------------------------------------------------------------• AC6323 NETWORK AC6323 BEA5 B 0.51 0.41 2.04• AC6323 -------------------------------------------------------------------• AC6323 MODELED GRAV- 979,314.4 (mgal) NAVD 88• AC6323• AC6323 VERT ORDER - FIRST CLASS II

NGS Datasheet – Leveling

• AC6323.This is a reference station for the BEAUMONT RRP• AC6323.National Continuously Operating Reference Station (BEA5).• AC6323• AC6323.The horizontal coordinates were established by GPS observations• AC6323.and adjusted by the National Geodetic Survey in February 2007.• AC6323• AC6323.The datum tag of NAD 83(2007) is equivalent to NAD 83(NSRS2007).• AC6323.See National for more information.• AC6323.The horizontal coordinates are valid at the epoch date displayed above.• AC6323.The epoch date for horizontal control is a decimal equivalence• AC6323.of Year/Month/Day.• AC6323• AC6323.The orthometric height was determined by differential leveling• AC6323.and adjusted in July 2002.• AC6323• AC6323.Photographs are available for this station.• AC6323• AC6323.The X, Y, and Z were computed from the position and the ellipsoidal ht.• AC6323• AC6323.The Laplace correction was computed from DEFLEC99 derived deflections.• AC6323• AC6323.The ellipsoidal height was determined by GPS observations• AC6323.and is referenced to NAD 83.• AC6323• AC6323.The geoid height was determined by GEOID03.• AC6323.The dynamic height is computed by dividing the NAVD 88• AC6323.geopotential number by the normal gravity value computed on the• AC6323.Geodetic Reference System of 1980 (GRS 80) ellipsoid at 45• AC6323.degrees latitude (g = 980.6199 gals.).

NGS Datasheet – Leveling

• AC6323.The modeled gravity was interpolated from observed gravity values.• AC6323• AC6323; North East Units Scale Factor Converg.• AC6323;SPC TXSC - 4,266,792.245 1,064,130.299 MT 0.99996526 +2 21 39.8• AC6323;SPC TXSC -13,998,634.22 3,491,234.16 sFT 0.99996526 +2 21 39.8• AC6323;UTM 15 - 3,336,466.860 386,299.039 MT 0.99975949 -0 35 35.3• AC6323• AC6323! - Elev Factor x Scale Factor = Combined Factor• AC6323!SPC TXSC - 1.00000227 x 0.99996526 = 0.99996753• AC6323!UTM 15 - 1.00000227 x 0.99975949 = 0.99976176• AC6323• AC6323 SUPERSEDED SURVEY CONTROL• AC6323• AC6323 ELLIP H (10/23/00) -14.414 (m) GP( ) 4 2• AC6323 NAD 83(1993)- 30 09 15.31899(N) 094 10 50.43496(W) AD( ) B• AC6323 ELLIP H (08/21/98) -14.386 (m) GP( ) 5 2• AC6323 NAD 83(1993)- 30 09 15.31917(N) 094 10 50.43466(W) AD( ) B• AC6323 ELLIP H (01/23/98) -14.225 (m) GP( ) 3 2• AC6323 NAVD 88 (08/21/98) 13.12 (m) 43.0 (f) LEVELING 3 • AC6323 NAVD 88 (01/23/98) 13.26 (m) 43.5 (f) LEVELING 3 • AC6323• AC6323.Superseded values are not recommended for survey control.• AC6323.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums.• AC6323.See file dsdata.txt to determine how the superseded data were derived.• AC6323

NGS Datasheet – Leveling

• AC6323_U.S. NATIONAL GRID SPATIAL ADDRESS: 15RUP8629936467(NAD 83)• AC6323_MARKER: I = METAL ROD• AC6323_SETTING: 59 = STAINLESS STEEL ROD IN SLEEVE (10 FT.+)• AC6323_STAMPING: BEA5 B 1996• AC6323_MARK LOGO: NGS • AC6323_PROJECTION: FLUSH• AC6323_MAGNETIC: I = MARKER IS A STEEL ROD• AC6323_STABILITY: A = MOST RELIABLE AND EXPECTED TO HOLD• AC6323+STABILITY: POSITION/ELEVATION WELL• AC6323_SATELLITE: THE SITE LOCATION WAS REPORTED AS SUITABLE FOR• AC6323+SATELLITE: SATELLITE OBSERVATIONS - January 27, 2005• AC6323_ROD/PIPE-DEPTH: 9.9 meters• AC6323_SLEEVE-DEPTH : 0.9 meters• AC6323• AC6323 HISTORY - Date Condition Report By• AC6323 HISTORY - 1996 MONUMENTED NGS• AC6323 HISTORY - 19970313 GOOD NGS• AC6323 HISTORY - 20030522 GOOD JCLS• AC6323 HISTORY - 20050127 GOOD USPSQD• AC6323• AC6323 STATION DESCRIPTION• AC6323• AC6323'DESCRIBED BY NATIONAL GEODETIC SURVEY 1996 (GAS)• AC6323'IN BEAUMONT, AT THE INTERSECTION OF HIGHWAYS 69, 96, 287, AND CHINN• AC6323'LANE, 70.6 M (231.6 FT) NORTH OF BENCH MARK W 1496, 8.3 M (27.2 FT)• AC6323'EAST OF THE CENTERLINE OF THE NORTHBOUND LANES OF THE HIGHWAY, 0.7 M• AC6323'(2.3 FT) NORTHEAST OF A WITNESS POST, 0.5 M (1.6 FT) EAST OF THE NORTH• AC6323'END OF A GUARDRAIL, AND 0.4 M (1.3 FT) BELOW THE LEVEL OF THE HIGHWAY.

NGS Datasheet – Leveling

• BL0243; North East Units Scale Factor Converg.• BL0243;SPC TXSC - 4,258,727.981 1,072,609.864 MT 0.99994722 +2 24 08.7• BL0243;SPC TXSC -13,972,176.72 3,519,054.20 sFT 0.99994722 +2 24 08.7• BL0243;UTM 15 - 3,327,978.224 394,350.224 MT 0.99973771 -0 32 58.1• BL0243• BL0243! - Elev Factor x Scale Factor = Combined Factor• BL0243!SPC TXSC - 1.00000334 x 0.99994722 = 0.99995056• BL0243!UTM 15 - 1.00000334 x 0.99973771 = 0.99974105• BL0243• BL0243 SUPERSEDED SURVEY CONTROL• BL0243• BL0243 NAD 83(1993)- 30 04 42.20377(N) 094 05 46.46109(W) AD( ) 1• BL0243 ELLIP H (03/11/02) -21.256 (m) GP( ) 4 1• BL0243 NAVD 88 (03/11/02) 6.22 (m) 20.4 (f) LEVELING 3 • BL0243 NGVD 29 (??/??/??) 6.199 (m) 20.34 (f) ADJUSTED 1 1• BL0243• BL0243.Superseded values are not recommended for survey control.• BL0243.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums.• BL0243.See file dsdata.txt to determine how the superseded data were derived.• BL0243• BL0243 HISTORY - Date Condition Report By• BL0243 HISTORY - 1972 MONUMENTED NGS• BL0243 HISTORY - 1986 GOOD NGS• BL0243 HISTORY - 198902 GOOD USPSQD• BL0243 HISTORY - 19890821 MARK NOT FOUND USPSQD• BL0243 HISTORY - 1990 GOOD USPSQD• BL0243 HISTORY - 19940817 GOOD USPSQD•

NGS Datasheet – Leveling

• 1 National Geodetic Survey, Retrieval Date = APRIL 30, 2008• 1 National Geodetic Survey, Retrieval Date = JUNE 7, 2009• AC6388 ***********************************************************************• AC6388 DESIGNATION - BPT AP STA B2• AC6388 PID - AC6388• AC6388 STATE/COUNTY- TX/JEFFERSON• AC6388 USGS QUAD - PORT ACRES (1993)• AC6388• AC6388 *CURRENT SURVEY CONTROL• AC6388 ___________________________________________________________________• AC6388* NAD 83(2007)- 29 56 48.75724(N) 094 00 44.75163(W) ADJUSTED • AC6388* NAVD 88 - 1.4 (meters) 5. (feet) GPS OBS • AC6388 ___________________________________________________________________• AC6388 EPOCH DATE - 2002.00• AC6388 X - -387,032.470 (meters) COMP• AC6388 Y - -5,517,618.690 (meters) COMP• AC6388 Z - 3,165,259.584 (meters) COMP• AC6388 . . .

• AC6388.The orthometric height was determined by GPS observations and a• AC6388.high-resolution geoid model.• AC6388• AC6388.The X, Y, and Z were computed from the position and the ellipsoidal ht.• AC6388• AC6388.The Laplace correction was computed from DEFLEC99 derived deflections.• AC6388•

NGS Datasheet - GPS Height

• 1 National Geodetic Survey, Retrieval Date = JUNE 6, 2009• AJ8222 ***********************************************************************• AJ8222 HT_MOD - This is a Height Modernization Survey Station.• AJ8222 DESIGNATION - BASELINE• AJ8222 PID - AJ8222• AJ8222 STATE/COUNTY- TX/JEFFERSON• AJ8222 USGS QUAD - BEAUMONT WEST (1994)• AJ8222• AJ8222 *CURRENT SURVEY CONTROL• AJ8222 ___________________________________________________________________• AJ8222* NAD 83(2007)- 30 05 43.32312(N) 094 12 12.09959(W) ADJUSTED • AJ8222* NAVD 88 - 11.14 (meters) 36.5 (feet) GPS OBS • AJ8222 ___________________________________________________________________• AJ8222 EPOCH DATE - 2002.00• AJ8222 X - -404,813.933 (meters) COMP• AJ8222 . . .• AJ8222 The epoch date for horizontal control is a decimal equivalence• AJ8222 of Year/Month/Day.• AJ8222• AJ8222.The orthometric height was determined by GPS observations and a• AJ8222.high-resolution geoid model using precise GPS observation and• AJ8222.processing techniques.• AJ8222• AJ8222.The X, Y, and Z were computed from the position and the ellipsoidal ht.• AJ8222• AJ8222.The Laplace correction was computed from DEFLEC99 derived deflections.

NGS Datasheet - Height Mod GPS

• 1 National Geodetic Survey, Retrieval Date = JUNE 6, 2009• AC6323 ***********************************************************************• AC6323 DESIGNATION - BEA5 B• AC6323 PID - AC6323• AC6323 STATE/COUNTY- TX/JEFFERSON• AC6323 USGS QUAD - VOTH (1993)• AC6323• AC6323 *CURRENT SURVEY CONTROL• AC6323 ___________________________________________________________________• AC6323* NAD 83(2007)- 30 09 15.32032(N) 094 10 50.43369(W) ADJUSTED • AC6323* NAVD 88 - 13.115 (meters) 43.03 (feet) ADJUSTED • AC6323 ___________________________________________________________________• AC6323 EPOCH DATE - 2002.00• AC6323 X - -402,394.366 (meters) COMP• AC6323 Y - -5,504,987.110 (meters) COMP• AC6323 Z - 3,185,163.813 (meters) COMP• AC6323 LAPLACE CORR- 0.04 (seconds) DEFLEC99• AC6323 ELLIP HEIGHT- -14.472 (meters) (02/10/07) ADJUSTED• AC6323 GEOID HEIGHT- -27.51 (meters) GEOID03• AC6323 DYNAMIC HT - 13.098 (meters) 42.97 (feet) COMP• AC6323• AC6323 ------- Accuracy Estimates (at 95% Confidence Level in cm) --------• AC6323 Type PID Designation North East Ellip• AC6323 -------------------------------------------------------------------• AC6323 NETWORK AC6323 BEA5 B 0.51 0.41 2.04• AC6323 -------------------------------------------------------------------• AC6323 MODELED GRAV- 979,314.4 (mgal) NAVD 88• AC6323

NGS Datasheet – Leveling

N

H

h

NAVD88 – Ellipsoid Ht + Geoid Ht = …

13.115 – (-14.472) – 28.345 = -0.758 USGG2003

13.115 – (-14.472) – 27.506 = +0.081 GEOID03

How accurate is a GPS-derived Orthometric Height?

• Relative (local) accuracy in ellipsoid heights between adjacent points will be better than 2 cm, at 95% confidence level

• Network accuracy (relative to NSRS) in ellipsoid and orthometric heights will be better than 5 cm, at 95% confidence level

• Accuracy of orthometric height is dependent on accuracy of the geoid model – Currently NGS is improving the geoid model with more data, i.e. Gravity and GPS observations on leveled bench marks from Height Mod projects

(NAVD88) H

H = Orthometric Height (leveling)

H = h - N

TOPOGRAPHIC SURFACE

h = Ellipsoidal Height (GPS)

N = Geoid Height (model)

h (NAD83)

Ellipsoid

N

GeoidGeoid Height (GEOID03/09)

Ellipsoid, Geoid, and Orthometric Heights

?

In Search of the Geoid…

Courtesy of Natural Resources Canada www.geod.nrcan.gc.ca/index_e/geodesy_e/geoid03_e.html

Dr. Dan Roman

Dr. Yan Wang

Definitions: GEOIDS versus GEOID HEIGHTS

• “The equipotential surface of the Earth’s gravity field which best fits, in the least squares sense, (global) mean sea level.”*

• Can’t see the surface or measure it directly.• Can be modeled from gravity data as they are

mathematically related.• Note that the geoid is a vertical datum surface.• A geoid height is the ellipsoidal height from an ellipsoidal

datum to a geoid.• Hence, geoid height models are directly tied to the geoid

and ellipsoid that define them (i.e., geoid height models are not interchangeable).

*Definition from the Geodetic Glossary, September 1986

LEVEL SURFACES AND ORTHOMETRIC HEIGHTS

Level Surfaces

PlumbLine

“Geoid”

PO

P

Level Surface = Equipotential Surface (W)

H (Orthometric Height) = Distance along plumb line (PO to P)

Earth’s

Surface

Ocean

MeanSeaLevel WO

WP

High Resolution Geoid Models

• Start with gravity measurements

• Add Digital Elevation Data (Continental)

• Add a Global Earth Gravity Model

(EGM96, EGM08)

• Result – A purely Gravitational geoid

model – good for scientific applications

… But is this what we need?

To use GNSS you need a good geoid model

NGS makes 2 geoid models

• Gravitational model:• is good for scientific applications• uses gravity data collected from a variety of sources

• “Hybrid” model: • starts with gravitational model• uses GPS on bench marks to enable a fit to NAVD88

GRACE Satellite Gravity

GGPSBM2003: 14,185 total 579 Canada STDEV 4.8 cm (2σ)

GGPSBM1999: 6,169 total 0 Canada STDEV 9.2 cm (2σ)

0.876 M in Beaumont, TX– 1999 model

0.839 M in Beaumont, TX – 2003 model

Gravimetric Geoid systematic misfit to BM’s but best fits “true” heightsHybrid Geoid “converted” to fit local BM’s, so best fits NAVD 88 heightsConversion Surface model of systematic misfit derived from BM’s in IDB

Earth’s Surface

hh

h hh

HH

HH

H

NNNN

N

Ellipsoid

Hybrid Geoid

Gravimetric Geoid Geoid

N N NN N

Gravimetric vs. Hybrid Geoid surface

Note use of GPS on BMs

1.065M in Beaumont, TX– 2009 Beta model

• Access to accurate, reliable heights nationally

• Standards that are consistent across the nation

• Data, technology, and tools that yield consistent results regardless of terrain and circumstances

• A system/process that will stand the test of time – “Maintain-able”

Goal of NHMP

That was then….

• In the early years of Height Mod NGS felt the Gravimetric geoid was adequate as the base for the Hybrid geoid

What’s changed?• Better accuracy from GNSS-derived heights

– can use GNSS to monitor changes in heights

• Better understanding of poor condition of vertical network

• Gravity holdings at NGS evaluated

NGS Gravity Holdings

• Most of the historical NGS data is terrestrial

• Multiple observers, multiple processors over the past 60 years

• Numerous corrections and datums over time

• Metadata maintained in paper records• Very limited aerogravity• Deficit in near-shore gravity data

Ship gravity

Terrestrial gravity

New Orleans

20-100 km gravity gaps along coast

Gravity Coverage for USGG2003/USGG2008

Is NAVD 88 “Maintainable”?

• NAVD88 defined through leveling network

• Not practical to re-leveling the country – At $3000 / km, re-leveling NAVD 88 would cost $2.25 Billion– Does not include densifying poorly covered areas in Western

CONUS, Alaska– Does not include leveling needed for separate vertical

datums on island states and territories

• Leveling yields cross-country error build-up; problems in the mountains

• Leveling requires leaving behind marks – Bulldozers and crustal motion do their worst

• Official NGS policy as of Nov 14, 2007– $38.5M over 10 years

• Airborne Gravity Snapshot

• Absolute Gravity Tracking

• Re-define the Vertical Datum of the USA by 2017

Transition to the Future – GRAV-D

Gravity for the Redefinition of the American Vertical Datum

http:/www.ngs.noaa.gov/GRAV-D/

GRAV-D is the most ambitious project within the National Height Mod Program

• National Height Modernization needs to support the NAVD 88 while transitioning (via GRAV-D) to a new vertical datum in 10 years

• All National Height Mod funds (internal or grants) should support access to accurate heights, in general, but:– With emphasis on NAVD 88 today– With emphasis on the new vertical datum in 5

years

From NGS’ 10-Year Plan

NGS Mission: “Modernize the Geopotential (“Vertical”) Datum

“The gravimetric geoid, long used as the foundation for hybrid geoid models, becomes the most critical model produced by NGS.”

Questions

Renee ShieldsHeight Modernization Manager301-713-3231, x116Renee.shields@noaa.gov

Dan RomanResearch Geodesist301-713-3202, x161Dan.Roman@noaa.gov