Geoid Modeling and GRAVD: Gravity for the

1
Geoid Modeling and GRAV-D Gravity for the
Redefinition of the American Vertical
Datum Beaumont, Texas June 8-9, 2009 Renee
Shields Height Modernization Manager
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From the NGS 10-Year Plan

• 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
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Mission The NSRS is

• 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.

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Science of Height mod 3 kinds of Heights
Orthometric Heights (NAVD88) Traditionally
leveling Ellipsoid Heights (NAD83) GNSS Geoid
Heights (NAD83) gravity and modeling
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Ellipsoid, Geoid, and Orthometric Heights
H Orthometric Height (leveling)
H h - N
h Ellipsoidal Height (GPS)
N Geoid Height (model)

TOPOGRAPHIC SURFACE
h (NAD83)
(NAVD88) H
N
Geoid Height (GEOID03/09)
Ellipsoid
Geoid
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NAVD88 Heights - Conventional Leveling

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enter GNSS
Can we get accurate heights with GNSS?
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Guidelines to obtain accurate heights through
GNSS
Equipment requirements Field Procedures/Data
Collection Parameters Basic Control
Requirements Processing/Analysis Procedures
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Equipment Requirements

• Dual-frequency, full-wavelength GPS receiver
• Required - observations gt 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

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Equipment Requirements
Fixed Height Tripod
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Data Collection Parameters

• VDOP lt 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

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Two Days/Same Time
-10.254 -10.251
gt -10.253
Difference 0.3 cm
Truth -10.276
Difference 2.3 cm
Two Days/ Different Times
-10.254
gt -10.275
-10.295
Difference 4.1 cm
Truth -10.276
Difference 0.1 cm
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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.
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Four Basic Control Requirements

• Occupy stations with known NAVD 88 orthometric
  heights, evenly distributed throughout project
• Project areas lt 20 km on a side, surround project
  with at least 4 NAVD 88 bench marks
• Project areas gt 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

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Processing Five Basic Procedures

• 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

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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

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Primary Base Stations
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GPS-Usable Stations
Spacing Station
Primary Base Station
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Observation Sessions
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Texas Primary Control CORS
TXBM Beaumont, TX
TXME Memphis, TX
TXAN San Antonio RRP2, TX
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Primary Control - Texas High Accuracy Reference
Network (1993)
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GPS Ellipsoid Height Hierarchy
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Height Modernization Project
HARN - Average 50km, max 75km
Secondary - Average 12-15km, max 15km
Local Average 6-8 km, max 10km
Primary 20-25km, max 40km
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NGS Datasheet Leveling

• 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

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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

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NGS Datasheet Leveling

• AC6323.The modeled gravity was interpolated from
  observed gravity values.
• AC6323
• AC6323 North East
  Units Scale Factor Converg.
• AC6323SPC TXSC - 4,266,792.245
  1,064,130.299 MT 0.99996526 2 21 39.8
• AC6323SPC TXSC -13,998,634.22 3,491,234.16
  sFT 0.99996526 2 21 39.8
• AC6323UTM 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

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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
• AC6323STABILITY POSITION/ELEVATION WELL
• AC6323_SATELLITE THE SITE LOCATION WAS REPORTED
  AS SUITABLE FOR
• AC6323SATELLITE 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

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NGS Datasheet Leveling

• BL0243 North East
  Units Scale Factor Converg.
• BL0243SPC TXSC - 4,258,727.981
  1,072,609.864 MT 0.99994722 2 24 08.7
• BL0243SPC TXSC -13,972,176.72 3,519,054.20
  sFT 0.99994722 2 24 08.7
• BL0243UTM 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.

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NGS Datasheet - GPS Height

• 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 . . .

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NGS Datasheet - Height Mod GPS

• 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

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NGS Datasheet Leveling

• 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

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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

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Ellipsoid, Geoid, and Orthometric Heights
H Orthometric Height (leveling)
H h - N
h Ellipsoidal Height (GPS)
N Geoid Height (model)

TOPOGRAPHIC SURFACE
h (NAD83)
(NAVD88) H
N
Geoid Height (GEOID03/09)
Ellipsoid
Geoid
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In Search of the Geoid
Dr. Dan Roman
Dr. Yan Wang
Courtesy of Natural Resources Canada
www.geod.nrcan.gc.ca/index_e/geodesy_e/geoid03_e.h
tml
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Definitions GEOIDS versus GEOID HEIGHTS

• The equipotential surface of the Earths gravity
  field which best fits, in the least squares
  sense, (global) mean sea level.
• Cant 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

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LEVEL SURFACES AND ORTHOMETRIC HEIGHTS
Earths
Surface
WP
Level Surfaces
P
Plumb
Line
Mean
Geoid
Sea
Level
WO
PO
Ocean
Level Surface Equipotential Surface (W)
H (Orthometric Height) Distance along plumb
line (PO to P)
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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?
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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
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GGPSBM1999 6,169 total 0 Canada STDEV
9.2 cm (2s)
GGPSBM2003 14,185 total 579 Canada STDEV 4.8
cm (2s)
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Gravimetric vs. Hybrid Geoid surface

Earths Surface
Note use of GPS on BMs
Ellipsoid
N
N
N
N
0.876 M in Beaumont, TX 1999 model
0.839 M in Beaumont, TX 2003 model
1.065M in Beaumont, TX 2009 Beta model
Hybrid Geoid
Gravimetric Geoid

• Gravimetric Geoid systematic misfit to BMs but
  best fits true heights
• Hybrid Geoid converted to fit local BMs, so
  best fits NAVD 88 heights
• Conversion Surface model of systematic misfit
  derived from BMs in IDB

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Goal of NHMP

• 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

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That was then.

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

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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

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20-100 km gravity gaps along coast
Terrestrial gravity
New Orleans
Ship gravity
45
Gravity Coverage for USGG2003/USGG2008
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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

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Transition to the Future GRAV-D
Gravity for the Redefinition of the American
Vertical Datum

• 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

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

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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.

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Questions
Renee Shields Height Modernization
Manager 301-713-3231, x116 Renee.shields_at_noaa.gov
Dan Roman Research Geodesist 301-713-3202,
x161 Dan.Roman_at_noaa.gov