Transcript Slide 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
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
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.
Science of Height mod – 3
kinds of Heights
Orthometric Heights (NAVD88):
Traditionally leveling
Ellipsoid Heights (NAD83): GNSS
Geoid Heights (NAD83): gravity
and modeling
Ellipsoid, Geoid, and Orthometric Heights
H = Orthometric Height (leveling)
H =h-N
h = Ellipsoidal Height (GPS)
N = Geoid Height (model)
(NAVD88) H
h (NAD83)
TOPOGRAPHIC SURFACE
N
Ellipsoid
Geoid
Geoid Height
(GEOID03/09)
NAVD88 Heights - Conventional Leveling
ROD 1
ROD 2
ROD 1
Backsight
5’
BM
100’
Foresight
Backsight
105’
HI
6’
Foresight
96’
4.5’
7.5’
INSTR
99’
INSTR
BM
enter GNSS…
Can we get accurate heights with GNSS?
Guidelines to obtain accurate
heights through GNSS
Equipment requirements
NOAA Technical Memorandum NOS NGS-58
GUIDELINES FOR ESTABLISHING GPS-DERIVED ELLIPSOID HEIGHTS
(STANDARDS: 2 CM AND 5 CM)
VERSION 4.3
David B. Zilkoski
Joseph D. D'Onofrio
Stephen J. Frakes
Collection Parameters
Silver Spring, MD
November 1997
U.S. DEPARTMENT OF
COMMERCE
Field Procedures/Data
National Oceanic and
Atmospheric Administration
National Ocean
Service
National Geodetic
Survey
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
Equipment Requirements
“Fixed” Height
Tripod
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)
Two Days/Same Time
-10.254
> -10.253
-10.251
MOLA to RV22 10.8 Km
dh
(m)
Hours
Diff.
Day 265
27hrs
17:00-17:30
27hrs
17:30-18:00
27hrs
18:00-18:30
27hrs
18:30-19:00
27hrs
19:00-19:30
27hrs
19:30-20:00
27hrs
20:00-20:30
27hrs
20:30-21:00
21hrs
15:00-15:30
21hrs
15:30-16:00
21hrs
16:00-16:30
21hrs
16:30-17:00
18hrs
14:00-14:30
20:30-21:00
-10.281
-10.278
-10.281
-10.291
-10.274
-10.287
-10.279
-10.270
-10.277
-10.271
-10.277
-10.271
-10.259
-10.254
18hrs
14:00-21:00
-10.275
Day 264
14:00-14:30
14:30-15:00
15:00-15:30
15:30-16:00
16:00-16:30
16:30-17:00
17:00-17:30
17:30-18:00
18:00-18:30
18:30-19:00
19:00-19:30
19:30-20:00
20:00-20:30
dh
(m)
Day 264 *
minus diff Mean dh
Day 265 >2
(m)
(cm) cm
14:30-15:00
-10.279
-10.270
-10.278
-10.274
-10.274
-10.276
-10.261
-10.251
-10.270
-10.276
-10.278
-10.286
-10.278
-10.295
-0.2
-0.8
-0.3
-1.7
0.0
-1.1
-1.8
-1.9
-0.7
0.5
0.1
1.5
1.9
4.1
14:00-21:00
-10.276
0.1
*
-10.280
-10.274
-10.280
-10.283
-10.274
-10.282
-10.270
-10.261
-10.274
-10.274
-10.278
-10.279
-10.269
-10.275
"Truth"
-10.276
Mean dh *
minus diff
"Truth" >2
(cm) cm
-0.5
0.2
-0.4
-0.7
0.2
-0.6
0.6
1.5
0.2
0.2
-0.2
-0.3
0.7
0.1
Difference = 0.3 cm
“Truth” = -10.276
Difference = 2.3 cm
Two Days/
Different Times
-10.254
-10.295 > -10.275
Difference = 4.1 cm
“Truth” = -10.276
Difference = 0.1 cm
REPEAT BASELINE DIFFERENCES BY DISTANCE
172 BASELINES - 3% Above 3 cm
REPEAT DIFFERENCES (CM)
6
6669-6666
4.9
5
6669-0092
4.7
0113-H83A
4 3.1
1933-0121
3.1
0092-1933
3.1
3
2
1
0
0
5000
10000
15000
20000
25000
30000
35000
40000
BASELINE LENGTH (M)
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
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
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
38°20’N
CORS
HARN
NAVD’88 BM
New Station
D191
10CC
Primary Base Station
MART
LAKE
MOLA
122°35’W
LONGITUDE
121°40
’W
GPS-Usable Stations
CORS
HARN
NAVD’88 BM
New Station
Spacing Station
Primary Base Station
Observation Sessions
38°16’N
Session F
Session E
CORS
HARN
NAVD’88 BM
New Station
Spacing Station
Session D
Primary Base Station
Session G
Session A
Session B
122°20’W
LONGITUDE
Session C
121°40’W
Texas Primary
Control – CORS
TXBM – Beaumont, TX
TXME – Memphis, TX
TXAN – San Antonio RRP2, TX
Primary Control
- Texas High
Accuracy
Reference
Network
(1993)
GPS Ellipsoid Height Hierarchy
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
Height Modernization Project
HARN - Average 50km,
max 75km
Primary – 20-25km,
max 40km
=
=
=
=
HARN
Primary Base Network
Secondary Base Network
Local Base Network
Secondary - Average 12-15km, max 15km
Local – Average 6-8 km, max 10km
= Existing NGS Level line
= New HMP Level line
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NGS Datasheet – Leveling
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 CORR0.04 (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 GRAV979,314.4
(mgal)
NAVD 88
AC6323
AC6323 VERT ORDER - FIRST
CLASS II
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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
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
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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
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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
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.
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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 - GPS Height
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National Geodetic Survey,
Retrieval Date = APRIL 30, 2008
National Geodetic Survey,
Retrieval Date = JUNE 7, 2009
***********************************************************************
DESIGNATION - BPT AP STA B2
PID
- AC6388
STATE/COUNTY- TX/JEFFERSON
USGS QUAD
- PORT ACRES (1993)
AC6388
AC6388
AC6388
AC6388
AC6388
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 - Height Mod GPS
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National Geodetic Survey,
Retrieval Date = JUNE 6, 2009
***********************************************************************
HT_MOD
- This is a Height Modernization Survey Station.
DESIGNATION - BASELINE
PID
- AJ8222
STATE/COUNTY- TX/JEFFERSON
USGS QUAD
- BEAUMONT WEST (1994)
AJ8222
AJ8222
AJ8222
AJ8222
AJ8222
AJ8222
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.
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NGS Datasheet – Leveling
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 CORR0.04 (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 ------------------------------------------------------------------NAVD88(mgal)
– Ellipsoid Ht + Geoid Ht = NAVD
… 88
AC6323 MODELED GRAV979,314.4
AC6323
H
13.115 – (-14.472) – 28.345 = -0.758 USGG2003
13.115 – (-14.472) – 27.506 = +0.081 GEOID03
h
N
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
Ellipsoid, Geoid, and Orthometric Heights
H = Orthometric Height (leveling)
H =h-N
h = Ellipsoidal Height (GPS)
N = Geoid Height (model)
(NAVD88) H
h (NAD83)
TOPOGRAPHIC SURFACE
?N
Ellipsoid
Geoid
Geoid Height
(GEOID03/09)
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.html
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
WP
P
Plumb
Line
Mean
Sea
Level
PO
“Geoid”
WO
Ocean
Level Surface = Equipotential Surface (W)
H (Orthometric Height) = Distance along plumb line (PO to P)
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
GGPSBM1999: 6,169 total
0 Canada STDEV 9.2 cm (2σ)
GGPSBM2003: 14,185 total 579 Canada STDEV 4.8 cm (2σ)
Gravimetric vs. Hybrid Geoid surface
Earth’s Surface
Note use of GPS on BMs
h
h
h
h
h
H
H
H
N
N
N
N
Hybrid Geoid
N
H
Ellipsoid
H
N
N
N
N
N
0.876 Min
0.839
in
1.065M
Beaumont, TX
TX––
1999 Beta
2003
modelmodel
2009
Gravimetric Geoid
Gravimetric Geoid systematic misfit to BM’s but best fits “true” heights
Hybrid Geoid “converted” to fit local BM’s, so best fits NAVD 88 heights
Conversion Surface model of systematic misfit derived from BM’s in IDB
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”
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
Terrestrial gravity
20-100 km
gravity gaps
along coast
New Orleans
Ship gravity
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
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/
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 Shields
Height Modernization Manager
301-713-3231, x116
[email protected]
Dan Roman
Research Geodesist
301-713-3202, x161
[email protected]