Transcript New Nationwide Readjustment NSRS 2007

Horizontal Datums in the United
States: NAD 27 and flavors of
NAD 83 (1986), NAD 83(HARN)
and NAD 83 (NSRS2007)
GISC3325
10 March 2008
Class Update
• Exam Wednesday during class and lab periods.
• Covers all labs, required readings, lectures and
Chapters 5-7 of text.
• Computations like those demonstrated in class will
be part of the test.
www.ngs.noaa.gov
Gravity For the
American
Vertical Datum
What did NAD 83 replace?
NAD 27
• 1927 – NAD 27 becomes the 2nd continuous,
continental scale datum of the United States
– Terrestrial
measurements
– Distances measured
manually
– Origin point in Kansas
Meades Ranch
Problems with NAD 27
• First-order surveys designed with an
accuracy goal of 1: 25,000
• Network was adjusted piecewise rather than
simultaneously.
• Underlying measurements used were of
relatively poor precision.
• It was not geocentric
• Reference ellipsoid used (Clarke 1866) was
not appropriate for space-based positioning.
NAD 83 (1986)
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Took place from 1974 to 1986
Digitized observations from 1800’s to 1980’s
300 persons worked on the project
$37M (in 1974 dollars)
1 million simultaneous equations
– Helmert blocking
• Doppler and VLBI used but mostly terrestrial
observations.
NO GPS!!!!
• NAD 83 datum moved to Center-Of-Mass of
Earth
NAD83(86) DOPPLER STATIONS
GPS with NAD 83 (1986)
• There were no GPS observations in NAD 83
(1986)
• With GPS we see:
– Accuracy greatly exceeds EDM
– Allows connection between points that are not
intervisible
– Provides ellipsoid heights
• Required creation of geoid models (first:
Geoid 90) to link ellipsoid to NAVD 88
COMPARISON OF DATUM ELEMENTS
NAD 27
NAD 83
ELLIPSOID
CLARKE 1866
a=6,378,206.4 m
1/f = 294.9786982
GRS80
a=6,378,137 m
1/f = 298.257222101
DATUM POINT
Triangulation Station
Meades Ranch, Kansas
NONE
Earth Mass Center
ADJUSTMENT
25k Stations
few hundred Base lines
few hundred Astro Az
250k Stations
30k EDMI base lines
5k Astro Azimuths
Doppler and VLBI
NAD 27 and NAD 83
Results of NAD83 (1986)
• Network Accuracy - 1 Meter
• Local Accuracy – First-Order
(1 part in 100,000)
PROBLEMS with NAD 83 (1986)
Not “GPSABLE”
POOR STATION ACCESSIBILITY
IRREGULARLY SPACED
POSITIONAL ACCURACY inadequate for
GPS observations
HIGH PRECISION GPS NETWORKS
“GPSABLE”
Clear Horizons for use with Satellites
EASY ACCESSIBILITY
Few Special Vehicle or Property Access
Requirements
REGULARLY SPACED
Always within 20-100 Km
HIGH HORIZONTAL ACCURACY
A-Order (5 mm + 1:10,000,000)
B-Order (8mm + 1:1,000,000)
HARN/HPGN ADJUSTMENT
A-Order Adjusted to VLBI-Existing FBNCORS
B-Order Adjusted to A-Order
Existing Horizontal (Conventional & GPS)
Readjusted to A/B-Order
Adopted Adjustment Date Tags e.g.
NAD83(1991)
HIGH ACCURACY REFERENCE NETWORK
Old Datasheet format
DX = 0.5378
DY = -1.4980
DZ = 0.1726
- or -
DN = -0.6743
DE = 0.4393
DU = 1.3840
IMPROVING POSITIONAL ACCURACY
NETWORK
TIME
SPAN
NETWORK
ACCURACY
LOCAL
ACCURACY
NAD 27
1927-1986
10 METERS
(1 part in 100,000)
NAD83(1986)
1986-1990
1 METER
(1 part in 100,000)
HARN
1990-1997
0.1 METER
CORS
1996 -
0.01 meter
B-order (1.0 ppm)
A-order (0.1 ppm)
0.01 meter
New Adjustment Network Accuracies
• Median horizontal network accuracy: 1 cm
• Median vertical network accuracy: 2 cm
Reasons for
NAD 83 (2007) Readjustment
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Multiple epoch dates
Inconsistencies between states
Need to be Consistent with CORS
Compute Network and Local accuracies
September 24, 2003 NGS Executive
Steering Committee approved a plan for
the readjustment of the horizontal
positions and ellipsoid heights for GPS
stations in the contiguous United States.
Orthometric Height adjustment will not be
attempted
What about Orthometric Heights?
 Decision was made not to perform a National
Readjustment of orthometric heights at this time
 Control would be NAVD 88 not the CORS
 No analysis of NAVD 88 control
Heights
h – H – N = 0 + errors in components
GPS on Benchmark data for GEOID 2003
h – H – N = 0 + errors in components
“Perfect” CORS
Tectonic Plates NUVEL-1A
Geoid Modeling Issues
• The new ellipsoid heights will be used in the
next hybrid Geoid model.
• Future efforts will center on improvements to
our gravity data.
• NGS has acquired an airborne gravity meter.
• We plan a comprehensive set of observations
to capture gravity throughout the United States
and our possessions.
• Airborne gravity data supplements terrestrial
and satellite based gravity.
(L) New NGS airborne meter
(R) Portable absolute meter (A 10)
NAD 83 (NSRS 2007) READJUSTMENT
ONLY GPS DATA WAS USED
CORS (started in 1994)
FEDERAL BASE NETWORK (A & B)
COOPERATIVE BASE NETWORK (B)
USER DENSIFICATION NETWORK (First)
AIRPORT SURVEYS (B & First)
A simultaneous rather than layered
adjustment strategy was adopted using
Helmert blocking.
Readjustment General Comments
 The CORS/CGPS were the control
 Only GPS projects participated
 The FBN/CBN Surveys are a key element
 Tied to the CORS
 Provide more accurate ellipsoid heights
More Comments
 Computation and database loading of scale
factors for each project.
 Projects re-weighted in both horizontal
and vertical (ellipsoid).
 Statewide GPS Readjustments provided
extra information about the network on a
state-by-state basis.
 Work in NAD 83
 Transform to ITRF later
Network Accuracy
 Network accuracy of a control point
 A value that represents the uncertainty of its
coordinates with respect to the geodetic datum at
the 95-percent confidence level
 Datum is considered to be best expressed by the
Continuous Operating Reference Stations (CORS)
 Local and Network accuracy values at CORS sites are
considered to be infinitesimal (approach zero)
 These accuracies were implemented with the
National Readjustment
Local Accuracy
 Local Accuracy of a control point:
 A value that represents the uncertainty of its
coordinates relative to other directly connected,
adjacent control points at the 95-percent
confidence level
 An approximate average of the individual local
accuracy values between this control point and
other observed control points used to establish its
coordinates
NEW STANDARDS FOR GEODETIC CONTROL
(http://fgdc.er.usgs.gov/standards/status/swgstat.html)
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local accuracy -------------network accuracy ----------
adjacent points
relative to CORS
Numeric quantities, units in cm (or mm)
Both are relative accuracy measures
Will not use distance dependent expression
Order/Class codes will no longer be used
N HEIGHT is
precisely
determined but
only tied to one
published
benchmark
Computations of accuracies
This is an extract from an
OPUS extended solution.
Annoying details
X = ATWA-1*ATWL
V = A*X-L
σ2 = (VTWV)/DF
• A is the design matrix
• L is a matrix of
observations
• W is a weight matrix
• X is the vector of
unknowns
• Q is the variancecovariance matrix (ATA)-1
Accuracy of a quantity is determined by
multiplying standard deviation of
variances by diagonal element of the Q
matrix.
SHIFTS (National results)
< 5 cm. in the horizontal component with an
average shift of 2.2 cm.
< 10 cm. in the vertical component with
an average shift of 4.6 cm.
ISSUES
New realization of the CORS coordinates are not
available-we recommend using the 2002 coordinates as
published. The new Policy statement is posted on the
NSRS Readjustment webpage.
Another adjustment MAY be run after the next CORS
realization (2005) is released and new CORS
coordinates published.
ITRF and WGS 84
WGS 84 from 1987 to 1994, WGS 84(G730)
from 1994 to 1996, WGS 84(G873) from 1997 to
2002 and WGS 84(G1150) from 2002 to present.
ITRF to NAD 83 (????)
NAD 83 (NSRS 2007) v NAD 83 (CORS 96)
• “… approximates but is not equivalent to the more
rigorously defined NAD 83 (CORS 96) realization in
which CORS coordinates are distributed.”
• “ …should be consistent with corresponding NAD 83
(CORS 96) positional coordinates to within the
accuracy of GPS data used and the accuracy of the
corrections applied…”
• NAD 83 (NSRS 2007) coordinates do not account for
vertical crustal motion.
• NGS did not compute velocities for any of the passive
monuments involved in the adjustment.
NSRS 2007 adjustment
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3,411 projects
67,693 points
283,691 vectors
673 CORS fixed
No. of constrained parameters = 2055
No. of unknown parameters = 203,076
Degrees 0f Freedom = 650,049
 Constraints in adjustment: 471 National CORS, 3
Canadian CACS, 1 Mexican CORS and 213 California
CGPS sites
Overall NAD 83 NSRS 2007 results
• Median horizontal network accuracy: 1 cm
– Median hz. adj vector residual: 0.52 cm
• Median vertical network accuracy: 2 cm
– Median vt. adj vector residual: 0.54 cm
Same data in datasheet format
Time-line for National Readjustment
Positions and Ellipsoid Heights
All projects loaded in the NGS database prior to
November 15, 2005 were included in the
National Readjustment
Projects submitted after 11/15/2005 were
accepted and loaded into the database but
were not included in the readjustment
Completed adjustment by February 10, 2007
deadline.
New Datasheets including new positions and
network accuracies are available NOW.
Project Report (pending)