National Readjustment

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National Readjustment

• Dale Pursell
• Observation and Analysis Division
• National Geodetic Survey
• FGCS Workgroup Meeting
• February 14, 2007

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NAD 83(86) NETWORK PROBLEMS

• Not GPSABLE
• POOR STATION ACCESSIBILITY
• IRREGULARLY SPACED
• POSITIONAL ACCURACY

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HIGH PRECISION GPS NETWORKS

• GPSABLE
• Clear Horizons for Satellite Signal Acquisition
• EASY ACCESSIBILITY
• Few Special Vehicle or Property Entrance
  Requirements
• REGULARLY SPACED
• Always within 20-100 Km
• HIGH HORIZONTAL ACCURACY
• A-Order (5 mm 110,000,000)
• B-Order (8mm 11,000,000)

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HARN/HPGN ADJUSTMENT

• A-Order Adjusted to VLBI-Existing FBN-CORS
• B-Order Adjusted to A-Order
• Existing Horizontal (Conventional GPS)
  Readjusted to A/B-Order
• New Adjustment Date Tag e.g. NAD83(1991.35)

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HIGH ACCURACY REFERENCE NETWORK
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NETWORK STATUS
HPGN/HARN STATEWIDE
1991
GPS222
1996
1992
1996
1991
GPS1048
GPS341
1991
GPS197
1996
GPS805
reos 7/96
GP S291
1993
1994
reos-1996
GPS941
GPS1047

reobs 9/97
1996
GPS730
1995
1994
GPS606
1992
1995
GPS1121
1997
1997
GPS887
GPS404
1994
GPS394
1991
GPS882
GPS260
GPS
GPS1178
1997
1997
1995
1993
1200

GPS633
1992
GPS908
1992
1993
GPS1150
GPS1179
GPS610
reos-6/95-GPS904

GPS412
GPS611
1995
GPS419
1992
1990
-GPS120
1997
GPS852
GPS376
1993
GPS1133

1993
1992
1994
GPS725
GPS366
GPS721
1992
GPS585

1990
1992
GPS383
reos 1/96 GPS936
-GPS170
GPS450
1993
GPS667
1997
.
GPS CLASSICAL ADJUSTMENTS COMPLETED

GPS ADJUSTMENT COMPLETED
GPS577
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FEDERAL BASE NETWORK

• NGS FUNDED
• NGS MARK MAINTENANCE
• MINIMUM 100 KM STATION SPACING
• A and B-ORDER HORIZONTAL ACCURACY
• FIRST-ORDER CLASS II ORTHOMETRIC HEIGHTS
• or
• SECOND-ORDER CLASS I ORTHOMETRIC HEIGHTS
• SECOND-ORDER CLASS I ELLIPSOIDAL HEIGHTS
• REVISIT EVERY 3 TO 5 YEARS

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FBN/CBN SURVEYS
1998
GPS1340
2000

2002
1999
GPS1492

2001
GPS1686
GPS1394

GPS1641

2000
1998
GPS1506
GPS1333

2002
1999
1997
2000
GPS1687

2001
1999
GPS1377
GPS1207

2001
GPS1514
GPS1596
GPS1397
GPS1712

2003

2000
2000
GPS1752
1999

2000
2003
GPS1481

2003
GPS1463
1999

2002
GPS1519
GPS1356
GPS
GPS1731
GPS1825
GPS1370
1850

2001


2000
2000
MEANS
GPS1629

2003

2000

2003

GPS1462
1998
GPS1530
GPS1188
GPS1851
2002
GPS1726
GPS1288
GPS-1547
1999

2001
2003 GPS1753
1999
2000
1999
GPS1399
GPS1554
GPS1347
GPS1411

GPS1381
1997
1998
GPS1250
1998
GPS1273
1998
GPS1258
1998
GPS1267
GPS1264
1999
GPS1378

2003
GPS READJUSTMENT COMPLETED
GPS1682
WILLIAMS
FBN GPS ADJUSTMENT COMPLETED
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IMPROVING POSITIONAL ACCURACY

• TIME NETWORK LOCAL
• NETWORK SPAN ACCURACY ACCURACY
• NAD 27 1927-1986 10 METERS (1 part in 100,000)
• NAD83(86) 1986-1990 1 METER (1 part in 100,000)
• HARN 1990-1997 0.1 METER B-order (1
  part in 1 million)
• A-order (1 part
  in 10 million)
• CORS 1996 - 0.01 meter 0.01 meter

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Reasons for Readjustment

• 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

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What about Orthometric Heights?

• Decision was made not to perform a National
  Readjustment of orthometric heights at this time
• Control NAVD 88 not the CORS
• Since no change in control most orthometric
  elevations would not change significantly
• Network Local Accuracy numbers for the
  orthometric heights would be the primary reason
  for readjustment
• Would require relevant network accuracies for the
  NAVD 88 network-This would require a complete
  analysis of the NAVD 88 network

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NGS Adjustment Team (1986)
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NAD 83 READJUSTMENT

• ONLY GPS DATA WAS USED
• CONTINUOUSLY OPERATING REFERENCE STATIONS
• FEDERAL BASE NETWORK
• COOPERATIVE BASE NETWORK
• USER DENSIFICATION NETWORK
• AIRPORT SURVEYS

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Project Adjustment Analysis

• 3436 Projects
• Free Adjustment
• Residual Plot
• Outliers Rejected
• Connectivity to A/B Order Network Verified
• Summary sheet with Project Information Created
• 67693 stations

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TRASH

• Projects Not Recommended for Inclusion
• Currently 149 Projects with 9903 stations
• Many Third Order FAA Projects from 1980s
• Some Projects that have no ties to the Network
• Original TN HARN (Macrometer Data)
• Original Eastern Strain Network Project

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The National Readjustment General Comments

• The CORS/CGPS sites provided the control
• Only GPS projects participated
• The FBN/CBN Surveys were a key element since
  these were high accuracy (2 cm) surveys that tied
  the HARN to the CORS throughout the contiguous
  United States and provided more accurate values
  for the ellipsoid heights of most HPGN stations.
• Free adjustment analysis of each GPS project that
  contributed to the National Spatial Reference
  System.

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The National Readjustment More Comments

• Computation and database loading of scale factors
  for each project was completed.
• Statewide GPS Readjustments provided extra
  information about the network on a state by state
  basis.

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Concept of Operation

• Helmert Blocking strategy was used.
• NETSTAT- New Helmert Blocking software developed
  for the entire adjustment process incorporating
  more quality control and significant time saving
  features.
• Coordinates Produced and Published for both NAD83
  (NSRS) and ITRF
• Adjustment in NAD83-Transformed to ITRF
• Network and Local Accuracies Produced

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Helmert Blocks

• CORS/CGPS provided control at the top block
• CGPS coordinates were determined by Scripps
  Sector utility projected to the 2007.0 epoch.
• An attempt was made to create a separate block of
  data for each state to minimize the number of
  junction observations between blocks.
• California, Florida, Minnesota, North and South
  Carolina were broken into multiple blocks because
  of the number of stations located in these blocks.

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HELMERT BLOCKING STRATEGY
This is how each state fits into the National
Readjustment
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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 will be implemented with the
  National Readjustment

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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

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NEW STANDARDS FOR GEODETIC CONTROL

• (http//fgdc.er.usgs.
  gov/standards/status/swgstat.html)
• local accuracy -------------- adjacent
  points
• network accuracy ---------- 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

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AVERAGE SHIFTS

• Based on the final adjusted results from the
  original published positions and ellipsoid
  heights
• lt Average horizontal shift 2.5 cm.
• lt Average ellipsoid height shift 2.7 cm.

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ISSUES

• New realization of the CORS coordinates based on
  ITRF2005 will not be available- The published
  NAD83(CORS96) coordinates based on ITRF2000 will
  define the new NAD83 (NSRS2007) coordinate
  system. This new Policy statement is posted on
  the NSRS Readjustment webpage..
• To be decided-will another adjustment be run
  after the next CORS realization (20??) is
  released and new CORS coordinates published.

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FINAL STATISTICS

• Free adjustment
• No. of Observations 851073
• No. of constrained parameters 3
• No. of unknown parameters 203,076
• Degrees 0f Freedom 647997
• Standard error of unit weight 1.276619
• Constrained adjustment
• (486 National CORS and 195 CGPS sites)
• No. of Observations 851073
• No. of constrained parameters 2055
• No. of unknown parameters 203,076
• Degrees 0f Freedom 650049
• Standard error of unit weight 1.375490

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www.ngs.noaa.gov/NationalReadjustment
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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 will be
  accepted and loaded into the database but will
  not be included in the readjustment
• Completed February 8, 2007
• Project Report
• 6 months-February, 2007 through July, 2007

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For Stations not Included

• NGS recommends that NAD 83 data that is NOT part
  of the NSRS readjustment be readjusted by
  contractor/user with the original observations.
• Because of the relatively small shifts
  anticipated, a model such as NADCON will not be
  developed between previously determined GPS
  coordinates.

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Post National Readjustment

• New adjustment procedures will be developed that
  incorporate local and network accuracies.
• Design and Publish new data sheets.
• Provide error ellipses for every station
• A reference frame maintenance strategy (NAD83,
  ITRF, either or both) will be developed.

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Important Facts

• COMPLETION DATE February 08, 2007
• NEW DATUM TAG - NAD83 (NSRS2007)
• For the latest information- visit the
  national readjustment webpage at
• (http//www.ngs.noaa.gov/NationalReadjust
  ment

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OTHER ISSUES

• We will streamline the process of project
  acceptance and database loading
• OPUS DB
• Require that all projects (submitted after the
  November 15, 2005 deadline) be tied to CORS
• Accept/require more data in digital format-e.g..
  reports, logs
• Photographs

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