National Spatial Reference System

1
National Spatial Reference System

• NORTH AMERICAN VERTICAL DATUM OF 1988
• (NAVD 88)
• SEMINAR
• January 15, 2003
• Catskill, New York
• Edward J. McKay

2
OUTLINE

• Vertical Datums
• Height Systems
• NAVD 88 Project
• NAVD 88 Implementation
• FEMA NAVD 88
• NAVD 88 Conversion Techniques

NATIONAL OCEAN SERVICE
3
NATIONAL SPATIAL REFERENCE SYSTEM

• The National Spatial Reference System (NSRS) is
  the name given to all geodetic control contained
  in the National Geodetic Survey (NGS) Data Base.
  This includes A, B, First, Second and
  Third-Order horizontal and vertical control,
  Geoid models such as GEOID 99, precise GPS orbits
  and Continuously Operating Reference Stations
  (CORS), observed by NGS as well as data submitted
  by other Federal, State, and local agencies,
  academic institutions and the private sector

NATIONAL OCEAN SERVICE
4
VERTICAL DATUMS

• SEA LEVEL DATUM OF 1929
• NATIONAL GEODETIC VERTICAL DATUM OF 1929
• (As of July 2, 1973)
• NORTH AMERICAN VERTICAL DATUM OF 1988
• (As of June 24, 1993)

NATIONAL OCEAN SERVICE
5
COMPARISON OF VERTICAL DATUM ELEMENTS

• 
  NGVD 29
  NAVD 88
• DATUM DEFINITION 26 TIDE GAUGES
  FATHERS POINT/RIMOUSKI
• 
  IN THE U.S. CANADA
  QUEBEC, CANADA
• BENCH MARKS 100,000
  450,000
• LEVELING (Km)
  102,724
  1,001,500
• GEOID FITTING Distorted to Fit
  MSL Gauges Best Continental
  Model

6
NORTH AMERICAN VERTICAL DATUM 88

• WHAT IS A VERTICAL CONTROL NETWORK?
• An Interconnected System of Bench Marks
• Each Bench Mark Is Assigned A height Referenced
  To A Common Surface

7
NORTH AMERICA VERTICAL

• WHY DO WE NEED A VERTICAL CONTROL NETWORK?
• Reduces The Amount Of Future Leveling Required
• Enables Surveyors To Check Their New Leveling
• Provides Backups For Destroyed Or Disturbed Bench
  Marks
• Assists In Monitoring Changes In Local Areas
• Provides A Common Framework

8
HEIGHT SYSTEMS

• FIVE STEPS TO CREATING A VERTICAL CONTROL NETWORK
• Recon level line and set new bench marks
• Observe height differences between bench marks
• Correct observations for known systematic effects
• Minimize discrepancies in the results obtained by
  leveling along different routes between the same
  two points
• Define the surface datum to which heights may be
  referred

9
(No Transcript)
10
Leveled Height vs. Orthometric Height
? h local leveled differences
?H relative orthometric heights
Equipotential Surfaces
B
Topography
? hAB
? hBC
A
C
HA
HC
?HAC ? ?hAB ?hBC
Reference Surface (Geoid)
Observed difference in orthometric height, ?H,
depends on the leveling route.
11
Leveling - Derived Orthometric Heights
Earths
Surface
Level Surfaces
P
Plumb
Line
Mean
Sea
Geoid
Level
PO
Ocean
Level Surface Equipotential Surface
H (Orthometric Height) Distance along Plumb
line (PO to P)
12
Heights Based on Geopotential Number

• Normal Height (NGVD29) H C / ?
• ? Average normal gravity along plumb line
• Dynamic Height (IGLD55,85) Hdyn C / ?45
• ?45 Normal gravity at 45 latitude
• Orthometric Height H C / g
• g Average gravity along the plumb line
• Helmert Height (NAVD 88) H C / (g 0.0424 H)
• g Surface gravity measurement (mgals)

13
The Geoid

• The geoid is the equipotential surface of the
  earths attraction and rotation which, on the
  average, coincides with mean sea level in the
  open ocean.

14
Execution of Surveys Sources of Error

• Errors may be characterized as random,
  systematic, or blunders
• Random error represents the effect of
  unpredictable variations in the instruments, the
  environment, and the observing procedures
  employed
• Systematic error represents the effect of
  consistent inaccuracies in the instruments or in
  the observing procedures
• Blunders or mistakes are typically caused by
  carelessness and are detected by systematic
  checking of all work through observational
  procedures and methodology designed to allow
  their detection and elimination

15
Geodetic Control

• Network of Monumented Points
• Precisely Measured in Accordance with Standard
  Procedures
• Meet Accuracy Specifications
• Adjusted to Tie Together
• Documented for Multiple Use

16
HEIGHT SYSTEMSGEOPOTENTIAL NUMBERS

• Although geopotential numbers are useful for the
  adjustment of vertical networks, for many
  purposes true orthometric heights above a
  physically defined reference surface are still
  necessary
• A geopotential number can be converted to a
  true orthometric height
• by dividing the geopotential number by the mean
  value of gravity along the plumb line between the
  point and the reference surface
• H C/gm
• Since the mean value of gravity cannot be
  directly measured (because the reference surface
  lies within the Earth beneath the point), a model
  must be used to derive the value as a point, and
  other variables

17
HEIGHT SYSTEMSGEOPOTENTIAL NUMBERS

• The geopotential number of a point is a measure
  of the difference in potential from the reference
  surface to the equipotential surface passing
  through the point
• The geopotential number is numerically equivalent
  to the work required to raise a mass of 1 Kg
  against gravity (g) through the orthometric
  height (H) to the point
  H
• Geopotential number (C) g dH
• 0
• The difference in height (dh) measured during
  each setup of leveling can be converted to a
  difference in potential by multiplying dh by the
  mean value of gravity (gm) for the setup
• Geopotential difference gmdh

18
HEIGHT SYSTEMSGEOPOTENTIAL NUMBERS

• The geopotential number C is measured in
  geopotential units (gpu)
• 1 gpu 1 Kgal meter 1000 gal meter
• g 0.98 Kgal ? c ? 0.98 H
• (Reference Physical Geodesy by Heiskanen and
  Moritz)

19
HEIGHT SYSTEMSSEA LEVEL HEIGHTS

• Heights measured above local mean sea level
• The National Tidal Datum epoch is a particular 19
  - year series over which the phases (such as mean
  lower low water) are determined.
• Encompasses all significant tidal periods
• Including the 18.6 - year period for the
  regression of the Moons nodes
• Averages out practically all of the
  meteorological, hydrological, and oceanographic
  variability
• Leveling is used to determine the relationship
  between bench marks and tidal gages

20
HEIGHT SYSTEMSDATUMS

• Any surface defined as the reference surface from
  which heights are measured, can be called a datum
• International Great Lakes Datum (IGLD)1955
• Defined by one height (Father Point)
• Water - level transfers used to connect leveling
  across the Great Lakes
• Dynamic heights
• H - C/G? G? - 980.6294 gals
• (Normal gravity at 45 degrees latitude as defined
  in 1955)

21
HEIGHT SYSTEMSDATUMS

• National Geodetic Vertical Datum of 1929 (NGVD
  29)
• Defined by heights of 26 tidal stations in the
  U.S. and Canada
• Tide gages were connected to the vertical network
  by leveling from tide gage staffs to bench marks
• Water - Level transfers used to connect leveling
  across the Great Lakes
• Normal orthometric heights
• H - C/Ga Ga - Normal gravity based on formula

22
HEIGHT SYSTEMSDATUMS

• North American Vertical Datum of 1988 (NAVD 88)
• Defined by one height (Father Point/Rimouski)
• Water-level transfers used to connect leveling
  across the Great Lakes
• Geopotential Numbers
• Helmert orthometric heights
• Hhel - C/Ga Ga Mean value of gravity along
  the plumb line
• between the geoid and surface,
  estimated using
• Helmerts reduction, I.e., g
  0.0424xHo.
• g gravity at the surface in gals
• Ho approximate height in kilometers

23
HEIGHT SYSTEMSDATUMS

• INTERNATIONAL Great Lakes Datum (IGLD) 1985
• Same as NAVD 88, except published in Dynamic
  Heights
• Dynamic Heights
• Hdym C/Go Go 980.6199 gals
• (Normal gravity at 45 degrees latitude as defined
  in 1985)

24
(No Transcript)
25
EXECUTION OF SURVEYSSOURCES OF ERROR

• Errors may be characterized as random,
  systematic, or blunders
• Random error in leveling results represent the
  effect of unpredictable variations in the
  instruments, the environment, and the procedure
  of leveling
• Random error cannot be completely eliminated,
  although it can be kept small
• Therefore, it represents the noise level, a
  limit on the accuracy with which leveling may
  measure elevation differences

26
EXECUTION OF SURVEYSSOURCES OF ERROR

• Errors may be characterized as random,
  systematic, or blunders
• Systematic error represents the effect of
  consistent inaccuracies in the instruments or in
  the leveling procedures
• Systematic error may be small in a single
  measurement it accumulates when measurements
  made under similar circumstances are totaled
• Therefore, it can result in a significant
  discrepancy in the height differences measured
  between two control points by different leveling
  systems and/or routes
• For leveling to provide accurate height
  differences, systematic error must be minimized,
  either by procedure or by applying corrections to
  the data

27
VERTICAL DATA REDUCTION COMPUTATIONS

• Systematic errors which cannot be sufficiently
  controlled by instrumentation or observational
  techniques are minimized by applying appropriate
  corrections to the observed data.
• (See Balazs and Young, 1982).
• NGS applies seven corrections
• Level Collimation
• Scale Imperfections
• Refraction
• Curvature
• Tidal Accelerations
• Gravity Field
• Magnetic Fields

28
EXECUTION OF SURVEYSSOURCES OF ERROR

• Blunders
• The sources of error in leveling can be
  classified into three groups
• Those affecting the line of sight
• Those affecting the heights computed
• Blunders

29
Error Sources Associated With Differential
Leveling

• Error Source
  Typical Size of Error
• 
  in mm Per 1 km Section
• Blunders
• Forward pin or plate movement between setups
  10.0
• One rod unit or larger error in reading the
  rod.. 5.0
• Systematic Errors
• Rod verticality error ... 1.0
• Rod scale error. 2.0
• Thermal expansion of Invar rod .
  0.2
• Rod index error .. 1.0
• Movement of tripod during setup (if set up
  correctly) 0.2
• Gradual movement of turning points
• during setups 0.6
• between setups 0.6

30
Error Sources Associated With Differential
Leveling

• Error Source
  Typical Size of Error
• 
  in mm Per 1 km Section
• Systematic Errors Continued
• Collimation .. . 2.4
• Under and over compensation ..
  0.4
• Refraction .. 2.0
• Refraction change during setup .
  0.6
• Diurnal Earth tides 0.1
• Earths magnetic field .. 1.0
• NI 002 parallax . 0.6

31
Error Sources Associated With Differential
Leveling

• Error Source
  Typical Size of Error
• 
  in mm Per 1 km Section
• Quasi Random Errors
• Scintillation, short-period . 1.0
• Scintillation, long-period .....
  5.0
• Pointing error (experienced observer)......
  . 0.4
• Rod error in individual graduations ...
  0.1
• NOTE Assumes 50 meter sight lengths and 10
  setups per 1 kilometer section.

32
NAVD 88 DATUM DEFINITION AND RESULTS
33
(No Transcript)
34
NAVD 88 PROGRAM DEFINITION

• NAVD 88 is a program which combined 1,300,00
  kilometers of leveling surveys held in the NGS
  National Spatial Reference System (NSRS) data
  base, into a single least squares adjustment to
  provide users with improved heights for over
  500,000 vertical control points distributed
  throughout the United States, on a common datum.

35
(No Transcript)
36
(No Transcript)
37
PRESENT NETWORK FOR NAVD 88

• ORIGINAL LEVELING 700,000 KM
• REPEAT LEVELING 200,000 KM
• NEW BNA LEVELING 81,500 KM
• NEW OUTSIDE LEVELING 20,000 KM
• TOTAL FOR NAVD 88 1,001,500 KM
• (620,000
  MILES)

38
(No Transcript)
39
(No Transcript)
40
NEW YORK VERTICAL NETWORK

• NGVD 29 bench marks . . . . . . . . . 12,927
• NAVD 88 bench marks . . . . . . . . . 14,529
• (INCLUDES POSTED DATA)
• POSTED bench marks . . . . . . . . . . 609
• Bench marks without
• NAVD 88 heights . . . . . . . . . .
  599
• Includes TBMs, some RESETS, and new marks on
  lines not included in NAVD 88 general adjustment

41
(No Transcript)
42
(No Transcript)
43
(No Transcript)
44
NORTH AMERICALN VERTICAL DATUMOF 1988 (NAVD 88)

• THE U.S. PORTION OF THE PROJECT INCLUDED THE
  REMONUMENTATION AND REOBSERVATION OF AN 80,000
  KILOMETER SUBSET OF THE VERTICAL CONTROL PORTION
  OF THE NATIONAL SPATIAL REFERENCE SYSTEM.
• A MINIMUM-CONSTRAINT LEAST SQUARES ADJUSTMENT OF
  LEVELING DATA INVOLVING 709,000 MARKS WAS
  PERFORMED.

45
NORTH AMERICAN VERTICAL DATUMOF 1988 (NAVD 88)

• IN ORDER TO MINIMIZE THE EFFECTS ON USGS NATIONAL
  MAPPING PRODUCTS (NMPs), AS REQUESTED BY USERS,
  NGS SELECTED THE NEW INTERNATIONAL GREAT LAKES
  DATUM OF 1985 (IGLD 85) LOCAL MEAN SEA LEVEL
  HEIGHT VALUE AT MINIMUM-CONSTRAINT DATUM POINT
  FOR NAVD 88. THE DATUM POINT IS LOCATED AT THE
  MOUTH OF THE ST. LAWRENCE RIVER IN QUEBEC,
  CANADA.
• USING FATHER POINT/RIMOUSKI AS THE DATUM POINT
  FOR BOTH IGLD 85 AND NAVD 88 MINIMIZES THE IMPACT
  ON NMPs, AND ALLOWS NAVD 88 TO REPLACE BOTH NGVD
  29 AND IGLD 55.

46
NORTH AMERICAN VERTICAL DATUMOF 1988 (NAVD 88)

• FISCAL THE GENERAL ADJUSTMENT DID NOT INCLUDE
  APPROXIMATELY 25 PERCENT OF THE VERTICAL CONTROL
  NETWORK. BENCH MARKS IN STABLE AREAS WHICH
  WERE REMOVED FROM THE ADJUSTMENT (DENOTED AS
  POSTED) BECAUSE OLDER DATA DID NOT FIT WITH
  THE LATEST DATA. THIS DATA WAS INCORPORATED INTO
  THE NAVD 88 DURING YEARS 1992-1993.

47
NORTH AMERICAN VERTICAL DATUMOF 1988 (NAVD 88)

• NAVD 88 DOES NOT CONTAIN USGS, COE, OR STATE DOT
  THIRD-ORDER LEVELING DATA.
• USGS PERSONNEL HAVE PERFORMED PILOT STUDIES TO
  DETERMINE HOW TO BEST INCORPORATE THEIR
  THIRD-ORDER DATA INTO NAVD 88 (ABOUT A 5-10 YEAR
  PROGRAM)

48
NAVD 88 DATUM DEFINITION

• Vertical datum based upon an equipotential
  surface
• Minimally constrained adjustment held fixed at
  one point, Father Point/Riouski (Point-au-Pere)
• 1.3 million kms of leveling data used
• Heights of 585,000 permanent bench marks
  estimated.
• Both orthometric heights and geopotential numbers
  have been published

49
NAVD 88 GENERAL ADJUSTMENT COMPLETION DATE OF
JUNE 1991WHAT DOES THIS REALLY MEAN?

• The general adjustment of NAVD 88 was completed
  in June 1991. This means that bench marks
  included in the NAVD 88 Helmert blocking phase
  (approximately 80 percent of the total) have
  final adjusted heights available.
• Bench marks in stable areas which were removed
  from the adjustment (denoted as POSTed) because
  older data did not fit with the latest data was
  incorporated into NAVD 88 during fiscal years
  1992-1993.

50
NAVD 88 GENERAL ADJUSTMENT COMPLETION DATE OF
JUNE 1991WHAT DOES THIS REALLY MEAN?

• Bench marks POSTed in large crustal movement
  areas, e.g., southern California, Phoenix,
  Arizona, Houston, Texas, and southern Louisiana
  was published as special reports after the final
  adjustment was completed. This is an on-going,
  long-term task which was started in January 1992.
  It is important to note that some bench marks in
  crustal movement areas, i.e., bench marks which
  were included in the NAVD 88 Helmert blocking
  phase, is available. The heights of these bench
  marks will be based on the latest available data,
  but still may be influenced by crustal movement
  effects.

51
Most surveying applications should not be
significantly affected because the changes in
relative height between adjacent bench marks
should be less than 1 cm. As stated above, the
absolute height values will change much more, but
this should not be a major concern to the
surveyor. The greatest problem the surveyor will
have is ensuring that all height values of bench
marks in the project area are referenced to the
same vertical datum, preferably NAVD 88 and
labeled correctly (metadata). Other agencies
bench marks, e.g., COE, FL Department of
Transportation, FL Department of Environmental
Protection, and USGS, were incorporated into NAVD
88 by NGS as these agencies provided, and still
do, their data in computer-readable form.
However, the leveling data associated with over
500,000 third-order bench marks established by
USGS have not been placed in computer-readable
form and do not have NAVD 88 heights. In
addition, COE has established hundreds of
thousands of bench marks across the nation which
do not have NAVD 88 heights.
52
IMPACT OF NAVD 88

• Data Bases containing heights referenced to NGVD
  29 will have to be updated to NAVD 88
• Depending upon the accuracy required, in many
  areas a Bias Factor could be used for bench
  marks not included in the readjustment
• In Moving areas a Bias Factor probably will not
  be sufficient for most applications

53
IMPACT OF NAVD 88

• Published Heights of Bench Marks Have Changed
• Published height values has shifted as much as 5
  decimeters
• In Stable areas, Relative height changes
  between adjacent bench marks should only be
  millimeters
• In Moving areas, Relative height changes have
  been dependent upon the reasons for the movements.

54
IMPACT OF NAVD 88

• Maps depicting NGVD 29 Heights will have to be
  modified for NAVD 88 Heights
• In many areas a Single Bias Factor, Describing
  the Difference between NGVD 29 and NAVD 88, could
  be used for most Mapping Applications
• In Moving areas, maps depicting the rates of
  movements will have to be compiled

55
REASONS TO CONVERT PRODUCTS TO NAVD 88

• Surveys between bench marks will often close
  better
• NAVD 88 has provided a better reference to
  compute GPS-Derived Orthometric Heights
• 40,000 Additional bench marks of First-Order
  accuracy is available on NAVD 88
• Data and NAVD 88 adjusted height values is
  readily available and accessible in a convenient
  format from NGSs web site http//www.ngs.noaa.go
  v
• Federal Surveying and Mapping agencies will stop
  publishing on NGVD 29 and will publish only on
  NAVD 88
• Surveys performed for the Federal Government
  requires the use of NAVD 88

56
REASONS TO CONVERT PRODUCTS TO NAVD 88

• THE AMERICAN CONGRESS ON SURVEYING AND MAPPING
  (ACSM) AND THE FEDERAL GEODETIC CONTROL
  SUBCOMMITTEE (FGCS) RECOMMEND NAVD 88.
• National Geodetic Survey no longer adjust to NGVD
  29

57
BENEFITS OF NAVD 88

• Improved set of heights on a single vertical
  datum for North America
• Improved FGCS Leveling procedures with higher
  production and lower error rates
• All NGS National Spatial Reference System data is
  validated in a single data base, with easy access
  by users for crustal motion studies, adjustments,
  latest official heights, and descriptions
• Removal of height discrepancies caused by
  inconsistent adjustment constraints

58
BENEFITS OF NAVD 88(CONTINUED)

• Detection and Removal of height errors due to
  blunders
• Minimization of effects of systematic errors in
  leveling data
• Replacement of both NGVD 29 and IGLD 55 with a
  single datum
• Remonumentation and incorporation of 80,000 km of
  new leveling data not previously adjusted to NGVD
  29
• Orthometric Heights compatible with GPS-Derived
  Orthometric Heights computed using the
  High-Resolution Geoid Model called Geoid99

59
(No Transcript)
60
NAVD 88 IMPLEMENTATION
61
NAVD 88 IMPLEMENTATION

• Published and distributed NAVD 88 height values
• Processed and distributed height values for
  POSTed data
• FGCS Vertical Workgroup input from ACSM Ad Hoc
  Committee
• USGS third-order vertical data
• FEMA/National Flood Insurance program

62
FGCS VERTICAL WORK GROUP

• MEMBERS
• National Geodetic Survey (Chair)
• U.S. Geological Survey
• Federal Highway Administration
• International Boundary Commission
• Bureau of Land Management
• U.S. Army Corps of Engineers
• U.S. Forest Service
• Federal Emergency Management Agency

63
ACSM AD HOC COMMITTEEGEOGRAPHIC MAKEUP

• East Coast (Florida to Massachusetts)
• Gulf Coast
• Interior Southern States
• Great Lakes area
• Plains and Mountain States
• Pacific Coast (California to Washington)

64
ACSM AD HOC COMMITTEEDISCIPLINE MAKEUP

• Land Surveyors
• Geodetic Surveyors
• Mappers
• ACSM Private Members
• ACSM Government Members

65
FEMAS RESPONSETO NAVD 88
66
FEMAS Response to NAVD 88

• Local Mean Sea Level (LMSL)
• determined at individual tide gages
• Sea Level Datum (SLD) of 1929
• constrained at 26 tide gages in the U.S. and
  Canada
• National Geodetic Vertical Datum of 1929 (NGVD
  29)
• renamed from SLD of 1929 to avoid confusion with
  LMSL
• North American Vertical Datum of 1988 (NAVD 88)
• constrained only at Pointe au Pere gage on St.
  Lawrence River

67
FEMAs Response to NAVD 88

• FEMA mapped and prepared Flood Insurance Studies
  (FISs) for thousands of communities with flood
  elevations
• vertical reference is the datum as defined by NGS
• FISs contain flood profiles
• Flood Insurance Rate Maps (FIRMs) contain flood
  elevations and Elevation Reference Marks (ERMs)
• Letters of Map Amendment and Revision (LOMAs and
  LOMRs) are issued based on elevation comparisons

68
FEMAs Response to NAVD 88

• FEMA Users Include
• Banks and mortgage institutions (lenders)
• Flood insurance agents
• Surveyors, engineers, architects, and planners
• Community floodplain, planning, and zoning
  officials
• FEMA Contractors Include
• Federal and State water resources agencies
• Regional water resources commissions
• Private architectural and engineering firms

69
FEMAs Response to NAVD 88

• Lenders initiate flood insurance purchase
  requirement based on FIRMs
• Surveyors provide Elevation Certificates for
  flood insurance agents and lenders
• Community officials enforce floodplain management
  regulations, which are based on FIS and FIRM
• Federal contractors must know how and when to
  implement conversion

70
FEMAs Response to NAVD 88

• Responsibility of Map Users
• ensure use of datum consistent with FIS and FIRM
• Responsibility of FEMA Contractors
• adherence to FEMA guidelines for conversion
• documentation of datum used in FIS and FIRM
• ensure datum consistency throughout FIS and FIRM

71
FEMAs Response to NAVD 88

• How Will FEMA Accomplish Conversion?
• Educate staff
• Educate contractors
• Educate users
• Close coordination with NGS
• FEMA has published two documents
• Appendix 6, Conversion to the North American
  Vertical Datum 1988
• Converting the NFIP to the NAVD 88

72
FEMAs Response to NAVD 88

• FEMAs Original Plan
• New Studies - FY 93 FISs
• (scope of work April 92)
• Map actions FY93 as practicable
• FEMAs Current Proposal
• Update Appendix 6, Conversion to the NAVD 88
• Refine strategy for an orderly transition of FISs
  and FIRMs to NAVD 88
• Gradually convert based on opportunities to
  republish FISs and FIRMs for other reasons.
• Ultimate goal is to convert all FISs to NAVD 88

73
(No Transcript)
74
(No Transcript)
75
(No Transcript)
76
(No Transcript)
77
(No Transcript)
78
(No Transcript)
79
(No Transcript)
80
(No Transcript)
81
(No Transcript)
82
(No Transcript)
83
(No Transcript)
84
(No Transcript)
85
(No Transcript)
86
NGS RESPONSIBILITIES

• Performed procedures to officially replace NGVD
  29 with NAVD 88
• Compiled documentation to brief Congress and
  State officials on NAVD 88 impacts and benefits
  to minimize problems with uniformed users
• Provided documentation and publication of NAVD 88
  final results

87
NGS RESPONSIBILITIES

• Estimated conversion (bias) shifts between NGVD
  29 and NAVD 88
• Analyzed bias shift computations to determine
  where other data, e.g., COE and/or USGS data, may
  be required (in computer-readable form) to
  improve the estimate of the bias factor
• Analyzed the vertical control network to
  determine local areas where height changes are
  due to crustal movement

88
NGS RESPONSIBILITIES

• Analyzed the vertical control network to separate
  bias shifts into components changes due to datum
  definition, crustal movement, improved
  corrections applied to leveling data to account
  for systematic errors, and removal of adjustment
  distortions in NGVD 29
• Incorporate other data, e.g., COE and/or USGS
  data, into NAVD 88 (data must be in
  computer-readable form)
• Educate NAVD 88 users

89
NAVD 88 USERS RESPONSIBILITIES

• Provide Kinds Of Data, Reports, Routines, and
  Training Required To Implement NAVD 88
• Relay (In A Timely Manner) To NGS Problems with
  Implementation Of NAVD 88

90
NAVD 88 CONVERSION TECHNIQUES
91
BASIC CONVERSION TECHNIQUES

• Estimation of bench mark heights by incorporating
  the original leveling data into NAVD
  88 using least squares adjustment techniques
• A rigorous transformation of bench mark heights
  for a particular project using datum conversion
  correctors estimated from the projects original
  adjustment constraints and their differences
  between NAVD 88 and NGVD 29
• A simplified transformation of bench mark heights
  using an average bias shift for the area (VERTCON)

92
CONVERSION TECHNIQUES(CONTINUED)

• Technique number 1 is the most rigorous technique
  because the bench mark heights will retain their
  original relative accuracy. These heights will
  be useful to all users. In addition, NGS will
  adjust and publish the results if the data are
  submitted to NGS in computer-readable form.
  Technique number 2 may meet many users
  requirements, but depending upon the accuracy
  requirements and the complexity of the userss
  leveling network, may prove to NGS to process.
  Technique number 3 should be the easiest method
  to implement, but in general is only sufficiently
  accurate enough to meet mapping requirements.

93
CONVERSION TECHNIQUES

• The use of GPS data and a high-resolution geoid
  model (Geoid99) to estimate accurate GPS-derived
  orthometric heights will be directly associated
  with the implementation of NAVD 88. It is
  important that users initiate a program to
  convert their products to NAVD 88. The
  conversion process is not a difficult one, but
  will require time and resources. There will be
  several different conversion techniques
  available. The technique used will depend on the
  accuracy requirement of the user, I.e.,
  procedures developed for conversion of less
  accurate GIS/LIS products will be different than
  procedures developed for conversion of USGS NGVD
  29 published height values.

94
(No Transcript)
95
Review

• Vertical Datums
• Height Systems
• NAVD 88 Project
• NAVD 88 Implementation
• FEMA NAVD 88
• NAVD 88 Conversion Techniques

NATIONAL OCEAN SERVICE
96
The End!!!!!