GPS-Derived Heights Part 1 Development and Description of NGS Guidelines

1
GPS-Derived HeightsPart 1Development and
Description of NGS Guidelines
2
To understand how to achieve GPS-derived
orthometric heights at centimeter-level accuracy,
three questions must be answered

• 1) What types of heights are involved?
• Orthometric heights
• Ellipsoid heights
• Geoid heights
• 2) How are these heights defined and related?
• 3) How accurately can these heights be
  determined?

3
Leveled Height Differences
B
Topography
A
C
4
Level Surfaces and Orthometric Heights
Earths
Surface
WP
Level Surfaces
P
Plumb
Line
Mean
Geoid
Sea
Level
WO
PO
Level Surface Equipotential Surface (W)
Ocean
Geopotential Number (CP) WP -WO
H (Orthometric Height) Distance along plumb
line (PO to P)
5
Heights Based on Geopotential Number (C)

• Normal Height (NGVD 29) H C / ?
• ? Average normal gravity along plumb line
• Dynamic Height (IGLD 55, 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 H0)
• g Surface gravity measurement (mgals)

6
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.
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Global Positioning System
27 Satellites 6 Planes, 55 Rotation 4/5
Satellites /Plane 20,183 km Orbit 1 Revolution
/12 Hrs
8
Z
Zero Meridian
-X
-Y
Y
X
Mean Equatorial Plane
-Z
9
Earth-Centered-Earth-Fixed Coordinates
Conventional
Z Axis
Terrestrial
Pole
(X,Y,Z)
P
Earths
Surface
Zero
Meridian
Z
Origin
Y Axis
(0,0,0)
Center of Mass
X
Y
Mean Equatorial Plane
X Axis
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The Ellipsoid
a Semi major axis
N
b Semi minor axis
f a-b Flattening a
b
a
Geodetic Reference System 1980
S
a 6,378,137.000 meters (semi-major axis)
b 6,356,752.3141403 m (semi-minor axis)
1/f 298.25722210088 (flattening)
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GPS - Derived Ellipsoid Heights
Z Axis
(X,Y,Z) P (?,?,h)
P
h
Earths
Surface
Zero
Meridian
Reference Ellipsoid
Y Axis
?
?
Mean Equatorial Plane
X Axis
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Ellipsoid, Geoid, and Orthometric Heights
h H N
Earths
Surface
P
Plumb Line
Ellipsoid
h
Q
N
Mean
Sea
Geoid
Level
PO
Ocean
h (Ellipsoid Height) Distance along ellipsoid
normal (Q to P)
N (Geoid Height) Distance along ellipsoid
normal (Q to PO)
H (Orthometric Height) Distance along plumb
line (PO to P)
13
Ellipsoid Heights (NAD 83 vs. ITRF 97)

• NAD 83 Origin and ellipsoid (GRS-80)
• a 6,378,137.000 meters (semi-major axis)
• 1/f 298.25722210088 (flattening)
• ITRF 97 Origin (best estimate of earths C.O.M.)
• NAD 83 is non-geocentric relative to ITRF97
  origin by 1 - 2 meters
• ITRF 97 ellipsoid heights Use a NAD 83 shaped
  ellipsoid centered at the ITRF97 origin
• Ellipsoid height differences between NAD 83 and
  ITRF97 reflect the non-geocentricity of NAD 83

14
Simplified Concept of ITRF 97 vs. NAD 83
h83
h97
Earths
Surface
ITRF 97
Origin
2.2 meters
NAD 83
Identically shaped ellipsoids (GRS-80) a
6,378,137.000 meters (semi-major axis) 1/f
298.25722210088 (flattening)
Origin
15
NAD83(86) to ITRF97(97) Ellipsoid Heights
(meters)
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High Resolution Geoid Models

• G96SSS
• 1.8 million gravity measurements (marine, land,
  altimetry)
• 30 second DTED updated with Canadian Rockies data
• Earth Gravity Model of 1996 (EGM96)
• 2 min x 2 min spacing
• International Terrestrial Reference Frame ITRF94
  (1996.0)
• GEOID96
• Begin with G96SSS model
• 2951 GPS/Level Bench Marks (NAD83/NAVD88)
• Converts NAD83 (86) into NAVD 88
• Relative to non-geocentric GRS-80 ellipsoid

17
High Resolution Geoid ModelsG99SSS (Scientific
Model)

• 2.6 million terrestrial, ship, and altimetric
  gravity measurements
• 30 arc second Digital Elevation Data
• 3 arc second DEM for the Northwest USA
• Decimated from 1 arc second NGSDEM99
• Earth Gravity Model of 1996 (EGM96)
• Computed on 1 x 1 arc minute grid spacing
• GRS-80 ellipsoid centered at ITRF97 origin

18
NGSDEM99 is a 1 x 1 arc-second Digital Elevation
Model (DEM) of the Northwest United States,
covering the region 39 - 49N latitude, and 234 -
265E longitude.
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High Resolution Geoid ModelsGEOID99

• Begin with G99SSS model
• 6169 NAD83 GPS heights on NAVD88 leveled
  benchmarks
• Determine national bias and trend relative to
  GPS/BMs
• Create grid to model local (state-wide) remaining
  differences
• ITRF97/NAD83 transformation
• Compute and remove conversion surface from G99SSS
• Relative to non-geocentric GRS-80 ellipsoid
• 4.6 cm RMS nationally when compared to BM data
• RMS ?16 improvement over GEOID96

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GEOID99
For the conterminous United States (CONUS),
GEOID99 heights range from a low of -50.97
meters (magenta) in the Atlantic Ocean to a high
of 3.23 meters (red) in the Labrador Strait.
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Tidal Datums

• Heights Measured Above Local Mean Sea Level
• National Tidal Datum epoch 19 year series
• Encompasses all significant tidal periods
  including 18.6 year period for regression of
  Moons nodes
• Averages out nearly all meteorological,
  hydrological, and oceanographic variability
• Leveling is used to determine relationship
  between bench marks and tidal gauges

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Importance of Shoreline
AL, AK, CA, CT, FL, GA, LA, MD, MS, NJ, NY, NC,
OR, RI, SC, WA
Territorial Seas
Privately Owned Uplands
State Owned Tidelands
Contiguous Zone
Exclusive Economic Zone
State Submerged Lands
Federal Submerged Lands
High Seas
3 n. mi.
12 n. mi.
MHHW
200 n. mi.
MHW
MLLW
Chart Datum
Privately Owned
State Owned
Privately Owned
State Owned
TX
DE, MA, ME, NH, PA, VA
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National Geodetic Vertical Datum 1929(NGVD 29)

• Defined by heights of 26 tidal stations in U.S.
  and Canada
• Tide gages were connected to the network by
  leveling from tide gage staffs to bench marks
• Water-level transfers used to connect leveling
  across Great Lakes
• Normal Orthometric Heights
• H C / ?
• C model (normal) geopotential number
• ? from normal gravity formula
• H 0 level is NOT a level surface

27
First-Order Leveling Network NGVD 29
28
North American Vertical Datum 1988(NAVD 88)

• Defined by one height (Father Point/Rimouski)
• Water-level transfers connect leveling across
  Great Lakes
• Adjustment performed in Geopotential Numbers
• Helmert Orthometric Heights
• H C / (g 0.0424 H0)
• C geopotential number
• g surface gravity measurement (mgals)
• H0 approximate orthometric height (km)
• H 0 level is nearly a level surface
• H 0 level is biased relative to global mean sea
  level

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Vertical Control Network NAVD 88
30
NGVD 29 Versus NAVD 88

• Datum Considerations NGVD 29
  NAVD 88
• Defining Height(s) 26 Local MSL
  1 Local MSL
• Tidal Epoch Various 1960-78
• 
  (18.6 years)

• Treatment of Leveling Data
• Gravity Correction Ortho Correction
  Geopotential Nos.
• (normal gravity) (observed
  gravity)
• Other Corrections Level, Rod, Temp.
  Level, Rod, Astro,
• Temp, Magnetic,
• and Refraction
• 
  

31
NGVD 29 Versus NAVD 88 (continued)

• Adjustments Considerations NGVD 29 NAVD
  88
• Method Least-squares
  Least-squares
• Technique Condition Eq.
  Observation Eq.
• Units of Measure Meters
  Geopotential Units
• Observation Type Links Between
  Height Differences
• Junction Points
  Between Adjacent BMs

32
NGVD 29 Versus NAVD 88 (continued)

• Adjustments Statistics NGVD 29
  NAVD 88
• No. of Bench Marks 100,000 (est)
  450,000 (US only)
• Km of Leveling Data 75,159 (US)
  1,001,500
• 31,565 (Canada)

• Published Information
• Orthometric Height Type Normal Helmert
  
• Orthometric Height Units Meters
  Meters
• Gravity Value Normal Actual

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Height Differences Between NAVD 88 and NGVD 29
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Expected Accuracies

• GPS-Derived Ellipsoid Heights
• 2 centimeters
• Geoid Heights (GEOID99)
• 2.5 cm correlated error (randomizing at 40 km)
• Relative differences typically less than 1 cm in
  10 km
• 4.6 cm RMS about the mean
• Leveling-Derived Heights
• Less than 1 cm in 10 km for third-order leveling

36
GPS-Derived Ellipsoid HeightGuidelines

• GPS related error sources
• Pilot projects were used to develop guidelines
• NOAA Technical Memorandum NOS NGS-58

37
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

38
GPS Error Sources

• Precise or broadcast orbit error
• Satellite relationship center-of-mass to L1
  antenna phase center
• Satellite clock error (nominal)
• SA dither (minimized)
• Satellite inter-channel bias
• L1-L2 antenna phase center offset
• Transmission multipath
• Ionospheric effects
• Tropospheric effects
• Dry (hydrostatic) troposphere delay
• Wet troposphere delay

• Multipath
• Antenna phase center variation
• Circular polarization
• L1-L2 phase center offset
• Receiver clock offset
• Receiver inter-channel bias
• Height of phase center above mark
• Marker stability
• Earth tides - direct effect
• Ocean tide loading
• Atmospheric loading
• Crustal motion

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The images compare the accuracy of GPS with and
without selective availability (SA). Each plot
shows the positional scatter of 24 hours of data
(0000 to 2359 UTC) taken at one of the
Continuously Operating Reference Stations (CORS)
operated by the NCAD Corp. at Erlanger, Kentucky.
On May 2, 2000, SA was set to zero. The plots
show that SA causes 95 of the points to fall
within a radius of 45.0 meters. Without SA, 95
of the points fall within a radius of 6.3 meters.
As illustration, consider a football stadium.
With SA activated, you really only know if you
are on the field or in the stands at that
football stadium with SA switched off, you know
which yard marker you are standing on.
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Atmospheric Error Sources

• Ionosphere
• Greatest at 1400 (local time)
• Typical 5 to 15 m at zenith
• Extreme 0.15 to 50 m at zenith
• Higher frequencies have less effect
• Error correction by dual frequency
• Wet Troposphere
• 10 of total effect
• Model accuracy only 10 to 50
• Need humidity along path
• About 20 cm at zenith

• Hydrostatic (Dry) Troposphere
• 90 of total effect
• Model accuracy only 2 to 5
• Need surface atmospheric pressure and
  temperatures
• Accurate pressure is critical
• About 2.2 m at zenith

43
Signal Multipath

• Satellite signal arriving at receiver via
  multiple paths due to reflection (Leick 1995)
• Quasi-periodic signal 5 to 50 minutes
• Maximum multipath is a fraction of wavelength
• (L1 19 cm L2 24 cm) typically 2 cm to 5 cm
• Geometric relationship between satellite,
  antenna, and surroundings
• Same pattern in same satellite geometry on
  consecutive days produces similar results
  similar effects

44
h
ø
ø
Figure 1 Multipath Description
August 1987 -Ionospheric refraction and Multipath
Effects in GPS Carrier Phase Observations Yola
Georgiadou and Alfred Kleusberg
IUGG XIX General
Assembly Meeting, Vancouver, Canada
45
Equipment Requirements

• Dual-frequency, full-wavelength GPS receivers
• Required for all observations greater than 10 km
• Preferred type for 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

46
SV 20
SV 20
SV 14
SV 14
Different Phase Patterns
Note that SV elevation and varying phase patterns
affect signal interpretation differently
Antenna Type A
Antenna Type B
47
Analyses of Data from Pilot Projects

• Northridge Earthquake Project 1994
• GPS on leveling-derived bench marks
• Two 3-hour sessions
• On different days
• Different times of day
• Provided 2 cm results for short lines, i.e. 5 to
  10 km

48
Northridge Earthquake Project
49
Analyses of Data from Pilot Projects

• Harris-Galveston Coastal Subsidence Districts
  CORS and PAMs
• 7 stations in a 25 km radius collecting data 24
  hours a day for 2 years
• Various length baselines
• 24, 6, 3, 2 and 1 hour solutions
• 20 minute to epoch-by-epoch solutions
• Real-life influences due to multipath,
  atmosphere, and satellite geometry

50
Harris-Galveston Coastal Subsidence District
51
Port-a-Measure PAM
52
Std. Dev. (0.91 cm)
53
24 - Hour Solutions
Day 300 -5.15
Day 301 -5.95
Day 302 -5.70
Day 303 -5.97
Day 304 -5.80
Mean -3.1
Day 302
? 0.5
Day 300
Mean -7.5
? 0.3
Day 304
Day 301
Day 303
54
Day 130 Mean (1.33 cm) / Std. Dev. (0.83 cm)
Day 131 Mean (1.01 cm) / Std. Dev. (0.47 cm)
1.3
0.9
0.8
0.6
55
Day 130 Mean (1.33 cm) / Std. Dev. (1.15 cm)
Day 131 Mean (1.03 cm) / Std. Dev. (0.58 cm)
2.0
1.2
0.5
0.7
56
Day 130 Mean (1.02 cm) / Std. Dev. (1.54 cm)
Day 131 Mean (1.05 cm) / Std. Dev. (0.96 cm)
2.0
1.6
0.2
0.3
57
Results from Pilot Project

• 24 hour solutions of data taken during bad
  atmospheric conditions may not always provide 2
  cm results
• 1, 2, 3, and 6 hour solutions will repeat very
  well from day to day when observations are
  collected at about the same time on different
  days, but may produce significantly different
  results using data collected during different
  times of the days, i.e. having significantly
  different satellite geometry
• Increasing the elevation cut-off angle will
  decrease the effects due to multipath, but it
  will also decrease the number of available
  satellites which may significantly decrease
  accuracy of short observing sessions

58
Analyses of Data from Pilot Projects

• FGCS 48 Hour Pseudo-Kinematic Network,
  Gaithersburg, MD (June 13 - 15, 1995)
• 12 stations occupied in network
• Two TCORS and one rover
• 10 minute observing sessions at each site
  continuously over 48 hour time span
• 10 different occupations at each site
• Baselines ranged from 100 meters to 26.1 km

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FGCS 48 Hour Pseudo-Kinematic Network
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Distances to Stations from TCORS
61
F

F
H
F


F
H

F - Could not fix integers - (FLOAT solution) -
Not included in statistics
H - RMS value greater than 1.5
Summary
- Difference greater than 5 cm
FLOAT Solutions - Large Residuals
High RMS Values - Large Residuals
62
Analyses of Data from Pilot Projects

• FGCS 24 Hour Pseudo-Kinematic Network,
  Gaithersburg, MD (November 28 - 29, 1995)
• 12 stations occupied in network
• Two TCORS and two rovers
• Simultaneous 10 minute observing sessions between
  rovers continuously over 24 hour time span
• 8 different occupations each site
• Baselines ranged from 100 meters to 26.1 km

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FGCS 24 Hour Pseudo-Kinematic Network
64
H
H
H
H
H
F
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
F
H
H
H
H
H
H
F
H
H
H
H
H
H
F
H
H
H
H
H
H
F - Could not fix integers - (FLOAT solution) -
Not included in statistics
H - RMS value greater than 1.5

Bad Weather

High RMS Values
65
Results from Pilot Project

• Base lines with high RMS produced outliers
• Base lines where integers could not be fixed
  produced outliers
• Standard deviation of a single 10 minute
  occupation during good atmospheric conditions
  was 2.1 cm
• Standard deviation of the mean of two 10 minute
  occupations during good atmospheric conditions
  was 1.4 cm
• Standard deviation of a single 10 minute
  occupation during bad atmospheric conditions
  was larger than a single 10 minute occupation
  obtained during good atmospheric conditions,
  i.e. 3.0 cm versus 2.1 cm

66
Analyses of Data from Pilot Projects

• San Francisco Bay Demonstration Project
• Test of guidelines and modifications based on
  results
• Phase I Static survey test for 2 cm accuracy
• Mixture of base line lengths 2 to 50 km
• Long observing sessions minimum of 3 hours
• Phase II Kinematic survey test for 5 cm accuracy
• Short base line lengths less than 5 km
• Short observing sessions 15 minutes
• Real-world conditions, i.e., traffic, trees,
  buildings

67
San Francisco Bay Demonstration Project
R 1393
MOLATE
941 4873 TIDAL 17
BRIONE
941 4863 TIDAL 5
RV 223
CORS
PT. BLUNT
YACHT
941 4819 TIDAL 32
GPS Site
941 4290 N
0 5 10
941 4779 ASFB
PORT 1
KM
S 1320
941 4750 TIDAL 7
CHABOT
M 554
U 1320
M 148
N 1197
L 1241
WINTON
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Two Days/Same Time
-10.254
gt -10.253
-10.251
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
71
Two Days/Same Time
20.660
gt 20.661
20.662
Difference -0.2 cm
Truth 20.615
Difference 4.6 cm
Two Days/Different Times
20.660
gt 20.637
20.614
Difference 4.6 cm
Truth 20.615
Difference 2.3 cm
72
Two Days/Different Times
-9.184
gt -9.185
-9.185
Difference 0.1 cm
Truth -9.218
Difference 3.3 cm
Need a Network!
Line is greater than 10 km
73
Results from Pilot Project

• Base lines with high RMS produced outliers
• Base lines where integers could not be fixed
  produced outliers
• Standard deviation of a 30 minute session for
  short lines, i.e. less than 10 km, was 1.2 cm
• Standard deviation of the mean of two 30 minute
  occupations on two different days and different
  times of day was 0.8 cm
• Standard deviation of a 10 minute session for
  short lines, i.e. less than 10 km, was 1.7 cm
• Standard deviation of the mean of two 10 minute
  occupations on two different days and different
  times of day was 1.1 cm

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Establish / Monitor Project Control
CORS
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Precision With CORS

• How GPS positioning is affected by baseline
  length
• Minimum occupation time required to meet
  established specifications
• Twice the rms _at_ 2 cm for horizontal components
• Twice the rms _at_ 4 cm for vertical components
• Twice the rms being approximately equal to a 95
  confidence region
• Varying length baselines formed from 19 CORS
• 10 days data from each site various session
  lengths

77
Design of Research
Sessions Processed

• 6 - 4 hour sessions ? 10 days ? 12 baselines
  720
• 4 - 6
  480
• 3 - 8
  360
• 2 - 12
  240
• 1 -24
  120
• (Total of Sessions) 1920

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Processing

• One station constrained
• Second station computed
• ITRF97 (X, Y, and Z) _at_ epoch January 1, 1999
• X, Y, and Z position of second station is source
  of results
• Automatic integer fixing (on)
• Tropospheric model (on)
• Antenna phase patterns (ant_info.001)
• Precise ephemerides (NGS ephemeredes)

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Time Scatter Plots (Horizontal)
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Multiple Occupation Estimates
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Comments About Results With CORS Data

• Majority of time you are less than 300 km from
  CORS (continental U.S.)
• Baseline length has little effect on positional
  accuracy
• No setup error or antenna measurement blunders
• lt 300 kilometers
• Using NGS PAGES software
• Precise ephemeris, Tropo models, and antenna
  patterns
• Horizontal and vertical specifications can be met
  in one 4-hour session

88
Recommendations to GuidelinesBased on These Tests

• Must repeat base lines
• Different days
• Different times of day
• Detect, remove, reduce effects due to multipath
  and having almost the same satellite geometry
• Must FIX integers
• Base lines must have low RMS values, i.e., lt 1.5
  cm

89
Available On-Line at the NGS Web
Site www.ngs.noaa.gov
90
Station Selection and Reconnaissance

• Assure accurate connections to control stations
• NGS approved CORS
• TCORS (temporary or project CORS)
• HPGN / HARN
• Federal Base Network (FBN)
• Cooperative Base Network (CBN)
• User Densified Network (UDN)
• NAVD 88 Bench Marks
• NGS Database and data sheets
• Identify GPS-usable stations

91
NGS Internet Page
www.ngs.noaa.gov
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Primary or SecondaryStation Selection Criteria

• 1. HPGN / HARN either FBN or CBN
• Level ties to A or B stability bench marks during
  this project
• 2. Bench marks of A or B stability quality
• Or HPGN / HARN previously tied to A or B
  stability BMs
• 3. UDN stations
• Level ties to A or B stability bench marks during
  this project
• 4. Bench marks of C stability quality
• Special guidelines for areas of subsidence or
  uplift

99
Four Basic Control Requirements

• BCR-1 Occupy stations with known NAVD 88
  orthometric heights
• Stations should be evenly distributed throughout
  project
• BCR-2 Project areas less than 20 km on a side,
  surround project with NAVD 88 bench marks
• i.e., minimum number of stations is four one in
  each corner of project
• BCR-3 Project areas greater than 20 km on a
  side, keep distances between GPS-occupied NAVD 88
  bench marks to less than 20 km
• BCR-4 Projects located in mountainous regions,
  occupy bench marks at base and summit of
  mountains, even if distance is less than 20 km

100
Obstruction Visibility Diagram
101
Equipment Requirements

• Dual-frequency, full-wavelength GPS receivers
• Required for all observations greater than 10 km
• Preferred type for 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

102
http//www.grdl/GRD/GPS/Projects/ANTCAL/index.shtm
l
103
Trimble Geodetic L1/L2 Antenna (TRM 22020.00)
Ashtech Geodetic III Antenna U.S.C.G. V Antenna
(ASH 700829.A1)
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Data Collection Parameters

• VDOP lt 6 for 90 or longer of 30 minute session
• Shorter session lengths stay lt 6 always
• Schedule travel during periods of higher VDOP
• Session lengths gt 30 minutes collect 15 second
  data
• Session lengths lt 30 minutes collect 5 second
  data
• Track satellites down to 10 elevation angle

109
Meteorological Data

• Weather data must be collected at control,
  primary, and secondary base stations at height of
  antenna PC
• Wet and dry temperatures, atmospheric pressure
• Sessions gt 2 hrs record beginning, midpoint,
  ending
• Sessions lt 2 hrs gt 30 min record beginning and
  ending
• Sessions lt 30 min record at midpoint
• Note on obs log where recorded and unusual
  conditions
• Stabilize equipment to ambient conditions
• Check equipment prior to observations

110
Antenna Setup

• Fixed-height tripods required for 2 cm standard
• Shade plumbing bubbles at least 3 min prior to
  plumbing
• Check perpendicularity of poles at beginning of
  project
• Fixed-height poles preferred for 5 cm standard
• Alternate tripod setups antenna heights MUST be
  measured carefully and accurately
• Check measuring system before project
• Check and adjust tribrachs at beginning of
  project
• Perform totally independent meter and feet
  measurements
• Have measurement computations checked by someone
  else

111
Fixed Height Tripod
112
Slip-leg Tripod and Slant Height Measurement
113
Table 1. -- Summary of Guidelines
114
Appendix B. - - GPS Ellipsoid Height Hierarchy
HARN/Control Stations (75 km) Primary Base (40
km) Secondary Base (15 km) Local Network
Stations (7 to 10 km)
115
HARN/Control Stations
CS1
75 km
CS3
CS2
116
Primary Base Stations
CS1
PB1
PB2
40 km
PB3
CS3
CS2
117
Primary Base Stations

• Basic Requirements
• 5 Hour Sessions / 3 Days
• Spacing between PBS cannot exceed 40 km
• Each PBS must be connected to at least its
  nearest PBS neighbor and nearest control station
• PBS must be traceable back to 2 control stations
  along independent paths i.e., base lines PB1 -
  CS1 and PB1 - PB2 plus PB2 - CS2, or PB1 - CS1
  and PB1 - PB3 plus PB3 - CS3

118
Secondary Base Stations
CS1
PB1
SB1
SB2
15 km
SB4
SB3
PB2
PB3
CS3
CS2
119
Secondary Base Stations

• Basic Requirements
• 30 Minute Sessions / 2 Days /Different times of
  day
• Spacing between SBS (or between primary and SBS)
  cannot exceed 15 km
• All base stations (primary and secondary) must
  be connected to at least its 2 nearest primary or
  secondary base station neighbors
• SBS must be traceable back to 2 PBS along
  independent paths i.e., base lines SB1 - PB1
  and SB1 - SB3 plus SB3 - PB2, or SB1 - PB1 and
  SB1 - SB4 plus SB4 - PB3
• SBS need not be established in surveys of small
  area extent

120
Local Network Stations
CS1
PB1
LN1
SB2
SB1
LN3
LN2
LN5
LN4
7 km
LN6
LN7
SB4
SB3
PB2
PB3
CS3
CS2
121
Local Network Stations

• Basic Requirements
• 30 Minute Sessions / 2 Days / Different times of
  the day
• Spacing between LNS (or between base stations and
  local network stations) cannot exceed 10 km
• All LNS must be connected to at least its two
  nearest neighbors
• LNS must be traceable back to 2 primary base
  stations along independent paths i.e., base
  lines LN1 - PB1 and LN1 - LN2 plus LN2 - SB1 plus
  SB1 - SB3 plus SB3 - PB2, or LN1 - PB1 and LN1 -
  LN3 plus LN3 - SB2 plus SB2 - SB4 plus SB4 - PB3

122
Sample Project Showing Connections
CS2
CS1
LN4
LN3
LN1
LN2
PB2
PB1
SB2
LN5
SB1
SB3
SB5
SB4
PB4
PB3
CS3
CS4
123
Sample Project
124
Project Information

• 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

125
GPS-Usable Stations
3820N
CORS HARN NAVD88 BM New Station
Spacing Station
Primary Base Station
LATITUDE
8.2km
3750N
12235W
12140W
LONGITUDE
126
Primary Base Stations
3820N
CORS HARN NAVD88 BM New Station
D191
10CC
19.0km
Primary Base Station
28.7km
25.7km
LATITUDE
38.3km
31.6km
38.7km
25.8km
LAKE
MART
29.6km
MOLA
3750N
12235W
12140W
LONGITUDE
127
East Bay Project Points
3816N
CORS HARN NAVD88 BM New Station Spacing Station
D191
TIDD
10LC
X469
Primary Base Station
MONT
Z190
DROU
BM20
Q555
LATITUDE
04KU
CATT
TOLA
TIDE
5144
ZINC
8.2km
PT14
R100
P371
04HK
LAKE
MART
3755N
12140W
12220W
LONGITUDE
128
Observation Sessions
3816N
Session F
Session E
CORS HARN NAVD88 BM New Station Spacing Station
Session D
Primary Base Station
Session G
LATITUDE
Session A
Session C
Session B
3755N
12140W
12220W
LONGITUDE
129
Independent Base Lines
3816N
CORS HARN NAVD88 BM New Station Spacing Station
Primary Base Station
LATITUDE
8.2km
3755N
12140W
12220W
LONGITUDE
130
Observation Schedule
131
Field Observations

• Observation logs
• Record complete receiver/antenna manufacturer,
  model part number, and serial numbers
• Record meteorological data and unusual conditions
• Record station and observer information
• Record height of antenna and measurement
  computations
• Obtain a clear station rubbing
• Rubbing for each occupation of station
• Make complete plan sketch of mark when rubbing
  not feasible

132
Sample Observation Log (Front Side)
133
Sample Observation Log (Back Side)
134
Sample Station Rubbing
135
Basic Concept of Guidelines

• Stations in local 3-dimensional network connected
  to NSRS to at least 5 cm uncertainty
• Stations within a local 3-dimensional network
  connected to each other to at least 2 cm
  uncertainty
• Stations established following guidelines are
  published to centimeters by NGS

136
Network / Local Accuracy
137
Points of Contact

• National Geodetic Survey NOAA, N/NGS12
  Geodetic Services Division
  Bldg. SSMC3, Station 9202
  1315 East-West Highway
  Silver Spring, MD 20910-3282
  Phone [phone] Fax
  301-713-4171
• Internet Web Site
• www.ngs.noaa.gov

• Curtis L. Smith Phone
  208-332-7197 E-mail
  Curt.Smith_at_noaa.gov