Achieving Great Height – with GNSS is Achieving_Great_… · Achieving Great Height – with GNSS...
Transcript of Achieving Great Height – with GNSS is Achieving_Great_… · Achieving Great Height – with GNSS...
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Michael Dennis, RLS, PEOregon State University
National Geodetic Survey
Oregon GPS Users Group 2015 Annual MeetingJanuary 23, 2015 ● Salem, OregonSalem Convention Center
Achieving Great Height – with GNSS
The Fundamental Problem
What is “height”?
GNSS provides ellipsoid heights
• Purely geometric
• Not referenced to gravity
But we usually need physical heights
• Heights referenced to gravity
• E.g., to tell where water will go
heights matter!Physical
Heights are not merely fascinating… Heights are complicated!Heights to suit your every need (and mood)! Ellipsoid heights
• Referenced to different datums and datum realizations
Geoid heights• Gravimetric? “Hybrid”? Quasi‐geoid?
Physical heights• Orthometric heights
Helmert (NAVD 88), Niethammer, Mader, New Brunswick, etc.
• Dynamic heights• Normal heights• Normal orthometric heights (e.g., NGVD 29)• Leveled heights
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PB0630 ***********************************************************************PB0630 DESIGNATION - MARSHPB0630 PID - PB0630PB0630 STATE/COUNTY- OR/KLAMATHPB0630 COUNTRY - USPB0630 USGS QUAD - CHEMULT (1967)PB0630PB0630 *CURRENT SURVEY CONTROLPB0630 ______________________________________________________________________PB0630* NAD 83(1991) POSITION- 43 07 45.66845(N) 121 48 18.75991(W) ADJUSTED PB0630* NAVD 88 ORTHO HEIGHT - 1413.441 (meters) 4637.26 (feet) ADJUSTED PB0630 ______________________________________________________________________PB0630 LAPLACE CORR - 0.77 (seconds) DEFLEC12APB0630 GEOID HEIGHT - -21.65 (meters) GEOID12APB0630 DYNAMIC HEIGHT - 1412.660 (meters) 4634.70 (feet) COMPPB0630 MODELED GRAVITY - 980,018.2 (mgal) NAVD 88PB0630PB0630 HORZ ORDER - FIRSTPB0630 VERT ORDER - FIRST CLASS IIPB0630PB0630.The dynamic height is computed by dividing the NAVD 88PB0630.geopotential number by the normal gravity value computed on thePB0630.Geodetic Reference System of 1980 (GRS 80) ellipsoid at 45PB0630.degrees latitude (g = 980.6199 gals.).PB0630PB0630.The modeled gravity was interpolated from observed gravity values.
Physical heights: Orthometric and dynamicThe NGS Datasheet
= HD
= H
Ortho and dynamic heights not equal
H ≠ HD
e.g., 176.311 – 176.264 = 0.047 m
Rod 1
Rod 2
B
Backsight Foresight
F
Δn
SB
SSF
Setup of Leveling, Δn = B – F and S = SB + SF
Leveling is simple… right?
Leveled Height Differences
AC
B Topography
Physical heights related to gravity
**
g
CH
""gravityofvaluesome
energypotentialGravityheightPhysical
• C is the geopotential number [m2/s2]– Gravity potential energy relative to a reference potential– Reference gravity potential is usually the geoid
• H* is some type of physical height [m]– Type depends on type of gravity value used for g* [m/s2]
• Why?– To get unique andmeaningful heights
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No gravity, no height.Know gravity,
know height.
The ellipsoid, the geoid, and you
Earth surface
Orthometric height, H
Geoid height, NG
Ellipsoid height, h
Deflection of the vertical
Mean sea level
h ≈ H + NG
Note: Geoid height is negative everywhere in the coterminous US
h = H + NG
You are here
GEOID09 (CONUS) GEOID09 (CONUS)
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GEOID09 (CONUS) GEOID09 (CONUS)
“Geoid model? We don’t need no stinkin’ geoid model!”
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It doesn’t matter which geoid model you use
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GNSS Computation Flowchart
Baseφb, λb, hb
Base
Rover
Convertto ECEF
ConvertfromECEF
(user inputcoordinates)
φ, λ
hXb , Yb , Zb
X, Y, Zφ’, λ’
N, E
h’
“Map projection”
N’, E’
Datum transformation(if necessary)
Grid translation,rotation, & scaling
(not recommended)
H
“Geoid model”
H’Vertical shift, inclinedplanar correction, and/orother vertical transformation(often unavoidable)
Earth-Centered Earth-Fixed Cartesian Coordinates
Small residualsdo not guarantee
good results!
GNSS‐derived ortho height tips
Be consistent with NAD 83 and hybrid geoid• Use NAD 83 (2011) with GEOID12A• Use NAD 83 (2007) with GEOID09• Use NAD 83 control consistent with earlier models
Reference vertical CONTROL• Tie to published NGS vertical control stations
Leveled NAVD 88 (NOT VERTCON) For GNSS‐derived, ONLY use Height Mod
• Use relative accuracy of hybrid geoid (1‐2 ppm)• Avoid using geoid by itself (e.g., OPUS ortho heights)
GEOID12A accuracy in CONUS 4‐9 cm (0.1‐0.3 ft), 95% conf.
GNSS‐derived ortho height tips
Vertical calibration/localization: be careful! Single point calibration/localization:
• Check into other vertical control
Multiple point:• Small residuals do NOT guarantee good results!• Slope of correction surface < 5 ppm (1 arc‐sec)• If no geoid model, slope ≈ geoid slope
If > 100 ppm, something is probably wrong
Network adjustments• Depends on software used• Usually adjust ellipsoid and ortho heights separately
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What is “Height Modernization”?
Height Modernization is an NGS program• Improve access to accurate, reliable, consistent “heights”
• Focus usually on GNSS‐derived orthometric heights
Done with campaign‐style GNSS surveys
Occupy bench marks with accurate leveled ortho heights
Perform network adjustment to get ortho heights
Constrained to leveled heights using hybrid geoid model
• Height Mod more than just GNSS‐derived ortho heights
Also includes leveling projects, GRAV‐D Project, and research
Additional information and associated documents• Height Mod website: www.ngs.noaa.gov/heightmod/
• Guidelines for GNSS‐derived ellipsoid and orthometric heights
What is a “Height Mod” survey?
A way to get accurate “elevations” with GNSS• “Elevations” are orthometric heights (usually NAVD 88)
• Requires ties to leveled bench marks (at least indirectly)
Over 13,000 “Height Mod” stations in NGS database• Stations with GNSS‐derived ortho heights published to cm
Determined in accordance with NGS 58 and 59 guidelines
• A leveled bench mark is not considered a Height Mod mark
Plus ~26,000 GPS bench marks (for GEOID12A)• An essential part of Height Mod surveys
• Typically tied as part of Height Mod surveys
• Required to make “hybrid” geoid (NAVD 88 datum surface)
“Height Mod” stations “Height Mod” stations
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“Height Mod” stations Height Modernization originsHeight Mod Study Report to Congress (June 1998) GPS a viable means to determine
accurate heights (to 2 cm) Cost savings from 25% to > 90%
when compared to conventional methods
Cost savings greatest for distances > 4 km between control marks
Estimated benefits > $12 billion A reliable, consistent vertical datum
is essential (including an accurate hybrid geoid model)
Guidelines are needed to promote best practices and ensure high‐quality results
Height Modernization guidelines
NOS NGS 58GPS‐derived ellipsoidheights(November 1997)
NOS NGS 59GPS‐derived orthometricheights(March 2008)
Factors to evaluate include:• Distance to passive & active control
• Number of simultaneous GPS occupations
• Occupation duration and data logging rate
• Number of multiple occupations and time between them
• Strategies and software for processing and adjustment
• Effects of multipath, signal propagation, antenna calibrations• Viability of real‐time data (both single‐base and network)
• Effect of GLONASS observables on results
• Impact of network design (including “trivial” vectors)
Objective: Improve speed and reliability of obtaining orthometric heights with GNSS
A new & improved NGS 58/59
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2013 project (Florida and South Carolina)• Fieldwork
Gustin, Cothern & Tucker, Inc. (GCT) and NGS
South Carolina Geodetic Survey (SCGS)
• Data Analysis: Ohio State University (OSU)
• All data collected, organized, and submitted for analysis• Two phases
Phase 1: Vertical Precision (Florida)
Phase 2: GPS Bench Marks (South Carolina)
2014 project (Oregon)• Data collection and analysis by Oregon State University
• Fieldwork completed, analysis underway
Two NGS‐58/59 update projects How “Height Mod” surveys performed
Need identified for vertical control in an area
Perform research on available NAVD 88 control
Must have ties to valid vertical control• Published to nearest mm or cm (usually leveled)
• Can use existing Height Mod stations as vertical control
• VERTCON‐ and GNSS‐derived airport control typically NOT valid
Perform in accordance with NGS 58/59 guidelines• Three 5‐hr occupations for baselines > ~20 km
Minimum 30‐minute sessions permitted for shorter baselines
• Simultaneous (session) GNSS processing required (sort of)
• Vector ties between adjacent stations required (also sort of)
• For “Bluebooking” must use ADJUST for network adjustment
Opinions about “Bluebooking” vary…
Friends …don’t let friends …
Bluebook
I’d rather beWhat is “Bluebooking” anyway?
The NGS “Bluebook” is a set of data formats• For electronic transfer of geodetic data to NGS database
• Defined by NGS through Federal Geographic Data Committee
• So “Bluebooking” is just putting data into specified formats
A means for our customers to help build the NSRS• Users get the control they want, where they want it
• NGS gets data for small fraction of total cost of performing work
• Great example of Fed, local gov’t, and private cooperation
“Bluebooking” now viewed as much more than formats• Standards & specifications for performing control work
• Helps ensure high‐quality data and results
• But needs to be streamlined to improve efficiency, reduce cost
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The Bluebooking Path
Requires specific software• ADJUST program suite REQUIRED
• WinDesc program REQUIRED to create and descriptions
• No specific GNSS baseline processing software required• GIS GPS Toolset: Display and analysis of ADJUST results
Run using Esri ArcGIS software (ArcMap and ArcToolbox)
OPUS‐Projects (OP)• Performs GPS baseline processing online (using PAGES)• Creates the files used by ADJUST• Goal is for OPUS‐Projects to make overall Bluebooking simpler
Longer term goal is to “Re‐invent” Bluebooking (by 2018)• In interim will continue to refine and streamline existing process
National Height Mod vertical adjustment
2011 national adjustment of passive control• Orthometric heights (“elevations”) NOT determined
• GNSS‐derived ortho heights based on mix of NAD 83 and geoid models
• More consistent if all are based on NAD 83(2011) and GEOID12A
Nationwide “Height Mod” vertical adjustment• NAD 83(2011) ellipsoid heights, GEOID12A, and national GNSS network
• Constrain to leveled NAVD 88 bench marks
• Determine GNSS‐derived NAVD 88 heights on non‐leveled marks
• Estimate accuracies on GNSS‐derived ortho heights
• Results will be used as part of study to update/replace NGS 58/59
• Likely will greatly increase number of Height Mod stations
• Will require significant analysis and computation
Similar project performed in Arizona (completed in 2010)• Used GEOID09 with NAD 83(2007) ellipsoid heights
• Significant changes in ortho heights; more consistent results statewide
Results Results
10 cm
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Height Mod adjustment sequence
Typically 4 adjustments performed1. Minimally constrained (“free”) geometric
Constrain single mark 3‐D
Internal integrity (mainly vector residuals)
2. Geometrically (“horizontally”) constrained Constrain multiple mark 3‐D, 2‐D, or 1‐D (ellipsoid ht)
3. Minimally constrained (“free”) vertical Constrain orthometric height for single mark
Hybrid geoid model used to transfer ortho heights
4. Vertically constrained Constrain vertical control 1‐D (orthometric height)
Why do OP and ADJUST results differ?
OP does not adjust both ellipsoid and ortho heights• OP always uses relationship h = H + N
OP uses different sigmas on constraints• Sigmas in XYZ rotated to NEU without covariances• Passive mark sigmas from OPUS peak‐to‐peak
ADJUST scales horizontal and vertical errors separately ADJUST multiplies final sigmas by std dev unit weight
• Not sure if OP does or not…
OP adjusted observables are double differences• ADJUST adjusted observables are GPS vector components• But should this cause a difference?
Additional things to think about
Where is NGS going with all this? How do you analyze OPUS‐Projects results?
• There are no vector residuals to analyze• There are no vectors to reject
Dependent (“trivial”) vectors Impact of network design on OP results Impact of OP session processing constraints on gfile
Bluebooking with commercial GNSS baseline processing software
Wrap‐up
Height Modernization Program• Enables efficiently obtaining accurate heights
• Combinws GNSS, geoid modeling, and traditional leveling
Height Modernization surveys• Augment and densify height control with user‐contributed data
• Refine and streamline submittal process (“re‐invent” Bluebooking)
New height‐determination standards (“guidelines”)• Update and replace existing NGS 58/59 with modern standards
• Analsys currently underway on two projects
A national Height Mod vertical adjustment• Obtain consistent GNSS‐derived ortho heights nationwide
Objective: Move Height Mod into the future…
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NAVD 88 to new vertical datumEstimated as NAVD 88 "zero" (datum) surface minus NGS gravimetric geoid
Orthometric height change (meters)
Questions?Achieving Great Height
– with GNSS