Achieving Great Height with GNSS

Transcription

1 Achieving Great Height with GNSS Oregon GPS Users Group 2015 Annual Meeting January 23, 2015 Salem, Oregon Salem Convention Center Michael Dennis, RLS, PE Oregon State University National Geodetic Survey 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 are not merely fascinating Physical heights matter! 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 1

2 Physical heights: Orthometric and dynamic The NGS Datasheet PB0630 *********************************************************************** PB0630 DESIGNATION - MARSH PB0630 PID - PB0630 PB0630 STATE/COUNTY- OR/KLAMATH PB0630 COUNTRY - US PB0630 USGS QUAD - CHEMULT (1967) PB0630 Ortho and dynamic heights not equal H H D e.g., = m PB0630 *CURRENT SURVEY CONTROL PB0630 PB0630* NAD 83(1991) POSITION (N) (W) ADJUSTED PB0630* NAVD 88 ORTHO HEIGHT (meters) (feet) ADJUSTED = H PB0630 PB0630 LAPLACE CORR (seconds) DEFLEC12A PB0630 GEOID HEIGHT (meters) GEOID12A PB0630 DYNAMIC HEIGHT (meters) (feet) COMP = H D PB0630 MODELED GRAVITY - 980,018.2 (mgal) NAVD 88 PB0630 PB0630 HORZ ORDER - FIRST PB0630 VERT ORDER - FIRST CLASS II PB0630 PB0630.The dynamic height is computed by dividing the NAVD 88 PB0630.geopotential number by the normal gravity value computed on the PB0630.Geodetic Reference System of 1980 (GRS 80) ellipsoid at 45 PB0630.degrees latitude (g = gals.). PB0630 PB0630.The modeled gravity was interpolated from observed gravity values. Leveling is simple right? Rod 1 Setup of Leveling, Δn = B F and S = S B + S F Rod 2 Backsight Foresight B F Δn S B S F S Leveled Height Differences B Topography A C Physical heights related to gravity Gravity potential energy Physical height some value of " gravity " * H C * g C is the geopotential number [m 2 /s 2 ] 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/s 2 ] Why? To get unique and meaningful heights 2

3 The ellipsoid, the geoid, and you Deflection of the vertical No gravity, no height. Know gravity, know height. Mean sea level You are here Ellipsoid height, h Geoid height, N G h = H + N G Orthometric height, H Earth surface Note: Geoid height is negative everywhere in the coterminous US GEOID09 (CONUS) GEOID09 (CONUS) 3

4 GEOID09 (CONUS) GEOID09 (CONUS) Geoid model? We don t need no stinkin geoid model! 4

5 It doesn t matter which geoid model you use 5

6 Base X b, Y b, Z b Base φ b, λ b, h b GNSS Computation Flowchart Convert to ECEF (user input coordinates) Small residuals do not guarantee good results! X, Y, Z Rover Convert from ECEF Earth-Centered Earth-Fixed Cartesian Coordinates Grid translation, rotation, & scaling (not recommended) φ, λ h N, E Map projection Datum transformation (if necessary) Geoid model H Vertical shift, inclined planar correction, and/or other vertical transformation (often unavoidable) N, E φ, λ H h 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 ( 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 6

7 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: 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 7

8 Height Mod stations Height Modernization origins Height 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 highquality results Height Modernization guidelines NOS NGS 58 GPS derived ellipsoid heights (November 1997) NOS NGS 59 GPS derived orthometric heights (March 2008) A new & improved NGS 58/59 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 8

9 Two NGS 58/59 update projects 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 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 I d rather be Friends don t let friends Bluebook What 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 9

10 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 10

11 Height Mod adjustment sequence Typically 4 adjustments performed 1. 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 11

12 Questions? Achieving Great Height with GNSS Orthometric height change (meters) NAVD 88 to new vertical datum Estimated as NAVD 88 "zero" (datum) surface minus NGS gravimetric geoid 12