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Gravity and Geoid Modelling for Improved Heights

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Gravity and Geoid Modelling for Improved Heights Gravity and Geoid Modelling for Improved Heights Texas Height Modernization Workshop Houston, Texas August 28, 2015 Denis Riordan, PSM NOAA, National Geodetic Survey [email protected] Presentation Outline 1. - Introductions. 2. - Mission and Vision of the National Geodetic Survey. 3. - NAVD88 – Still good enough? 4. - GEOIDS – types and definitions. 5. - GRAV-D and what it’s about. 6. - NGS Gravity Survey Plan. 7. - New U.S. Datums for 2022. 8. - Questions. U.S. Department of Commerce National Oceanic & Atmospheric Administration National Geodetic Survey Mission: To define, maintain & provide access to the National Spatial Reference System (NSRS) to meet our Nation’s economic, social & environmental needs National Spatial Reference System * Latitude * Scale * Longitude * Gravity * Height * Orientation & their variations in time NGS MISSION - The NSRS Define - National Coordinate Sys. (NSRS) Maintain - the NSRS Provide Access The National Geodetic Survey 10 year plan Mission, Vision and Strategy 2013 – 2023 http://www.ngs.noaa.gov/INFO/NGS10yearplan.pdf • Official NGS policy as of Jan 9, 2008 (updated in 2013) – Modernized agency – Attention to accuracy – Attention to time-changes – Improved products and services – Integration with other fed missions • 2022 Targets: – Replace NAD 83 and NAVD 88 – Cm-accuracy access to all coordinates – Customer-focused agency – Global scientific leadership GEODETIC DATUMS VERTICAL 1 D (Orthometric Height) (e.g. NGVD 29, NAVD 88, Tidal) HORIZONTAL 2 D (Latitude and Longitude) (e.g. NAD 27, NAD 83 (1986)) GEOMETRIC 3 D (Latitude, Longitude and Ellipsoid Height) Fixed and Stable - Coordinates seldom change (e.g. NAD 83 (1994), NAD 83 (2007)) also 4 D (Latitude, Longitude, Ellipsoid Height, Velocities) Coordinates change with time (e.g. ITRF00, ITRF05, ITRF08) Datums - Vertical National Geodetic Vertical Datum of 1929 - Based on heights at 26 tide stations along US & Canadian coast. - Used 75,000 Km (US) & 30,000 Km (Can.) of leveling data. - Adjustment constrained to the 26 tide stations. North American Vertical Datum of 1988 - Approx. 650,000 Km of new leveling since NGVD 1929. - Based on (constrained to) agreed upon mark in Great Lakes Region (Father’s Point). - Greatly improved vertical accuracy over NGVD 1929 datum. - Defined only on the conterminous North American continent - Does not (and cannot) exist for any place you can’t level to from Father Point, such as: • Guam, American Samoa, Hawaii, Puerto Rico, American Virgin Islands, Commonwealth of the Northern Marianas, Aleutian Islands VERTICAL CONTROL USED FOR NGVD 1929 DATUM North American Vertical Datum of 1988 – June 24, 1993 Federal Register / Vol. 58, No. 120 SUMMARY: This Notice announces a decision by the Federal Geodetic Control Subcommittee (FGCS) to affirm the North American Vertical Datum of 1988 (NAVD 88) as the official civilian vertical datum for surveying and mapping activities in the United States performed or financed by the Federal Government, and to the extent practicable, legally allowable, and feasible, require that all Federal agencies using or producing vertical height information undertake an orderly transition to NAVD 88. NGVD 29 NAVD 88 • Datum Definition 26 Tide Gauges in Father’s Point the U.S. & Canada Quebec, Canada • Bench Marks 100,000 450,000 • Leveling (Km) 102,724 1,001,500 • Geoid Fitting Distorted to Fit Best Continental Model MSL Gauges Problems with NAVD 88 (Nationally) • NAVD 88 relies upon bench marks that: – Disappear by the thousands every year – Are not funded for replacement – Are almost never re-checked for movement – Are affected by freeze / thaw, subsidence, uplift, etc. – Are not necessarily in convenient places – Cross-country error build up NAVD 88 suffers from a zero height surface that: – Has been proven to be ~50 cm biased from the latest, best geoid models (GRACE satellite) – Has been proven to be ~ 1 meter tilted across CONUS (again, based on the independently computed geoid from the GRACE satellite) 10 Can NAVD 88 be fixed? • Long term fix: Re-level some/all of NAVD 88 –81,500 km of 1st order leveling at least –625,000 km of mixed 1st and 2nd order leveling • Re-leveling NAVD 88 estimated to cost between $200 Million and $2 Billion • Time factor in that amount of leveling • Still would have problems related to passive control 12 Can NAVD 88 be fixed? Best long term fix: It has been determined that the best long term fix for having a national vertical datum that is accurate and available, is to replace the NAVD 88 datum with a geopotential reference frame (surface). ……..but how is that accomplished? 13 NEW VERTICAL DATUM (Rationale) • A move away from differentially leveled passive control as the defining mechanism of the reference surface • To be consistent with the shift in the geometric reference frame/ellipsoid (2022) • Improvement in our technical abilities in reference surface realization (geopotential gravimetric reference surface - 1cm accuracy of the geoid (GNSS/GRAV-D)) • Goal - ability to establish 2cm orthometric height anywhere in U.S. using a minimum of 15 min. of GNSS data. • The new geopotential reference surface will be aligned with the geometric reference frame/ellipsoid (i.e., no hybrid geoid) Vertical Datum (Status) • United States Gravimetric Geoid 2012 (USGG12) Made before GRAV-D data • Hybrid Geoid 2012 (Geoid 12B) available • 2022 Definition of the Vertical Reference Surface • Yearly Experimental Geoids that Include GRAV-D Airborne Data • The US is not the first adopter of a vertical datum based on a gravimetric geoid! • New Zealand • First adopter in the world (2009) • Finished airborne gravity over country in 2 months of 2014 • New datum available in late 2015 • Canada • Adopted in 2014 Can NAVD 88 be fixed? Best long term fix: Replace NAVD 88 with a 1cm vertically accurate geopotential reference frame (surface)……..but how? First, let’s talk about geoids. 16 Definitions: GEOIDS vs GEOID HEIGHTS • “The equipotential surface of the Earth’s gravity field which best fits, in the least squares sense, (global) mean sea level.”* • Can’t see the surface or measure it directly. • Can be modeled from gravity data as they are mathematically related. • Note that the geoid is a vertical datum surface • A geoid height is the height from an ellipsoidal datum to a geoid. • Hence, geoid height models are directly tied to the geoid and ellipsoid that define them (i.e., geoid height models are not interchangeable). *Definition from the Geodetic Glossary, September 1986 Why do we need Geoid Models? • The geoid surface is mathematically GRACE Satellite Gravity related to gravity • Understanding the gravity field is important because it − impacts survey measurements − dictates the direction water flows • Geoid models allow us to relate different kinds of heights Measuring and relating different kinds of heights • Ellipsoid heights – Inherent to GNSS measurements – Need accurate ellipsoid height control – Better field procedures = better heights • Orthometric heights – Measure by leveling surveys – Most accurate but most expensive – Difficult to maintain over time, esp. on national scale • Geoid heights – Derived from model developed from gravity observations – Can provide relationship between e.h. and o.h. The ellipsoid, the geoid, and you Deflection of the vertical You are here Earth surface Ellipsoid height, h Orthometric height, H Mean sea level Geoid height, NG h = H + NG Note: Geoid height is negative everywhere in the coterminous US Types Uses and History of Geoid Height Models • Gravimetric (or Gravity) Geoid Height Models – Defined by satellite gravity, EGM08, RTM model – Refined by terrain models (DEM’s) – Purely a scientific model for engineering applications – Does not provide for GPS to NAVD88 relationship • Composite (or Hybrid) Geoid Height Models – Gravimetric geoid defines most regions – Warped to fit available GPSBM control data – Defined by legislated ellipsoid (NAD 83) and local vertical datum (NAVD 88, PRVD02, etc.) – May be statutory for some surveying & mapping applications USGG2012(Gravimetric Geoid) • Satellite Gravity Models + EGM08 • 2.6 million terrestrial, ship-borne, altimetric gravity measurements • 30 arc second Digital Elevation Data • Computed on 1 x 1 arc minute grid spacing • GRS-80 ellipsoid Basic Concepts of Hybrid Geoid Modeling (NGS) • Start with a gravimetric geoid (USGG2012) • Use control data to fit to local datums – Appropriate versions of NAD 83 – Respective local Vertical Datum (if one exists) • Use LSC to determine correlated signal • For complex areas (e.g., CONUS), use MMLSC • Apply grid of correlated signal to USGG2012 • Results in GEOID12 with high frequency nature from USGG2012 but fit to local control GPS Bench Marks used for Geoid 12B (23,961) GPSBM1999: 6,169 total 0 Canada STDEV 9.2 cm (2σ) GPSBM2003: 14,185 total 579 Canada STDEV 4.8 cm (2σ) GPSBM2009: 18,291 total 576 Canada STDEV 2.8 cm (2σ) TX GPSBM Marks for Geoid 12B (377) Which Geoid for Which NAD 83? NAD 83(2011) --- Geoid12B NAD 83(2007) --- Geoid09 NAD 83(19xx) --- Geoid03 --- Geoid99 --- Geoid96 Current and Best NGS Geoid: USGG2012 • GRACE + 2 yrs. of GOCE + other satellite data • + World Gravity Model (EGM2008) • + Estimated Gravity from Topography • + Around Two Million Surface Gravity Points But even this isn’t good enough for a 1-2 cm accuracy vertical reference surface Problems with Gravity Holdings 20-100 km Terrestrial • Field is not sampled gravity gravity gaps along coast uniformly • Data range in age and quality, some w/o metadata • Some surveys
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