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The Canadian Geodetic Vertical Datum of 2013 Canadian Institute of Geomatics – Ottawa Branch 29 April 2014. Marc Véronneau Canadian Geodetic Survey, Surveyor General Branch. OUTLINE. The Canadian Spatial Reference System (CSRS) … 3 WHAT - Canada’s Height Modernization … 4
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The Canadian Geodetic Vertical Datum of 2013Canadian Institute of Geomatics – Ottawa Branch29 April 2014 Marc Véronneau Canadian Geodetic Survey, Surveyor General Branch
OUTLINE • The Canadian Spatial Reference System (CSRS) … 3 • WHAT - Canada’s Height Modernization … 4 • WHY - Canada’s Height Modernization … 5 • WHAT - Canadian Geodetic Vertical Datum of 1928 … 7 • WHAT - Canadian Geodetic Vertical Datum of 2013 … 11 • WHAT - Impact, Velocity, Labelling and Tools … 19 • SUMMARY … 23 • General concepts (optional) … 24
Canadian Spatial Reference System (CSRS) Horizontal network (f, l) 4-D Geometric frame Geopotential frame (f, l, h, H = h – N, t) 4-D Geometric frame (f, l, h, t) Dynamic coordinates NAD27 NAD83 NAD83(CSRS) Velocity models CGG2013 NAD83(CSRS) Velocity models Direct Transformation ITRF96 NAD83(CSRS) 3-DGeometric frame (f, l, h) CGVD2013 Vertical network (H) Adopted in U.S.A., but not in Canada CGVD28 NAVD 88
WHAT is Canada’s Height Modernisation? • It is the establishment of a new vertical datum for Canada that is integrated within the CSRS • The Canadian Geodetic Vertical Datum of 2013 (CGVD2013) • Released in November 2013 • It replaces the Canadian Geodetic Vertical Datum of 1928 (CGVD28) • Adopted in 1935 by an Order in Council • Three important changes: • New definition: from mean sea level at specific tide gauges to an equipotential surface • New realisation: from adjusting levelling data to integrating gravity data • New access: from benchmarks to a geoid model • CGVD2013 is compatible with Global Navigation Satellite Systems (GNSS) such as GPS Orthometric height determination by two techniques: levelling and combination of GPS measurements and a geoid model.
WHY Height Modernisation in Canada? • TECHNOLOGY, ACCESS & COST • Levelling is a precise technique that served Canada well over the last 100 years to realise and maintain a vertical datum, but for a country as wide as Canada … • It is prone to the accumulation of systematic errors over long distances; • It does not provide a national coverage (BMs only along major roads and railways); • It is a costly and time-consuming technique. Motorized Levelling
WHY Height Modernisation in Canada? • Modern technology in positioning • GNSS positioning is now mature and has gained widespread adoption by users. • It is a cost efficient technique in determining precise heights everywhere in Canada. • Satellite gravity missions offer unprecedented precision in the determination of the long and middle wavelength components of the geoid. • A geoid model realizes an accurate and homogeneous vertical reference surface all across Canada (land, lakes and oceans). • Our U.S. partners contributed GRAV-D data in the Great Lakes region. GPS GRAV-D GRACE GOCE
DH = BS - FS Foresight rod Backsight rod Levelling Backsight readingBS Foresight reading FS B DH WHAT is the Canadian Geodetic Vertical Datum of 1928 (CGVD28)? A
CGVD28: Levelling networks ? ~ 90 000 benchmarks Prince Rupert Pointe-au-Père Vancouver Halifax Yarmouth Rouses Point 1906-1928 1929-1939 1940-1965 1966-1971 1972-1981 1982-1989 1990-2007 Original constraints for Canada’s mainland Examples of later constraints
Levelling surveys over the years in Canada 1906-1928 1929-1939 1940-1965 1972-1981 1982-2012 1966-1971
WHAT are the error sources in CGVD28? • CGVD28: • Assume that oceans are at a same equipotential surface • Use entirely gravity values from a mathematical model • Omit systematic corrections on old levelling data • Neglect post-glacial rebound • Accumulation of systematic errors Earth surface St-Lawrence River (Pointe-au-Père) Pacific Ocean (near Vancouver) Atlantic Ocean Level surface wrt MSL in Vancouver (near Halifax) CGVD28 +36 cm -140 cm Level surface wrt MSL in Halifax NAVD 88 • NAVD 88 (not adopted in Canada): • Significant east-west systematic error (~1 m) of unknown sources in Canada (in the US too)
WHAT is the definition of CGVD2013? • U.S. NGS and NRCan’s GSD signed an agreement (16 April 2012) to realize and maintain a common vertical datum for USA and Canada defined by W0 = 62,636,856.0 m2/s2 • CGVD2013: Conventional equipotential surface (W0 = 62,636,856.0 m2/s2) averaging the coastal mean sea level for North America measured at Canadian and American tide gauges. • It also corresponds to the current convention adopted by the International Earth Rotation and Reference Systems Service (IERS) and International Astronomical Union (IAU). • Canada’s recommended definition for a World Height System Sea Surface Topography Canadian tide gauges American tide gauges
Ellipsoidal height and Orthometric height • GPS ellipsoidal heights require conversion to orthometric heights (heights above mean sea level) using a geoid model. • Heights are traditionally referred to mean sea level (BM, DEM, Topo maps). • Orthometric heights are consistent with the direction of water flow. Sept-Iles Ste-Anne-des-Monts Baie-Comeau Slope of the Saint-Lawrence River between Portneuf and Sept-Iles Rimouski Gros-Cacouna Ellipsoidal heights St-Joseph-de-la-Rive Orthometric heights St-Francois Portneuf Sept-Iles
Canadian Gravimetric Geoid of 2013 (CGG2013) Boundaries North: 90° South: 10° West: -170° East: -10° Resolution 2’ x 2’ Satellite model EIGEN-6C3stat (GFZ) Förste et al., IAG 2013 GOCE (until May 24, 2013) Transition zone Degrees: 120-180 (l = 333 km-222 km) Reference frames ITRF2008 and NAD83(CSRS) GRACE (2002 - ) DEM Land & ship gravity GOCE (2009 – 2013) Altimetry
Accuracy of the geoid model (CGG2013) • The accuracy considers data errors (gravity and DEM), grid resolution, discrepancies between gravity datasets and cut-off of satellite contribution. • 3 cm or better accuracy over 80% of Canada’s landmass • Decimeter level in areas with greater topography/mass distribution variability • Centimeter level relative precision over distances of 100 km or less Unit: cm 1 9 11 3 5 7 67% confidence (1 s)
WHAT is the difference between CGVD2013 and the mean sea level? Terrain Mean Sea Level Vancouver 17 cm Geoid (CGVD2013) -39 cm Halifax Mean Sea Level
WHAT is the difference between CGVD2013 and CGVD28? CGVD28(HTv2.0) – CGVD2013(CGG2010) Approximate values HCGVD2013– HCGVD28 St John’s -37 cm Halifax -64 cm Charlottetown -32 cm Fredericton -54 cm Montréal -36 cm Toronto -42 cm Winnipeg -37 cm Regina -38 cm Edmonton -04 cm Banff +55 cm Vancouver +15 cm Whitehorse +34 cm Yellowknife -26 cm Tuktoyaktuk -32 cm Banff CGVD2013 Vancouver Thunder Bay Difference (m) Regina CGVD28 Halifax Windsor Montréal Distance (km)
WHAT is the difference for Ontario? • Conversion CGVD28-CGVD2013 • GPS on BM • Published elevations at BMs • HTv2.0 - CGG2013 (image on the left) HCGVD2013– HCGVD28
HOW CGVD2013 impact heights in Canada? • All reference points (benchmarks) will have a new elevation. • Natural Resources Canada (NRCan) stopped levelling surveys for the maintenance of the vertical datum. • NRCan is NOT maintaining benchmarks by either levelling or GNSS technique. • However, the levelling networks is readjusted to conform with CGVD2013 using existing data. • NRCan is publishing CGVD28 and CGVD2013 heights at benchmarks. • NRCan cannot confirm the actual height of benchmarks in either CGVD28 or CGVD2013 (cannot confirm stability of benchmarks). • The Canadian Active Control Stations (CACS) and Stations of the Canadian Base Network (CBN) form the federal infrastructure for positioning. • 250 stations • Modern alternative techniques provide height determination. • NRCan’sPrecise Point Positioning (PPP) • Differential GNSS positioning • Public and Private Real-Time Kinematic (RTK) positioning • Levelling will remain the most efficient technique for most short distance work.
Vertical velocity (Glacial Isostatic Adjustment) Vertical velocity of the geoid (GRACE) Vertical velocity of the terrain (GPS) Colour scale: -14 mm/a to 14 mm/a Colour scale: -1.4 mm/a to 1.4 mm/a
Labelling Heights • Type of height: Orthometric (H), dynamic (Hd), normal (Hn), ellipsoidal (h), geoid (N) • Height Reference System: NAD83, ITRF, CGVD28, CGVD2013, NAVD 88 • Height Reference Frame: CSRS v., geoid model • Precision (e.g., ± 0.05 m) • Epoch (e.g., 2012.75) H N Height: 101.61 m Precision: ± 0.01 m Epoch: 2013.2 Type of height: Orthometric Height system: CGVD2013 Height frame: CGG2013 H = 23.126 ± 0.007 m CGVD2013(CGG2013) Epoch 2013.2 Geoid Height: -10.354 m Precision: ± 0.015 m Epoch: Static Model: CGG2013 Frame: NAD83(CSRS) N = -10.354 ± 0.015 m CGG2013, NAD83(CSRS) h Height: 91.256 m Precision: ± 0.007 m Epoch: 2013.2 Type of height: Ellipsoidal (geodetic) Height system: NAD83 Height frame: CSRS (version if available) h = 23.126 ± 0.007 m NAD83(CSRS) Epoch 2013.2
Tools available for Height Modernisation Precise Point Positioning (PPP): Process GPS RINEX files to provide coordinates (latitude, longitude, ellipsoidal height and orthometric height) GPS-H: Convert ellipsoidal heights to orthometric heights (makes use of any geoid models, works with different types of coordinate systems (geographic, UTM, MTM and Cartesian), and different geometric reference frames (NAD83(CSRS) and ITRF)) TRX: Transform coordinates between different geometric reference frames, epochs and coordinate systems.
SUMMARY • NRCan released a new vertical datum in November 2013 • Canadian Geodetic Vertical Datum of 2013 (CGVD2013) • Realised by geoid model CGG2013 (W0 = 62,636,856.0 m2/s2) • Compatible with GNSS positioning technique • Levelling networks were readjusted using existing data to conform with CGVD2013 • Why a new national vertical datum? • Cost of conducting levelling surveys at the national scale • To provide access to the vertical datum all across Canada • New space-based technologies (GNSS/Gravity) in positioning • The difference between CGVD2013 and CGVD28 • Separation ranging from -65 cm and 55 cm at the national scale. • US and Canada signed an agreement for the realisation of a unique height reference system between the two country by 2022. The common datum is defined by W0= 62,636,856.0 m2/s2 (adopted definition of CGVD2013).
General concepts: Datum • Datum • Reference point or surface against which position measurements are made • Vertical Datum • Reference for measuring the elevations of points Terrain H H H H Datum • The vertical datum should have physical meaning in order to i.e. properly manage water.
General concepts: Reference system and Reference frame • Reference System • Collection of abstract principles • Definitions, conventions, standards, fundamental parameters • Reference frame • Concrete realisation of the reference system • Levelling (e.g., CGVD28, NAVD88, IGLD85) • Geoid modelling (e.g., CGG2010, EGM08) • A reference system may have several realisations (reference frames) • New data • New processing approach • A new reference frame is generally an improvement (more accurate) with respect to the previous frame.
General concepts: Types of vertical datum • Tidal • Sea level at tide gauges • Levelling (benchmarks) • Advantage: Simple concept • Disadvantage: MSL stops at the coast, MSL is not level • Gravimetric • Potential • Geoid model • Advantage: Define everywhere • Disadvantage: Complex process • Geodetic • Ellipsoid • Geometric reference frame • Advantage: Efficient and precise method • Disadvantage: No physical meaning ? N = f(gsat, gter)
General concepts: Types of height W0 = Geoid surface W5 Wi = Equipotential surfaces W4= Lake surface P2 W4 Earth’s P1 South North W3 Ellipsoid Lake W2 W1 Q1 W0 Q2 Dynamic Height (Hd) Orthometric height (H) Ellipsoidal height (h) - gi is the mean gravity along the plumb line between Qi and Pi. Hd1 = Hd2 H1 < H2 h1 > h2 - As the lake is in the northern hemisphere, g1 is larger than g2 because gravity increases towards the pole. Hd1 = (W4 – W0)/g45 H1 = (W4 – W0)/g1 - Hd2 = (W4 – W0)/g45 H2 = (W4 – W0)/g2
General concepts: Relation between heights, potential and gravity • Gravity is the vertical gradient of potential (W) W = 60 m2/s2 W = 30 m2/s2 W = 40 m2/s2 W = 70 m2/s2 W = 50 m2/s2 W = 60 m2/s2 W = 80 m2/s2 W = 70 m2/s2 W = 80 m2/s2 W = 90 m2/s2 W = 90 m2/s2 g1 < g2
What is the Geoid? • Equipotential surface representing best, in a least-squares sense, the global mean sea level (MSL) • The actual shape of the Earth • Vertical datum (W0) • Gravity is perpendicular (vertical) to the geoid • Water stays at rest on the geoid (a level surface) The water drop stays in place geoid g g g
General concepts: Geometric reference frames (NAD83(CSRS), ITRF, WGS84) Geodetic (ellipsoidal) heights (h) and geoid heights (N) do not have the same magnitude in the ITRF and NAD83(CSRS) reference frames because the position of the origin is different. Orthometric heights (H) are independent to the 3D geometric reference frames. Earth’s surface h ITRF h NAD83(CSRS) Ellipsoid (e.g. GRS80) H H = h – N Geoid Ellipsoid(GRS80) NAD83(CSRS) origin N NAD83(CSRS) N ITRF ~2.2 m Note: WGS84 has the same origin as ITRF ITRF origin
General concepts: Mass and potential Wi: Equipotential surface W0: Equipotential surface (datum) W2 DW DW W1 DW Ht1= -1 A H = 0 DW W0 B C Ht1= 1 ht0 ht1 Ellipsoid gt1 gt0 0x 2x
General concepts: The real Earth Continent Ocean Ellipsoid (GRS80) a: 6,378,137 m b: 6,356,752.3141 m a-b: 21,384.6859 m Everest: 8848 m N max.: 100 m SST max.: 2 m Semi-minor axis: b Semi-major axis: a If a = 1 m b: 0.996647 m a-b: 3.35 mm Everest: 1.387 mm N max: 0.016 mm SST max: 0.3 mm Sphere Geoid Ellipsoid
NRCan Contacts: Philippe Lamothe (email@example.com) Marc Véronneau (firstname.lastname@example.org) Jianliang Huang (email@example.com) General information: Web: http://www.geod.nrcan.gc.ca Email: firstname.lastname@example.org Phone: 1-613-995-4410 Fax: 1-613-995-3215 QUESTIONS?